GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


archive:programming:dostech.pro

CHAPTER 1.

                       DOS TECHNICAL INFORMATION

Programming Technical Reference - IBM Copyright 1988, Dave Williams

SOME HISTORY

Development of MSDOS/PCDOS began in October 1980, when IBM began searching the market for an operating system for the yet-to-be-introduced IBM PC. Microsoft had no real operating system to sell, but after some research licensed Seattle Computer Products' 86-DOS, which had been written by a man named Tim Paterson for use on the company's line of 8086, S100 bus micros. This was hurriedly polished up and presented to IBM for evaluation. IBM had originally intended to use Digital Research's CP/M operating system, which was the industry standard at the time. Folklore reports everything from obscure legal entanglements to outright snubbing of the IBM representatives by Digital, irregardless, IBM found itself left with Microsoft's offering of "Microsoft Disk Operating System 1.0". An agreement was reached between the two, and "IBM PC-DOS 1.0" was ready for the introduction of the IBM PC in October 1981. IBM subjected the operating system to an extensive quality-assurance program, found well over 300 bugs, and decided to rewrite the programs. This is why PC-DOS is copyrighted by both IBM and Microsoft.

It is sometimes amusing to reflect on the fact that the IBM PC was not originally intended to run MSDOS. The target operating system at the end of the development was for a (not yet in existence) 8086 version of CP/M. On the other hand, when DOS was originally written the IBM PC did not yet exist! Although PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably have been the main operating system for the PC except for two things - Digital Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and many software developers found it easier to port existing CP/M software to DOS than to the new version of CP/M.

MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. There was an expansion board for the Apple ][ that allowed one to run (some) well - behaved DOS programs. There are expansion boards for the Commodore Amiga 2000, the Apple MacIntosh II, and the IBM RT PC allowing them to run DOS, and the IBM 3270 PC, which ran DOS on a 68000 microprocessor. The Atari STs can run an emulator program and boot MSDOS.

Specific Versions of MS/PC-DOS:

DOS version nomenclature: major.minor.minor. The digit to the left of the decimal point indicates a major DOS version change. 1.0 was the first version. 2.0 added subdirectories, etc. 3.0 added file handles and network support. The first minor version indicates customization for a major application. For example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not seem to have any particular meaning.

The main versions of DOS are:

PC-DOS 1.0 October 1981 original release PC-DOS 1.1 June 1982 bugfix, double sided drive support MS-DOS 1.25 June 1982 for early compatibles PC-DOS 2.0 March 1983 for PC/XT, many UNIX-like functions PC-DOS 2.1 October 1983 for PCjr, bugfixes for 2.0 MS-DOS 2.11 October 1983 compatible equivalent to 2.1 PC-DOS 3.0 August 1984 for PC/AT, network support PC-DOS 3.1 November 1984 bugfix for 3.0 MS-DOS 2.25 October 1985 compatible; extended foreign language support PC-DOS 3.2 July 1986 3.5 inch drive support for Convertible PC-DOS 3.3 April 1987 for PS/2 series

Some versions of MS-DOS varied from PC-DOS in the availible external commands. Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves or contracted them from outside software houses like Phoenix. Most of the external programs for DOS 3.x are written in "C" while the 1.x and 2.x utilities were written in assembly language. Other OEMs required customized versions of DOS for their specific hardware configurations, such as Sanyo 55x and early Tandy computers, which were unable to exchange their DOS with the IBM version.

At least two versions of DOS have been modified to be run entirely out of ROM. The Sharp PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1100 and some Tandy models have MSDOS 2.11 in ROM.

THE OPERATING SYSTEM HIERARCHY

The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer between the application program and the machine, and as a source of services to the application program. The system heirarchy may be thought of as a tree, with the lowest level being the actual hardware. The 8088 or V20 processor sees the computer's address space as a ladder two bytes wide and one million bytes long. Parts of this ladder are in ROM, parts in RAM, and parts are not assigned. There are also 256 "ports" that the processor can use to control devices. The hardware is normally addressed by the ROM BIOS, which will always know where everything is in its particular system. The chips may usually also be written to directly, by telling the processor to write to a specific address or port. This sometimes does not work as the chips may not always be at the same addresses or have the same functions from machine to machine.

DOS STRUCTURE

DOS consists of four components:

* The boot record * The ROM BIOS interface (IBMBIO.COM or IO.SYS) * The DOS program file (IBMDOS.COM or MSDOS.SYS) * The command processor (COMMAND.COM or aftermarket replacement)

* The Boot Record

The boot record begins on track 0, sector 1, side 0 of every diskette formatted by the DOS FORMAT command. The boot record is placed on diskettes to produce an error message if you try to start up the system with a nonsystem diskette in drive A. For hard disks, the boot record resides on the first sector of the DOS partition. All media supported by DOS use one sector for the boot record.

* Read Only Memory (ROM) BIOS Interface

The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS. This file provides a low-level interface to the ROM BIOS device routines and may contain extensions or changes to the system board ROMs. Some compatibles do not have a ROM BIOS to extend, and load the entire BIOS from disk. (Sanyo 55x, Viasyn)

* The DOS Program File

The actual DOS program is file IBMDOS.COM or MSDOS.SYS. It provides a high- level interface for user (application) programs. This program consists of file management routines, data blocking/deblocking for the disk routines, and a variety of built-in functions easily accessible by user programs. When a user program calls these function routines, they accept high-level information by way of register and control block contents. For device operations, the functions translate the requirement into one or more calls to IBMBIO.COM or MSDOS.SYS to complete the request.

* The Command Interpreter

The Command interpreter, COMMAND.COM, consists of these parts:

Resident Portion:

The resident portion resides in memory immediately following IBMDOS.COM and its data area. This portion contains routines to process interrupts 22h (Terminate Address), 23h (Ctrl-Break Handler), and 24h (Critical Error Handler), as well as a routine to reload the transient portion if needed. For DOS 3.x, this portion also contains a routine to load and execute external commands, such as files with exensions of COM or EXE.

When a program terminates, a checksum is used to determine if the application program overlaid the transient portion of COMMAND.COM. If so, the resident portion will reload the transient portion from the area designated by COMSPEC= in the DOS environment. If COMMAND.COM cannot be found, the system will halt.

NOTE: DOS 3.3 checks for the presence of a hard disk, and will default to

    COMSPEC=C:\. Previous versions default to COMSPEC=A:\. Under some DOS
    versions, if COMMAND.COM is not immediately availible for reloading
    (i.e., swapping to a floppy with COMMAND.COM on it) DOS may crash.

All standard DOS error handling is done within the resident portion of COMMAND.COM. This includes displaying error messages and interpreting the replies of Abort, Retry, Ignore, Fail.

An initialization routine is included in the resident portion and assumes control during startup. This routine contains the AUTOEXEC.BAT file handler and determines the segment address where user application programs may be loaded. The initialization routine is then no longer needed and is overlaid by the first program COMMAND.COM loads.

NOTE: AUTOEXEC.BAT may be a hidden file.

A transient portion is loaded at the high end of memory. This is the command processor itself, containing all of the internal command processors and the batch file processor. For DOS 2.x, this portion also contains a routine to load and execute external commands, such as files with extensions of COM or EXE.

This portion of COMMAND.COM also produces the DOS prompt (such as "A>"), reads the command from the standard input device (usually the keyboard or a batch file), and executes the command. For external commands, it builds a command line and issues an EXEC function call to load and transfer control to the program.

NOTE: COMMAND.COM may be a hidden file.

NOTE: For Dos 2.x, the transient portion of the command processor contains the

    EXEC routine that loads and executes external commands. For DOS 3.x, the
    resident portion of the command processor contains the EXEC routine. 
DOS Initialization

The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset (reset button), or by turning the computer on. The Intel 80x8x series processors always look for their first instruction at the end of their address space (0FFFF0h) when powered up or reset. This address contains a jump to the first instruction for the ROM BIOS. Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine status and run inspection programs of various sorts. Some machines set up a reserved RAM area with bytes indicating installed equipment (AT and PCjr). The ROM routine looks for a disk drive at A: or an option ROM (usually a hard disk) at absolute address C:800h. If no floppy drive or option ROM is found, the BIOS calls int 19h (ROM BASIC if it is an IBM) or displays error message. If a bootable disk is found, the ROM BIOS loads the first sector of information from the disk and then jumps into the RAM location holding that code. This code normally is a routine to load the rest of the code off the disk, or to "boot" the system. The following actions occur after a system initialization:

1. The boot record is read into memory and given control.

2. The boot record then checks the root directory to assure that the first

   two files are IBMBIO.COM and IBMDOS.COM. These two files must be the 
   first two files, and they must be in that order (IBMBIO.COM first, with 
   its sectors in contiguous order).
   NOTE: IBMDOS.COM need not be contiguous in version 3.x.

3. The boot record loads IBMBIO.COM into memory.

4. The initialization code in IBMBIO.COM loads IBMDOS.COM, determines

   equipment status, resets the disk system, initializes the attached 
   devices, sets the system parameters and loads any installable device 
   drivers according to the CONFIG.SYS file in the root directory (if 
   present), sets the low-numbered interrupt vectors, relocates IBMDOS.COM 
   downward, and calls the first byte of DOS.
   NOTE: CONFIG.SYS may be a hidden file.

5. DOS initializes its internal working tables, initializes the interrupt

   vectors for interrupts 20h through 27h, and builds a Program Segment 
   Prefix for COMMAND.COM at the lowest available segment. For DOS versions
   3.10 up, DOS initializes interrupt vectors for interrupts 0Fh through 3Fh.

6. IBMBIO.COM uses the EXEC function call to load and start the top-level

   command processor. The default command processor is COMMAND.COM.

CHAPTER 2

Programming Technical Reference - IBM Copyright 1988, Dave Williams

CPU Port Assignments, System Memory Map, BIOS Data Area, Interrupts 00h to 09h

                      SYSTEM MEMORY MAP - OVERALL

The IBM PC handles its address space in 64k segments, divided into 16k fractions and then further as nescessary.

start start end addr. addr. addr. usage (dec) (hex)

00000 640k * system data, drivers…. 0000:0000 hardware interrupt vectors 0000:0040 BIOS interrupt vectors 0k start of RAM | 0000:0080 DOS interrupt vector table 16k 00000-03FFF | 0000:0300 Stack area during POST and bootstrap routine 32k 04000-07FFF | 0000:0400 BIOS Data Area 48k 08000-0BFFF | 0000:04F0 Intra-Application Communications Area | 64k 10000-13FFF | 0000:0500 DOS reserved communication area 80k 14000-17FFF | xxxx:0000 IO.SYS - DOS interface to ROM I/O routines 96k 18000-1BFFF | xxxx:0000 MSDOS.SYS - DOS interrupt handlers, service 112k 1C000-1FFFF | routines (int 21 functions) | 128k 20000-23FFF | xxxx:xxxx DOS buffers, control areas, and installed 144k 24000-27FFF | device drivers. 160k 28000-2BFFF | xxxx:0000 resident portion of COMMAND.COM, interrupt 176k 2C000-2FFFF | handlers for int 22h, 23h,24h, and code to | reload the transient portion 192k 30000-33FFF | xxxx:0000 master environment block, default 64 bytes 208k 34000-37FFF | xxxx:0000 environment for next program 224k 38000-3BFFF | xxxx:0000 external commands or utilities (COM or EXE 240k 3C000-3FFFF | files) | 256k 40000-43FFF | —-:—- application programs 272k 44000-47FFF | xxxx:0000 user stack for COM files (256 bytes) 288k 48000-4BFFF | xxxx:0000 transient portion of COMMAND.COM 304k 4C000-4FFFF | | 320k 50000-53FFF | 336k 54000-57FFF | 352k 58000-5BFFF | 368k 5C000-5FFFF | | 384k 60000-63FFF | 400k 64000-67FFF | 416k 68000-6BFFF | 432k 6C000-6FFFF | | 448k 70000-73FFF | 464k 74000-77FFF | 480k 78000-7BFFF | 496k 7C000-7FFFF | | 512k 80000-83FFF | 528k 84000-87FFF | 544k 88000-8BFFF | original IBM PC-1 BIOS limited memory to 544k 560k 8C000-8FFFF | | 576k 90000-93FFF | 592k 94000-97FFF | 609k 98000-9BFFF | 624k 9C000-9FFFF | to 640k (top of RAM address space) A0000 * 64k * EGA address 640k A0000-A95B0 MCGA 320x200 256 color video buffer

            AF8C0  MCGA 640x480 2 color video buffer
           -A3FFF 

656k A4000-A7FFF 672k A8000-ABFFF 688k AC000-AFFFF

B0000 * 64k * mono and CGA address 704k B0000-B3FFF mono uses only 4k | The PCjr and early Tandy 1000 720k B4000-B7FFF | BIOSs revector direct writes to 736k B8000-BBFFF CGA uses entire 16k | the B8 area to the Video Gate 756k BC000-BFFFF | Array and reserved system RAM C0000 * 64k * expansion ROM 768k C0000-C3FFF 16k EGA BIOS C000:001E EGA BIOS signature (the letters IBM) 784k C4000-C5FFF

      C6000-C63FF  256 bytes Professional Graphics Display communication area
      C6400-C7FFF

800k C8000-CBFFF 16k hard disk controller BIOS, drive 0 default 816k CC000-CDFFF 8k IBM PC Network NETBIOS

      CE000-CFFFF

D0000 * 64k * expansion ROM | PCjr first ROM cartridge 832k D0000-D7FFF 32k IBM Cluster Adapter | address area. DA000 voice communications | 848k D4000-D7FFF | Common expanded memory board 864k D8000-DBFFF | paging area. 880k DC000-DFFFF | E0000 * 64k * expansion ROM | PCjr second ROM 896k E0000-E3FFF | cartridge address 912k E4000-E7FFF | area 928k E8000-EBFFF | 944k EC000-EFFFF |

F0000 * 64k * system | PCjr optional ROM 960k F0000-F3FFF reserved by IBM | cartridge address 976k F4000- | area (cartridge F6000 ROM BASIC Begins | BASIC) 992k F8000-FB000 | 1008k FC000-FFFFF ROM BASIC and original | BIOS (Compatibility BIOS | in PS/2) | 1024k FFFFF end of memory (1024k) for 8088 machines F000:FFF5 BIOS release date F000:FFFE PC model identification 384k 100000-15FFFF 80286/AT extended memory area, 1Mb motherboard 15Mb 100000-FFFFFF 80286/AT extended memory address space 15Mb 160000-FDFFFF Micro Channel RAM expansion (15Mb extended memory) 128k FE0000-FFFFFF system board ROM (PS/2 Advanced BIOS) PC Port Assignment, Intel 8088, 80C88, 8086, 80286, 80386 CPUs hex addr. Function 0000-000F 8237 DMA controller 0010-001F 8237 DMA controller (AT, PS/2) 0020-0027 8259A interrupt controller 0020-003F 8259A interrupt controller (AT) 0040-005F 8253-5 programmable timers (note: 0041 was memory refresh in PCs. Not used in PS/2) 0060-0067 8255 peripheral interface 0060-006F 8042 keyboard controller (AT) 0200-020F game-control adapter 0210-0217 expansion box (PC, XT) 0278-027F LPT3 02F8-02FF COM2 0300-031F prototype card 0320-032F hard disk controller 0378-037F LPT2 03BC-03BF LPT1 03D0-03DF CGA, MCGA, VGA adapter control 03F0-03F7 floppy disk controller 03F8-03FF COM1 note: These are functions common across the IBM range. The PCjr, PC Convertible and PS/2 (both buses) have enhancements. In some cases, the AT and PS/2 series ignore, duplicate, or reassign ports arbitrarily. If your code incorporates specific port addresses for video or system board control it would be wise to have your application determine the machine type and video adapter and address the ports as required. Reserved Memory Locations in the IBM PC addr. size description 000h-3FFh DOS interrupt vector table 30:00h- used as a stack area during POST and bootstrap routines. This 3F:FFh stack area may be revectored by an application program. The BIOS Data Area addr. from 400h to 4FFh 40:00 word COM1 port address | These addresses are zeroed out in the 40:02 word COM2 port address | OS/2 DOS Compatibility Box if any of 40:04 word COM3 port address | the OS/2 COM??.SYS drivers are loaded. 40:06 word COM4 port address | 40:08 word LPT1 port address 40:0A word LPT2 port address 40:0C word LPT3 port address 40:0E word LPT4 port address (not valid in PS/2 machines) 40:0E word PS/2 pointer to 1k extended BIOS Data Area at top of RAM 40:10 word equipment flag (see int 11h) bits: 0 1 if floppy drive present (see bits 6&7) 0 if not 1 1 if 80x87 installed (not valid in PCjr) 2,3 system board RAM (not used on AT or PS/2) 00 16k 01 32k 10 48k 11 64k 4,5 initial video mode 00 no video adapter 01 40 column color (PCjr) 10 80 column color 11 MDA 6,7 number of diskette drives 00 1 drive 01 2 drives 10 3 drives 11 4 drives 8 0 DMA present 1 DMA not present (PCjr) 9,A,B number of RS232 serial ports C game adapter (joystick) 0 no game adapter 1 if game adapter D serial printer (PCjr only) 0 no printer 1 serial printer present E,F number of parallel printers installed note 1) The IBM PC and AT store the settings of the system board switches or CMOS RAM setup information (as obtained by the BIOS in the Power-On Self Test (POST)) at addresses 40:10h and 40:13h. 00000001b indicates "on", 00000000b is "off". 2) CMOS RAM map, PC/AT: offset contents 00h Seconds 01h Second Alarm 02h Minutes 03h Minute Alarm 04h Hours 05h Hour Alarm 06h Day of the Week 07h Day of the Month 08h Month 09h Year 0Ah Status Register A 0Bh Status Register B 0Ch Status Register C 0Dh Status Register D 0Eh Diagnostic Status Byte 0Fh Shutdown Status Byte 10h Disk Drive Type for Drives A: and B: The drive-type bytes use bits 0:3 for the first drive and 4:7 for the other Disk drive types: 00h no drive present 01h double sided 360k 02h high capacity (1.2 meg) 03h-0Fh reserved 11h (AT):Reserved (PS/2):drive type for hard disk C: 12h (PS/2):drive type for hard disk D: (AT, XT/286):hard disk type for drives C: and D: Format of drive-type entry for AT, XT/286: 0 number of cyls in drive (0-1023 allowed) 2 number of heads per drive (0-15 allowed) 3 starting reduced write compensation (not used on AT) 5 starting cylinder for write compensation 7 max. ECC data burst length, XT only 8 control byte Bit 7 disable disk-access retries 6 disable ECC retries 5-4 reserved, set to zero 3 more than 8 heads 2-0 drive option on XT (not used by AT) 9 timeout value for XT (not used by AT) 12 landing zone cylinder number 14 number of sectors per track (default 17, 0-17 allowed) 13h Reserved 14h Equipment Byte (corresponds to sw. 1 on PC and XT) 15h-16h Base Memory Size (low,high) 17h-18h Expansion Memory Size (low,high) 19h-20h Reserved (PS/2) POS information Model 50 (60 and 80 use a 2k CMOS RAM that is not accessible through software) 21h-2Dh Reserved (not checksumed) 2Eh-2Fh Checksum of Bytes 10 Through 20 (low,high) 30h-31h Exp. Memory Size as Det. by POST (low,high) 32h Date Century Byte 33h Information Flags (set during power-on) 34h-3Fh Reserved 3) The alarm function is used to drive the BIOS wait function (int 15h function 90h). 4) To access the configuration RAM write the byte address (00-3Fh) you need to access to I/O port 70h, then access the data via I/O port 71h. 5) CMOS RAM chip is a Motorola 146818 6) The equipment byte is used to determine the configuration for the power-on diagnostics. 7) Bytes 00-0Dh are defined by the chip for timing functions, bytes 0Eh-3Fh are defined by IBM. 40:12 byte number of errors detected by infrared keyboard link (PCjr only) 40:13 word availible memory size in Kbytes (less display RAM in PCjr) this is the value returned by int 12h 40:17 byte keyboard flag byte 0 (see int 9h) bit 7 insert mode on 3 alt pressed 6 capslock on 2 ctrl pressed 5 numlock on 1 left shift pressed 4 scrollock on 0 right shift pressed 40:18 byte keyboard flag byte 1 (see int 9h) bit 7 insert pressed 3 ctrl-numlock (pause) toggled 6 capslock pressed 2 PCjr keyboard click active 5 numlock pressed 1 PCjr ctrl-alt-capslock held 4 scrollock pressed 0 40:19 byte storage for alternate keypad entry (not normally used) 40:1A word pointer to keyboard buffer head character 40:1C word pointer to keyboard buffer tail character 40:1E 32bytes 16 2-byte entries for keyboard circular buffer, read by int 16h 40:3E byte drive seek status - if bit=0, next seek will recalibrate by repositioning to Track 0. bit 3 drive D bit 2 drive C 1 drive B 0 drive A 40:3F byte diskette motor status bit 7 1, write in progress 3 1, D: motor on (floppy 3) 6 2 1, C: motor on (floppy 2) 5 1 1, B: motor on 4 0 1, A: motor on 40:40 byte motor off counter starts at 37 and is decremented 1 by each system clock tick. motor is shut off when count = 0. 40:41 byte status of last diskette operation where: bit 7 timeout failure bit 3 DMA overrun 6 seek failure 2 sector not found 5 controller failure 1 address not found 4 CRC failure 0 bad command 40:42 7 bytes NEC status 40:49 byte current CRT mode (hex value) 00h 40x25 BW (CGA) 01h 40x25 color (CGA) 02h 80x25 BW (CGA) 03h 80x25 color (CGA) 04h 320x200 color (CGA) 05h 320x200 BW (CGA) 06h 640x200 BW (CGA) 07h monochrome (MDA) extended video modes (EGA/MCGA/VGA or other) 08h lores,16 color 09h med res,16 color 0Ah hires,4 color 0Bh n/a 0Ch med res,16 color 0Dh hires,16 color 0Eh hires,4 color 0Fh hires,64 color 40:4A word number of columns on screen, coded as hex number of columns 20 col = 14h (video mode 8, low resolution 160x200 CGA graphics) 40 col = 28h 80 col = 46h 40:4C word screen buffer length in bytes (number of bytes used per screen page, varies with video mode) 40:4E word current screen buffer starting offset (active page) 40:50 8 words cursor position pages 1-8 the first byte of each word gives the column (0-19, 39, or 79) the second byte gives the row (0-24) 40:60 byte end line for cursor (normally 1) 40:61 byte start line for cursor (normally 0) 40:62 byte current video page being displayed (0-7) 40:63 word base port address of 6845 CRT controller or equivalent for active display 3B4h=mono, 3D4h=color 40:65 byte current setting of the CRT mode register 40:66 byte current palette mask setting (CGA) 40:67 5 bytes temporary storage for SS:SP during shutdown (cassette interface) 40:6C word timer counter low word 40:6E word timer counter high word 40:69 byte HD_INSTALL (Columbia PCs) (not valid on most clone computers) bit 0 = 0 8 inch external floppy drives 1 5-1/4 external floppy drives 1,2 = highest drive address which int 13 will accept (since the floppy drives are assigned 0-3, subtract 3 to obtain the number of hard disks installed) 4,5 = # of hard disks connected to expansion controller 6,7 = # of hard disks on motherboard controller (if bit 6 or 7 = 1, no A: floppy is present and the maximum number of floppies from int 11 is 3) 40:70 byte 24 hour timer overflow 1 if timer went past midnight it is reset to 0 each time it is read by int 1Ah 40:71 byte BIOS break flag (bit 7 = 1 means break key hit) 40:72 word reset flag (1234 = soft reset, memory check will be bypassed) PCjr keeps 1234h here for softboot when a cartridge is installed 40:74 byte status of last hard disk operation; PCjr special diskette control 40:75 byte # of hard disks attached (0-2) ; PCjr special diskette control 40:76 byte hd control byte; temporary holding area for 6th param table entry 40:77 byte port offset to current hd adapter ; PCjr special diskette control 40:78 4 bytes timeout value for LPT1,LPT2,LPT3,LPT4 40:7C 4 bytes timeout value for COM1,COM2,COM3,COM4 (0-FFh seconds, default 1) 40:80 word pointer to start of circular keyboard buffer, default 03:1E 40:82 word pointer to end of circular keyboard buffer, default 03:3E 40:84 byte rows on the screen (EGA only) 40:84 byte PCjr interrupt flag; timer channel 0 (used by POST) 40:85 word bytes per character (EGA only) 40:85 2 bytes (PCjr only) typamatic char to repeat 40:86 2 bytes (PCjr only) typamatic initial delay 40:87 byte mode options (EGA only) Bit 1 0 = EGA is connected to a color display 1 = EGA is monochrome. Bit 3 0 = EGA is the active display, 1 = "other" display is active. Mode combinations: Bit3 Bit1 Meaning 0 0 EGA is active display and is color 0 1 EGA is active display and is monochrome 1 0 EGA is not active, a mono card is active 1 1 EGA is not active, a CGA is active 40:87 byte (PCjr only) current Fn key code 40:88 byte feature bit switches (EGA only) 0=on, 1=off bit 3 = switch 4 bit 2 = switch 3 bit 1 = switch 2 bit 0 = switch 1 40:88 byte (PCjr only) special keyboard status byte bit 7 function flag 3 typamatic (0=enable,1=disable) 6 Fn-B break 2 typamatic speed (0=slow,1=fast) 5 Fn pressed 1 extra delay bef.typamatic (0=enable) 4 Fn lock 0 write char, typamatic delay elapsed 40:89 byte PCjr, current value of 6845 reg 2 (horiz.synch) used by ctrl-alt-cursor screen positioning routine in ROM 40:8A byte PCjrCRT/CPU Page Register Image, default 3Fh 40:8B byte last diskette data rate selected 40:8C byte hard disk status returned by controller 40:8D byte hard disk error returned by controller 40:8E byte hard disk interrupt (bit 7=working int) 40:90 4 bytes media state drive 0, 1, 2, 3 40:94 2 bytes track currently seeked to drive 0, 1 40:96 byte keyboard flag byte 3 (see int 9h) 40:97 byte keyboard flag byte 2 (see int 9h) 40:98 dword pointer to users wait flag 40:9C dword users timeout value in microseconds 40:A0 byte real time clock wait function in use 40:A1 byte LAN A DMA channel flags 40:A2 2 bytes status LAN A 0,1 40:A4 dword saved hard disk interrupt vector 40:A8 dword EGA pointer to parameter table 40:B4 byte keyboard NMI control flags (Convertible) 40:B5 dword keyboard break pending flags (Convertible) 40:B9 byte port 60 single byte queue (Convertible) 40:BA byte scan code of last key (Convertible) 40:BB byte pointer to NMI buffer head (Convertible) 40:BC byte pointer to NMI buffer tail (Convertible) 40:BD 16bytes NMI scan code buffer (Convertible) 40:CE word day counter (Convertible and after) to -04:8F end of BIOS Data Area 40:90-40:EF reserved by IBM 04:F0 16 bytes Intra-Application Communications Area (for use by applications 04:FF to transfer data or parameters to each other) 05:00 byte DOS print screen status flag 00h not active or successful completion 01h print screen in progress 0FFh error during print screen operation 05:01 Used by BASIC 05:02-03 PCjr POST and diagnostics work area 05:04 byte Single drive mode status byte 00 logical drive A 01 logical drive B 05:05-0E PCjr POST and diagnostics work area 05:0F BASIC: SHELL flag (set to 02h if there is a current SHELL) 05:10 word BASIC: segment address storage (set with DEF SEG) 05:12 4 bytes BASIC: int 1Ch clock interrupt vector segment:offset storage 05:16 4 bytes BASIC: int 23h ctrl-break interrupt segment:offset storage 05:1A 4 bytes BASIC: int 24h disk error interrupt vector segment:offset storage 05:1B-1F Used by BASIC for dynamic storage 05:20-21 Used by DOS for dynamic storage 05:22-2C Used by DOS for diskette parameter table. See int 1Eh for values 05:30-33 Used by MODE command 05:34-FF Unknown - Reserved for DOS At Absolute Addresses: 0008:0047 IO.SYS or IBMBIO.COM IRET instruction. This is the dummy routine that interrupts 01h, 03h, and 0Fh are initialized to during POST. C000:001E EGA BIOS signature (the letters IBM) F000:FFF5 BIOS release date F000:FFFE PC model identification date model byte submodel byte revision 04/24/81 FF = PC-0 (16k) – – 10/19/81 FF = PC-1 (64k) – – 08/16/82 FF = PC, XT, XT/370 – – (256k motherboard) 10/27/82 FF = PC, XT, XT/370 – – (256k motherboard) 11/08/82 FE = XT, Portable PC – – XT/370, 3270PC 01/10/86 FB = XT 00 01 01/10/86 FB = XT-2 (early) 05/09/86 FB = XT-2 (640k) 00 02 06/01/83 FD = PCjr – – 01/10/84 FC = AT – – 06/10/85 FC = AT 00 01 11/15/85 FC = AT 01 00 04/21/86 FC = XT/286 02 00 09/13/85 F9 = Convertible 00 00 09/02/86 FA = PS/2 Model 30 00 00 11/15/86 FC = AT, Enhanced 8mHz 02/13/87 FC = PS/2 Model 50 04 00 02/13/87 FC = PS/2 Model 60 05 00 1987 F8 = PS/2 Model 80 00 00 2D = Compaq PC (4.77) – – 9A = Compaq Plus (XT) – – 00FC 7531/2 Industrial AT 06FC 7552 Gearbox The IBM PC System Interrupts (Overview) The interrupt table is stored in the very lowest location in memory, starting at 0000:0000h. The locations are offset from segment 0, ie location 0000h has the address for int 0, etc. Each address is four bytes long and its location in memory can be found by multiplying the interrupt number by 4. For example, int 7 could be found by (7x4=28) or 1Bh (0000:001Bh). These interrupt vectors normally point to ROM tables or are taken over by DOS when an application is run. Some applications revector these interrupts to their own code to change the way the system responds to the user. Interrupt Address Function Number (Hex) 0 00-03 CPU Divide by Zero 1 04-07 CPU Single Step 2 08-0B CPU Nonmaskable 3 0C-0F CPU Breakpoint 4 10-13 CPU Overflow 5 14-17 BIOS Print Screen 6 18-1B hdw Reserved 7 1C-1F hdw Reserved 8 20-23 hdw Time of Day 9 24-27 hdw Keyboard A 28-2B hdw Reserved B 2C-2F hdw Communications [8259] C 30-33 hdw Communications D 34-37 hdw Disk E 38-3B hdw Diskette F 3C-3F hdw Printer 10 40-43 BIOS Video 11 44-47 BIOS Equipment Check 12 48-4B BIOS Memory 13 4C-4F BIOS Diskette/Disk 14 50-53 BIOS Serial Communications 15 54-57 BIOS Cassette, System Services 16 58-5B BIOS Keyboard 17 5C-5F BIOS Parallel Printer 18 60-63 BIOS Resident BASIC 19 64-67 BIOS Bootstrap Loader 1A 68-6B BIOS Time of Day 1B 6C-6F BIOS Keyboard Break 1C 70-73 BIOS Timer Tick 1D 74-77 BIOS Video Initialization 1E 78-7B BIOS Diskette Parameters 1F 7C-7F BIOS Video Graphics Characters, second set 20 80-83 DOS General Program Termination 21 84-87 DOS DOS Services Function Request 22 88-8B DOS Called Program Termination Address 23 8C-8F DOS Control Break Termination Address 24 90-93 DOS Critical Error Handler 25 94-97 DOS Absolute Disk Read 26 98-9B DOS Absolute Disk Write 27 9C-9F DOS Terminate and Stay Resident 28-3F A0-FF DOS Reserved for DOS 40-43 100-115 BIOS Reserved for BIOS 44 116-119 BIOS First 128 Graphics Characters 45-47 120-131 BIOS Reserved for BIOS 48 132-135 BIOS PCjr Cordless Keyboard Translation 49 136-139 BIOS PCjr Non-Keyboard Scancode Translation Table 50-5F 140-17F BIOS Reserved for BIOS 60-67 180-19F Reserved for User Software Interrupts 68-7F 1A0-1FF Reserved by IBM 80-85 200-217 ROM BASIC 86-F0 218-3C3 Used by BASIC Interpreter When BASIC is Running F1-FF 3C4-3FF Reserved by IBM For consistency in this volume, all locations and offsets are in hexadecimal unless otherwise specified. All hex numbers are prefaced with a leading zero if they begin with an alphabetic character, and are terminated with a lowercase H (h). The formats vary according to common usage. The IBM-PC System Interrupts (in detail) Interrupt 00h Divide by Zero (processor error). Automatically called at end (0:0000h) of DIV or IDIV operation that results in error. Normally set by DOS to display an error message and abort the program. Interrupt 01h Single step - Taken after every instruction when CPU Trap Flag (0:0004h) indicates single-step mode (bit 8 of FLAGS is 1). This is what makes the T command of DEBUG work for single stepping. Is not generated after MOV to segment register or POP of segment register. (unless you have a very early 8088 with the microcode bug). Interrupt 02h Non-maskable interrupt - Vector not disabled via CLI. Used by (0:0008h) parity check routine in POST, 8087 coprocessor, PCjr infrared keyboard link. Interrupt 03h Breakpoint - Taken when CPU executes the 1-byte int 3 (0CCh). (0:000Ch) Generated by opcode 0CCh. Similar to 8080's RST instruction. Generally used to set breakpoints for DEBUG. Interrupt 04h Divide overflow - Generated by INTO instruction if OF flag is (0:0010h) set. If flag is not set, INTO is effectively a NOP. Used to trap any arithmetic errors when program is ready to handle them rather than immediately when they occur. Interrupt 05h Print Screen - service dumps the screen to the printer. Invoked (0:0014h) by int 9 for shifted key 55 (PrtSc). Automatically called by keyboard scan when PrtSc key is pressed. Normally executes routine to print the screen, but may call any routine that can safely be executed from inside the keyboard scanner. Status and result byte are at address 0050:0000. entry AH 05h return absolute address 50:0 00h print screen has not been called, or upon return from a call there were no errors. 01h print screen is already in progress. 0FFh error encountered during printing. note 1) Uses BIOS services to read the screen 2) Output is directed to LPT1 3) Revectored into GRAPHICS.COM if GRAPHICS.COM is loaded Interrupt 06h Reserved by IBM (0:0018h) Interrupt 07h Reserved by IBM (0:00C0h) Interrupt 08h Timer - 55ms timer "tick" taken 18.2 times per second. Updates (0:0020h) (IRQ0) BIOS clock and turns off diskette drive motors after 2 seconds of inactivity. entry AH 08h return absolute addresses: 40:6C number of interrupts since power on (4 bytes) 40:70 number of days since power on (1 byte) 40:67 day counter on all products after AT 40:40 motor control count - gets decremented and shuts off diskette motor if zero note Int 1Ch invoked as a user interrupt. Interrupt 09h Keyboard - taken whenever a key is pressed or released. (0:0024h) (IRQ1) Stores characters/scan-codes in status at [0040:0017,18] entry AH 09h return at absolute memory addresses: 40:17 bit 0 right shift key depressed 1 left shift key depressed 2 control key depressed 3 alt key depressed 4 ScrollLock state has been toggled 5 NumLock state has been toggled 6 CapsLock state has been toggled 7 insert state is active 40:18 bit 0 left control key depressed 1 left alt key depressed 2 SysReq key depressed 3 Pause key has been toggled 4 ScrollLock key is depressed 5 NumLock key is depressed 6 CapsLock key is depressed 7 Insert key is depressed 40:96 bit 0 last code was the E1h hidden code 1 last code was the E0h hidden code 2 right control key down 3 right alt key down 4 101 key Enhanced keyboard installed 5 force NumLock if rd ID & kbx 6 last character was first ID character 7 doing a read ID (must be bit 0) 40:97 bit 0 ScrollLock indicator 1 NumLock indicator 2 CapsLock indicator 3 circus system indicator 4 ACK received 5 resend received flag 6 mode indicator update 7 keyboard transmit error flag 40:1E keyboard buffer (20h bytes) 40:1C buffer tail pointer 40:72 1234h if ctrl-alt-del pressed on keyboard AL scan code note 1) Int 05h invoked if PrtSc key pressed 2) Int 1Bh invoked if Ctrl-Break key sequence pressed 3) Int 15h, AH=85h invoked on AT and after if SysReq key is pressed 4) Int 15h, AH=4Fh invoked on machines after AT Interrupt 0Ah EGA Vertical Retrace (0:0028h) (IRQ2) used by EGA vertical retrace, hard disk Interrupt 0Bh Communications Controller (serial port) hdw. entry (0:002Ch) (IRQ3) Serial Port 2 (com2) note IRQ 3 may be used by SDLC (synchronous data-link control) or bisynchronous communications cards instead of a serial port. Interrupt 0Ch Communications Controller (serial port) hdw. entry (0:0030h) (IRQ4) Serial Port 1 (com1) note IRQ 4 may be used by SDLC (synchronous data-link control) or bisynchronous communications cards instead of a serial port. Interrupt 0Dh Alternate Printer, PC/AT 80287 (0:0034h) (IRQ5) used by hard disk, 60 Hz RAM refresh, LPT2 on AT, XT/286, and PS/2, dummy CRT vertical retrace on PCjr Interrupt 0Eh Diskette - indicates that a seek is in progress (0:0038h) (IRQ6) (sets bit 8 of 40:3E) Interrupt 0Fh Reserved by IBM (0:003Ch) (IRQ7) IRQ7 used by PPI interrupt (LPT1, LPT2) CHAPTER 3 THE PC ROM BIOS Programming Technical Reference - IBM Copyright 1988, Dave Williams Interrupt 10h Video I/O - services to handle video output (0:0040h) The ROM video routines in the original PC BIOS are designed for use with the Color Graphics Adapter and incorporate code to test for the horizontal retrace before writing. The check is performed no matter what actual display adapter is installed. The ROM character table for the first 128 characters is located at 0FA6Eh in the PC. Int 01Fh can be used to point to a second table of 128 characters. CS, SS, DS, ES, BX, CX, DX are preserved during call. All others are destroyed. Function 00h Determine or Set Video State entry AH 00h set video mode AL display mode: CGA|PCjr|MDA|MCGA|EGA|VGA|8514 00h 40x25 B/W text CGA|PCjr| | |EGA| | 16 color 01h 40x25 color text CGA|PCjr| | |EGA| | 02h 80x25 B/W text CGA|PCjr| | |EGA| | 16 color 03h 80x25 color text CGA|PCjr| | |EGA|VGA| 4 color 04h 320x200 color graphics CGA|PCjr| | |EGA| | 4 tone gray 05h 320x200 B/W graphics CGA|PCjr| | |EGA| | 2 color 06h 640x200 B/W graphics CGA|PCjr| | |EGA| | monochrome 07h 80x25 monochrome text | |MDA| |EGA| | 16 color 08h 160x200 color graphics CGA|PCjr| | | | | 16 color 09h 320x200 color graphics |PCjr| | | |VGA| 4 color 0Ah 640x200 color graphics |PCjr| | | | | N/A 0Bh BIOS font load | | | |EGA|VGA| N/A 0Ch BIOS font load | | | |EGA|VGA| 16 color 0Dh 320x200 graphics | | | |EGA|VGA| 16 color 0Eh 640x200 graphics | | | |EGA|VGA| monochrome 0Fh 640x350 graphics | | | |EGA|VGA| 16 & 64 color 10h 640x350 color hi-res | | | |EGA|VGA| 2 color 11h 640x480 graphics | | |MCGA| |VGA| 16 color 12h 640x480 graphics | | | | |VGA| 256 color 13h 320x200 graphics | | |MCGA| |VGA|8514 14h-20h used by EGA and VGA graphics modes 18h 132x44 8x8 char mono | Tseng Labs EVA 19h 132x25 8x14 char mono | Tseng Labs EVA 1Ah 132x28 8x13 char mono | Tseng Labs EVA monochrome 21h Hercules Graphics, Graphics Page 1 monochrome 22h Hercules Graphics, Graphics Page 2 22h 132x44 | Tseng, Ahead 23h 132x25 | Tseng Labs EVA 132x25 | Ahead Systems EGA2001 132x25 | ATI EGA Wonder 24h 132x28 | Tseng Labs EVA 25h 80x60 640x480 | Tseng Labs EVA 16 color 640x480 | VEGA VGA 26h 80x60 | Tseng Labs EVA 80x60 640x480 | Ahead Systems EGA2001 16 color 27h 720x512 | VEGA VGA monochrome 132x25 | ATI EGA Wonder 28h unknown | VEGA VGA 16 color 29h 800x600 | VEGA VGA 256 color 2Dh 640x350 | VEGA VGA 256 color 2Eh 640x480 | VEGA VGA 256 color 2Fh 720x512 | VEGA VGA 256 color 30h 800x600 | VEGA VGA unknown | AT&T 6300 16 color 36h 960x720 | VEGA VGA 16 color 37h 1024x768 | VEGA VGA monochrome 132x44 | ATI EGA Wonder 2 color 40h 640x400 | AT&T 6300 80x43 | VEGA VGA 16 color 41h 640x200 | AT&T 6300 132x25 | VEGA VGA 16 color 42h 640x400 | AT&T 6300 132x43 | VEGA VGA 43h unsupported 640x200 of 640x400 viewport AT&T 6300 80x60 | VEGA VGA 44h disable VDC and DEB output | AT&T 6300 100x60 | VEGA VGA 48h 80x50 640x400 | AT&T 6300 4Dh 120x25 | VEGA VGA 4Eh 120x43 | VEGA VGA 4Fh 132x25 | VEGA VGA monochrome 50h 132x25 | Ahead Systems EGA2001 16 color 640x480 | Paradise EGA-480 monochrome 80x43 | VEGA VGA 640x480 mono? | Taxan 565 EGA 51h 80x30 | Paradise EGA-480 monochrome 132x25 | VEGA VGA 640x480 ? | ATI EGA Wonder monochrome 52h 132x44 | Ahead Systems EGA2001 monochrome 132x43 | VEGA VGA 752x410 ? | ATI EGA Wonder 53h 800x560 ? | ATI EGA Wonder 54h 132x43 | Paradise EGA-480 16 color 132x43 | Paradise VGA 16 color 132x43 | Paradise VGA on multisync 132x43 | Taxan 565 EGA 55h 132x25 | Paradise EGA-480 16 color 132x25 | Paradise VGA 16 color 132x25 | Paradise VGA on multisync 132x25 | Taxan 565 EGA 56h 132x43 | NSI Smart EGA+ 132x43 | Paradise VGA 132x43 | Paradise VGA on multisync monochrome 132x43 | Taxan 565 EGA 57h 132x25 | NSI Smart EGA+ 132x25 | Paradise VGA 132x25 | Paradise VGA on multisync monochrome 132x25 | Taxan 565 EGA 58h 100x75 800x600 16/256k | Paradise VGA 59h 100x75 800x600 | Paradise VGA 5Eh 640x400 | Paradise VGA,VEGA VGA 5Fh 640x480 | Paradise VGA 60h 80x??? ???x400 | Corona/Cordata BIOS v4.10+ 752x410 | VEGA VGA 60h 400 line graphics+80 col text |(Corona/Cordata) 61h 400 line graphics | Corona/Cordata BIOS v4.10+ 720x540 | VEGA VGA 62h 800x600 | VEGA VGA 16 color 71h 100x35 800x600 | NSI Smart EGA+ 74h 640x400 graphics | Toshiba 3100 82h 80x25 B&W | AT&T VDC overlay mode * 83h 80x25 | AT&T VDC overlay mode * 86h 640x200 B&W | AT&T VDC overlay mode * C0h 640x400 2/prog pallet | AT&T VDC overlay mode * C4h disable output | AT&T VDC overlay mode * D0h 640x400 | DEC VAXmate AT&T mode unknown 640x225 | Z-100 unknown 640x400 | Z-100 note 1) If the high bit in AL is set, the display buffer is not cleared when a new mode is selected. This may be used to mix modes on the display; for example, characters of two difference sizes might be displayed 2) Modes 8-10 are available on the PCjr, Tandy 1000, and PS/2 3) IBM claims 100% software and hardware emulation of the CGA with the MCGA chipset. All registers may be read and written as CGA. All characters are double-scanned to give 80x25 with 400 line resolution. The attributes for setting border color may be set on MCGA, but the borders will remain the default color (they cannot actually be set) 4) The IBM Color Graphics Adapter (CGA) is too slow for the screen to be updated before the vertical retrace of the monitor is completed. If the video RAM is addressed directly, the screen will have "snow" or interference. IBM's default is to turn the adapter off when it is being updated, ie "flickering" when the display is scrolled. 5) The vertical retrace signal may be ignored when using the MCGA adapter. The MCGA will not generate snow when written to. There is no flicker with the MCGA. 6) The PCjr Video Gate Array uses a user-defined block of main system RAM from 4 to 32k in size instead of having dedicated memory for the display. Vertical retrace may be ignored when writing to the PCjr. There is no flicker with the PCjr display. 7) The Hercules Graphics Card has 750x348 resolution 8) The Hercules Graphics Card takes 32k beginning at B:000 (same as MDA) 9) The CGA, MCGA, and VGA adapters use hardware address B:800 10) The BIOS clears the screen when the mode is set or reset. 11) For AT&T VDC overlay modes, BL contains the DEB mode, which may be 06h, 40h, or 44h Function 01h Set Cursor Type - set the size of the cursor or turn it off entry AH 01h CH bit values: bits 0-4 top line for cursor in character cell bits 5-6 blink attribute 00 normal 01 invisible (no cursor) 10 slow (not used on original IBM PC) 11 fast CL bit values: bits 0-4 bottom line for cursor in character cell return none note 1) The ROM BIOS default cursors are: start end monochrome mode 07h: 11 12 text modes 00h-03h: 6 7 2) The blinking in text mode is caused by hardware and cannot be turned off, though some kludges can temporarily fake a nonblinking cursor 3) The cursor is automatically turned off in graphics mode 4) Another method of turning off the cursor in text mode is to position it to a nondisplayable address, such as (X,Y)=(0,25) 5) Buggy on EGA systems - BIOS remaps cursor shape in 43 line modes, but returns unmapped cursor shape Function 02h Set Cursor Position - reposition the cursor to (X,Y) entry AH 02h BH video page 00h graphics mode 03h modes 2 and 3 07h modes 0 and 1 DH row (Y=0-24) DL column (X=0-79 or 0-39) return none note 1) (0,0) is upper left corner of the screen Function 03h Read Cursor Position - return the position of the cursor entry AH 03h BH page number 00h in graphics modes 03h in modes 2 & 3 07h in modes 0 & 1 return CH top line for cursor (bits 4-0) CL bottom line for cursor (bits 4-0) DH row number (Y=0-24) DL column number (X=0-79 or 0-39) Function 04h Read Light Pen - fetch light pen information entry AH 04h return AH 00h light pen not triggered AH 01h light pen is triggered, values in resgisters BX pixel column (X=0-319 or 0-639) graphics mode CH raster line (Y=0-199) old graphics modes CX (EGA) raster line (0-nnn) new graphics modes DH row of current position (Y=0-24) text mode DL column of current position (X=0-79 or 0-39) text mode note Not supported on PS/2 Function 05h Select Active Page - set page number for services 6 and 7 entry AH 05h AL number of new active page 0-7 modes 00h and 01h (CGA) 0-3 modes 02h and 03h (CGA) 0-7 modes 02h and 03h (EGA) 0-7 mode 0Dh (EGA) 0-3 mode 0Eh (EGA) 0-1 mode 0Fh (EGA) 0-1 mode 10h (EGA) for PCjr only: AL 80h to read CRT/CPU page registers 81h to set CPU page register to value in BL 82h to set CRT page register to value in BH 83h to set both CPU and page registers (and Corona/Cordata BIOS v4.10+) BH CRT page number for subfunctions 82h and 83h BL CPU page register for subfunctions 81h and 83h return standard PC none PCjr if called with AH bit 7=1 then BH CRT page register BL CPU page register DX segment of graphics bitmap buffer (video modes 60h,61h; AL0Fh) note 1) Mono adapter has only one display page 2) CGA has four 80x25 text pages or eight 40x25 text pages 3) A separate cursor is maintained for each display page 4) Switching between pages does not affect their contents 5) Higher page numbers indicate higher memory positions Function 06h Scroll Page Up - scroll up or clear a display "window" entry AH 06h AL number of lines blanked at bottom of page 0 = blank entire window BH attributes to be used on blank line CH row (Y) of upper left corner or window CL column (X) of upper left corner of window DH row (Y) of lower right corner of window DL column (X) of lower right corner of window return none note 1) Push BP before scrolling, pop after 2) If in CGA text mode, affects current page only Function 07h Scroll Page Down - scroll down or clear a display "window" entry AH 07h AL number of lines to be blanked at top of page 0 = blank entire window BH attributes to be used on blank line CH row (Y) of upper left corner or window CL column (X) of upper left corner of window DH row (Y) of lower right corner of window DL column (X) of lower right corner of window return none note 1) Push BP before scrolling, pop after 2) If in CGA text mode, affects current page only Function 08h Read Character Attribute - of character at current cursor pos. entry AH 08h BH display page number - text mode return AH character attribute - text mode AL ASCII code of character at current cursor position Function 09h Write Character and Attribute - at current cursor position entry AH 09h AL ASCII code of character to display BH display page number - text mode BL attribute/color of character CX number of characters to write return none note 1) CX should not exceed actual rows availible, or results may be erratic 2) Setting CX to zero will cause runaway 3) All values of AL result in some sort of display; the various control characters are not recognized as special and do not change the current cursor position 4) Does not change cursor position when called - the cursor must be advanced with int 10 function 0Ah. 5) If used to write characters in graphics mode with bit 7 of AH set to 1 the character will by XORed with the current display contents. 6) In graphics mode the bit patterns for ASCII character codes 80h-0FFh are obtained from a table. On the standard PC and AT, the location is at interrupt vector 01Fh (0000:007C). For ASCII characters 00h-07Fh, the table is at an address in ROM. On the PCjr the table is at interrupt vector 44h (0000:00110) and is in addressable RAM (may be replaced by the user) 7) All characters are displayed, including CR, LF, and BS Function 0Ah Write Character - display character(s) (use current attribute) entry AH 0Ah AL ASCII code of character to display BH display page - text mode BL color of character (graphics mode, PCjr only) CX number of times to write character return none note 1) CX should not exceed actual rows availible, or results may be erratic 2) All values of AL result in some sort of display; the various control characters are not recognized as special and do not change the current cursor position 3) If used to write characters in graphics mode with bit 7 of AH set to 1 the character will by XORed with the current display contents. 4) In graphics mode the bit patterns for ASCII character codes 80h-0FFh are obtained from a table. On the standard PC and AT, the location is at interrupt vector 01Fh (0000:007C). For ASCII characters 00h-07Fh, the table is at an address in ROM. On the PCjr the table is at interrupt vector 44h (0000:00110) and is in addressable RAM (may be replaced by the user) 5) In EGA in graphics modes, replication count in CX works correctly only if all characters written are contained on the same row 6) All characters are displayed, including CR, LF, and BS Function 0Bh Set Color Palette - set palette for graphics or text border entry AH 0Bh BH 00h select border (text mode) BL color 0-15, 16-31 for high-intensity characters BH 01h set graphics palette with value in BL (CGA) BL 0 green/red/yellow 1 cyan/magenta/white (EGA) (graphics modes) BH 0 BL has border color (0-15) & high intensity bkgr'd color (16-31) BH 1 BL contains palette being selected (0-1) return none note 1) Valid in CGA mode 04h, PCjr modes 06h, 08h-0Ah 2) Although the registers in the MCGA may be set as if to change the border, the MCGA will not display a border no matter what register settings are used. Function 0Ch Write Dot - plot one graphics pixel entry AH 0Ch AL dot color code (0/1 in mode 6, 0-3 in modes 4 and 5) (set bit 7 to XOR the dot with current color) 0-3 mode 04h, 05h 0-1 mode 06h BH page number (ignored if adapter supports only one page) CX column (X=0000h - 027Fh) (0 - 319 in modes 4,5,13, 0 - 639 in modes 6,14,15,16) DX row (Y=0000h - 00C7h) (0 - 199 CGA) return none note Video graphics modes 4-6 only Function 0Dh Read Dot - determine the color of one graphics pixel entry AH 0Dh CX column (X=0000h - 027Fh) (0-319 or 639) DX row (Y=0000h - 00C7h) (0-199) return AL color of dot note Only valid in graphics mode Function 0Eh Write TTY - write one character and update cursor. Also handles CR (0Dh), beep (07h), backspace (10h), and scrolling entry AH 0Eh AL ASCII code of character to be written BH page number (text) BL foreground color (video modes 6 & 7 only) (graphics) return none note 1) The ASCII codes for bell, backspace, carriage return, and linefeed are recognized and appropriate action taken. All other characters are written to the screen and the cursor is advanced to the next position 2) Text can be written to any CGA page regardless of current active page 3) Automatic linewrap and scrolling are provided through this function 4) This is the function used by the DOS CON console driver. 5) This function does not explicitly allow the use of attributes to the characters written. Attributes may be provided by first writing an ASCII 27h (blank) with the desired attributes using function 09h, then overwriting with the actual character using this function. While clumsy this allows use of the linewrap and scrolling services provided by this function Function 0Fh Return Current Video State - mode and size of the screen entry AH 0Fh return AH number of character columns on screen AL mode currently set (see AH=00h for display mode codes) BH current active display page note If mode was set with bit 7 set ("no blanking"), the returned mode will also have bit 7 set Function 10h Set Palette Registers PCjr, Tandy 1000, EGA, MCGA, VGA entry AH 10h AL 00h set individual palette register 01h set border color palette register 02h set all palette registers and overscan 03h toggle blink/intensity bit (EGA, MCGA, VGA) 04h unknown 05h unknown 06h unknown 07h read individual palette register (VGA) 08h read overscan (order color) (VGA) 09h read all palette registers and overscan register (VGA) 10h set individual video DAC color register (MCGA, VGA) 11h unknown 12h set block of video DAC color registers (MCGA, VGA) 13h set video DAC color page (VGA) 14h unknown 15h read individual video DAC color register (MCGA, VGA) 16h unknown 17h read block of video DAC color registers (MCGA, VGA) 18h unknown 19h unknown 1Ah read video DAC color-page state (VGA) 1Bh perform gray-scale summing (VGA) BH color value BL if AL=00h palette register to set (00h-0Fh) if AL=03h 0 to enable intensity 1 to enable blinking ES:DX if AL=02h pointer to 16-byte table of register values followed by the overscan value: bytes 0-15 values for palette registers 0-15 byte 16 value for border register return none note DAC is Digital to Analog Convertor circuit in MCGA/VGA chips Function 11h Character Generator Routine (EGA and after) entry AH 11h The following functions will cause a mode set, completely resetting the video environment, but without clearing the video buffer. AL 00h, 10h load user-specified patterns ES:BP pointer to user table CX count of patterns to store DX character offset into map 2 block BL block to load in map 2 BH number of bytes per character pattern AL 01h, 11h load ROM monochrome patterns (8 by 14) BL block to load AL 02h, 12h load ROM 8 by 8 double-dot patterns BL block to load AL 03h set block specifier BL block specifier AL 04h load 8x16 text characters (MCGA, VGA) AL 14h set 8x16 text characters (MCGA, VGA) The routines called with AL=1x are designed to be called only immediately after a mode set and are similar to the routines called with AL=0x, except that: Page 0 must be active. Bytes/character is recalculated. Max character rows is recalculated. CRT buffer length is recalculated. CRTC registers are reprogrammed as follows: reg09h bytes/char-1; max scan line (mode 7 only) reg0Ah bytes/char-2; cursor start reg0Bh 0 ; cursor end reg12h 1)-1 ; vertical display end reg14h bytes/char ; underline loc (* BUG: should be 1 less *) The following functions are meant to be called only after a mode set: AL 20h user 8 by 8 graphics characters (INT 1FH) ES:BP = pointer to user table AL 21h user graphics characters ES:BP pointer to user table CX bytes per character BL row specifier 0 user set - DL = number of rows 1 14 rows 2 25 rows 3 43 rows AL 22h ROM 8 by 14 set BL row specifier AL 23h ROM 8 by 8 double dot BL row specifier AL 24h load 8x16 graphics characters (MCGA, VGA) AL 30h return information BH pointer specifier 0 int 1Fh pointer 1 int 44h pointer 2 ROM 8 by 14 character font pointer 3 ROM 8 by 8 double dot font pointer 4 ROM 8 by 8 DD font (top half) 5 ROM text alternate (9 by 14) pointer return ES:BP specified pointer value CX bytes/character DL character rows on screen Function 12h Alternate Select (EGA and after) entry AH 12h AL 00h unknown 01h unknown 02h select 400 line mode (VGA) BL 10h return EGA information 20h select alternate print screen routine 30h select vertical resolution for text modes (VGA) 31h enable/disable default palette loading (MCGA, VGA) 32h enable/disable video addressing (MCGA, VGA) 33h enable/disable default gray scale summing (MCGA, VGA) 34h enable/diable text cursor emulation (VGA) 35h display-switch interface return BH 00h if color mode is in effect 01h if mono mode is in effect BL 00h if 64k EGA memory 01h if 128k EGA memory 02h if 192k EGA memory 03h if 256k EGA memory CH feature bits CL switch settings Function 13h Write String, Don't Move Cursor (AT, XT/286, PS/2) entry AH 13h AL 00h BH display page number BL attribute CX length of string DX starting cursor position ES:BP pointer to start of string return none Function 13h Write String, Move Cursor (AT, XT/286, PS/2) entry AH 13h AL 01h BH display page number BL attribute DX starting cursor position CX length of string ES:BP pointer to start of string return none Function 13h Write String of Alternating Characters and Attributes; Don't Move Cursor (AT, XT/286, PS/2) entry AH 13h AL 02h bit 0: set in order to move cursor after write bit 1: set if string contains alternating chars and attributes BH display page number BL attribute if AL bit 1 clear CX length of string DH row of starting cursor position DL column of starting cursor position ES:BP pointer to start of string return none Function 13h Write String of Alternating Characters and Attributes; Move Cursor (AT, XT/286, PS/2) entry AH 13h AL 03h bit 0: set in order to move cursor after write bit 1: set if string contains alternating characters and attributes BH display page number BL attribute if AL bit 1 clear CX length of string DH,DL row,column of starting cursor position ES:BP pointer to start of string return none note Recognizes CR, LF, BS, and bell Function 14h Load LCD Character Font (Convertible) entry AH 14h AL 00h load user specified font BH number of bytes per character BL 00h load main font (block 0) 01h load alternate font (block 1) AL 01h load system ROM default font BL 00h load main font (block 0) 01h load alternate font (block 1) AL 02h set mapping of LCD high intensity attribute BL 00h ignore high intensity attribute 01h map high intensity to underscore 02h map high intensity to reverse video 03h map high intensity to seleected alternate font ES:DI pointer to character font CX number of characters to store DX character offset into RAM font area Function 15h Return Physical Display Parameters (Convertible) return AX Alternate display adapter type ES:DI pointer to parameter table: word # information 01h monitor model number 02h vertical pixels per meter 03h horizontal pixels per meter 04h total number of vertical pixels 05h total number of horizontal pixels 06h horizontal pixel separation in micrometers 07h vertical pixel separation in micrometers Function 1Ah Display Combination Code (PS/2) Using the compatibility BIOS of the PS/2 Models 50, 60, 80 there is a way to determine which video controller and attached display are on the system. The Display Combination Code (DCC) is a Video BIOS function that provides the capability. entry AH 1Ah AL 00h read display combination code 01h write display combination code return AL 1Ah indicates Compatibility BIOS is supported, any other value is invalid BH alternate display device where: 00h no display 01h IBM monochrome display and printer adapter 02h IBM color/graphics monitor adapter 03h reserved 04h IBM EGA (color display) 05h IBM EGA (monochrome) 06h IBM PGA 07h VGA (analog monochrome display) 08h VGA (analog color display) 09h reserved 0Ah reserved 0Bh IBM PS/2 Model 30 (analog monochrome display) 0Ch IBM PS/2 Model 30 (analog color display) BL active display device Function 1Bh Functionality/State Information (MCGA, VGA) entry AH 1Bh return unknown Function 1Ch Save/Restore Video State (VGA) entry AH 1Ch AL 00h return state buffer size 01h save video state 02h restore video state return unknown note VGA only Function 70h Get Video RAM Address (Tandy 1000) entry AH 70h return AX segment addresses of the following BX offset address of green plane CX segment address of green plane DX segment address of red/blue plane note (red offset = 0, blue offset = 4000) Function 71h Get INCRAM Addresses (Tandy 1000) entry AH 71h return AX segment address of the following BX = segment address of INCRAM CX = offset address of INCRAM Function 72h Scroll Screen Right (Tandy 1000) entry AH 72h AL number of columns blanked at left of page 00h blank window BH attributes to be used on blank columns CH,CL row, column address of upper left corner DH,DL row, column address of lower right corner Function 73h Scroll Screen Left (Tandy 1000) entry AH 73h AL number of columns blanked at right of page 00h blank window BH attributes to be used on blank columns CH,CL row, column address of upper left corner DH,DL row, column address of lower right corner Function 81h DESQview video - Get something? entry AH 81h DX 4456h ('DV') return ES segment of DESQview data structure for video buffer byte ES:[0] = current window number note This function is probably meant for internal use only, due to the magic value required in DX Function 82h DESQview - Get Current Window Info entry AH 82h DX 4456h ('DV') return AH unknown AL current window number BH unknown BL direct screen writes 0 program does not do direct writes 1 program does direct writes, so shadow buffer not usable CH unknown CL current video mode DS segment in DESQview for data structure in DV 2.00, byte DS:[0] = window number word DS:[1] = segment of other data structure word DS:[3] = segment of window's object handle ES segment of DESQview data structure for video buffer note This function is probably meant for internal use only, due to the magic value required in DX Function 0F0h Microsoft Mouse driver EGA support - Read One Register entry AH 0F0h BL register number DX group index Pointer/data chips 00h CRT Controller (25 reg) 3B4h mono modes, 3D4h color modes 08h Sequencer (5 registers) 3C4h 10h Graphics Controller (9 registers) 3CEh 18h Attribute Controller (20 registers) 3C0h Single registers 20h Miscellaneous Output register 3C2h 28h Feature Control register (3BAh mono modes, 3DAh color modes) 30h Graphics 1 Position register 3CCh 38h Graphics 2 Position register 3CAh return BL data Function 0F1h Microsoft Mouse driver EGA support - Write One Register entry AH 0F1h DX group index (see function F0h) BL register number BH value to write return BL data Function 0F2h Microsoft Mouse driver EGA support - Read Register Range entry AH 0F2h CH starting register number CL number of registers (>1) DX group index 00h CRTC (3B4h mono modes, 3D4h color modes) 08h Sequencer 3C4h 10h Graphics Controller 3CEh 18h Attribute Controller 3C0h ES:BX pointer to buffer, CL bytes Function 0F3h Microsoft Mouse driver EGA support - Write Register Range entry AH 0F3h CH starting register CL number of registers (>1) DX group index 00h CRTC (3B4h mono modes, 3D4h color modes) 08h Sequencer 3C4h 10h Graphics Controller 3CEh 18h Attribute Controller 3C0h ES:BX pointer to buffer, CL bytes Function 0F4h Microsoft Mouse driver EGA support - Read Register Set entry AH 0F4h CX number of registers (>1) ES:BX pointer to table of records in this format: bytes 1-2 group index Pointer/data chips 00h CRTC (3B4h mono modes, 3D4h color modes) 08h Sequencer 3C4h 10h Graphics Controller 3CEh 18h Attribute Controller 3C0h Single registers 20h Miscellaneous Output register 3C2h 28h Feature Control register (3BAh mono modes, 3DAh color) 30h Graphics 1 Position register 3CCh 38h Graphics 2 Position register 3CAh byte 3 register number (0 for single registers) byte 4 register value Function 0F5h Microsoft Mouse driver EGA support - Read Register Set entry AH 0F5h CX number of registers (>1) ES:BX pointer to table of records in this format: bytes 1-2 port number Pointer/data chips 00h CRTC (3B4h mono modes, 3D4h color modes) 08h Sequencer 3C4h 10h Graphics Controller 3CEh 18h Attribute Controller 3C0h Single registers 20h Miscellaneous Output register 3C2h 28h Feature Control register (3BAh mono modes, 3DAh color) 30h Graphics 1 Position register 3CCh 38h Graphics 2 Position register 3CAh byte 3 register number (0 for single registers) byte 4 register value Function 0F6h Microsoft Mouse driver EGA support Revert to Default Registers entry AH 0F6h return unknown Function 0F7h Microsoft Mouse driver EGA support Define Default Register Table entry AH 0F7h DX port number Pointer/data chips 00h CRTC (3B4h mono modes, 3D4h color modes) 08h Sequencer 3C4h 10h Graphics Controller 3CEh 18h Attribute Controller 3C0h Single registers 20h Miscellaneous Output register 3C2h 28h Feature Control register (3BAh mono modes, 3DAh color modes) 30h Graphics 1 Position register 3CCh 38h Graphics 2 Position register 3CAh ES:BX address of table of one byte entries, one byte to be written to each register Function 0FAh Microsoft Mouse driver EGA support - Interrogate Driver entry AH 0FAh BX 00h return BX 00h if mouse driver not present ES:BX pointer to EGA Register Interface version number, if present: byte 1 major release number byte 2 minor release number Function 0FEh Get Alternate Screen Buffer Address (text mode only) (Topview/DesQview/Taskview) entry AH 0FEh ES:DI segment:offset of assumed video buffer return: ES:DI segment:offset of actual video buffer note 1) This alternate video buffer can be written to directly, in the same manner as writing to B:000 or B:800. The MT program will manage the actual display. 2) There is no need to synchronize vertical retrace when writing to the alternate buffer; this is managed by the MT program 3) If TopView or DESQview is not running, ES:DI is returned unchanged. 4) TopView requires that function 0FFh be called every time you write into the buffer to tell TopView that something changed Function 0FFh Update Real Display (text mode only) (TopView) Update Video Buffer (Topview/DesQview/Taskview) entry AH 0FFh CX number of sequential characters that have been modified DI offset of first character that has been modified ES segment of video buffer return unknown note 1) DesQview supports this call, but does not require it 2) Avoid CX=0 Interrupt 11h Equipment Check (0:0044h) fetch a code describing active peripherals. entry AH 11h return AX Equipment listing word Bits are: 0 number of floppy drives 0 no drives 1 bootable diskette installed 1 math chip 0 no math coprocessor (80x87) present 1 math coprocessor (80x87) present (PS/2) 2 0 mouse not installed 1 mouse installed (PC) 2,3 system board RAM 0,0 16k (PC-0, PC-1) 1,1 64k (PC-2, XT) note 1) not commonly used. Set both bits to 1 2) both bits always 1 in AT 4,5 initial video mode 0,0 no video installed (use with dumb terminal) 0,1 40x25 color (CGA) 1,0 80x25 color (CGA) 1,1 80x25 monochrome (MDA or Hercules) 6,7 number of diskette drives (only if bit 0 1) 0,0 1 drives 0,1 2 drives 1,0 3 drives 1,1 4 drives 8 0 DMA present 1 no DMA on system (PCjr, some Tandy 1000s) 9,A,B number of RS232 serial ports (0-3) 0,0,0 none 0,0,1 1 0,1,0 2 0,1,1 3 1,0,0 4 C 0 no game I/O attached 1 game I/O attached (default for PCjr) D serial accessory installation 0 no serial accessories installed 1 Convertible - internal modem installed 1 PCjr - serial printer attached E,F number of parallel printers 0,0 none 0,1 one (LPT1, PRN) 1,0 two (LPT2) 1,1 three (LPT3) note Models before PS/2 would allow a fourth parallel printer. Remapping of the BIOS in the PS/2s does not allow the use of LPT4. Interrupt 12h Memory Size (0:0048h) get system memory return AX number of contiguous 1K RAM blocks note 1) This service does not depend on the setting of the motherboard switches 2) This is the same value stored in absolute address 04:13h Interrupt 13h Disk I/O - access the disk drives (floppy and hard disk) (0:004Ch) does not try rereading disk if an error is returned Function 00h Reset - reset the disk controller chip entry AH 00h DL drive (if bit 7 is set both hard disks and floppy disks reset) return AH status note 1) Forces controller chip to recalibrate read/write heads 2) Some systems (Sanyo 55x) this resets all drives Function 01h Get Status of disk system entry AH 01h DL drive (hard disk if bit 7 set) return AL status of most recent operation 00h successful completion 01h bad command 02h address mark not found 03h tried to write on write-protected disk 04h sector not found 05h reset failed (hard disk) 06h diskette removed or changed 07h bad parameter table (hard disk) 08h DMA overrun 09h attempt to DMA across 64K boundary 0Ah bad sector detected (hard disk) 0Bh bad track detected (hard disk) 0Ch unsupported track 0Dh invalid number of sectors on format (hard disk) 0Eh control data address mark detected (hard disk) 0Fh DMA arbitration error (hard disk) 10h bad CRC/EEC on read 11h data ECC corrected 20h controller failure 40h seek failed 80h timeout 0AAh drive not ready (hard disk) 0BBh undefined error (hard disk) 0CCh write fault (hard disk) 0E0h status error (hard disk) 0FFh sense operation failed (hard disk) Function 02h Read Sectors - read one or more sectors from diskette entry AH 02h AL number of sectors to read BX address of buffer (ES=segment) CH track number (0-39 or 0-79 for floppies) (for hard disk, bits 8,9 in high bits of CL) CL sector number (1 to 18, not value checked) DH head number (0 or 1) DL drive (0=A, 1=B, etc.) (bit 7=0) (drive 0-7) ES:BX address to store/fetch data (buffer to fill) [0000:0078] dword pointer to diskette parms return CF clear (0) for successful set (1) failure AH status (00h, 02h, 03h, 04h, 08h, 09h, 10h, 0Ah, 20h, 40h, 80h) AL number of sectors transferred note 1) Number of sectors begins with 1, not 0 2) Trying to read zero sectors is considered a programming error; results are not defined Function 03h Write Sectors - write from memory to disk entry AH 03h AL number of sectors to write (1-8) CH track number (for hard disk, bits 8,9 in high bits of CL) CL beginning sector number (if hard disk, high two bits are high bits of track #) DH head number DL drive number (0-7) ES:BX address of buffer for data return CF set if error AH status (see above) AL number of sectors written note 1) Number of sectors begins with 1, not 0 2) Trying to write zero sectors is considered a programming error; results are not defined Function 04h Verify - verify that a write operation was successful entry AH 04h AL number of sectors to verify (1-8) CH track number (for hard disk, bits 8,9 in high bits of CL) CL beginning sector number DH head number DL drive number (0-7) return CF set on error AH status (see above) AL number of sectors verified Function 05h Format Track - write sector ID bytes for 1 track entry AH 05h AL number of sectors to create on this track CH track (or cylinder) number CL sector number DH head number (0, 1) DL drive number (0-3) ES:BX pointer to 4-byte address field (C-H-R-N) byte 1 = (C) cylinder or track byte 2 = (H) head byte 3 = (R) sector byte 4 = (N) bytes/sector (0 = 128, 1 = 256, 2 = 512, 3 = 1024) return CF set if error occurred AH status code (see above) note Not valid for ESDI hard disks on PS/2 Function 06h Hard Disk - format track and set bad sector flags (PC2, PC-XT, and Portable) entry AH 06h AL interleave value (XT only) CH cylinder number (bits 8,9 in high bits of CL) CL sector number DH head DL drive ES:BX 512 byte format buffer the first 2*(sectors/track) bytes contain f,n for each sector f 00Fh for good sector 80h for bad sector n sector number return AH status code Function 07h Hard Disk - format the drive starting at the desired track (PC2, PC-XT and Portable) entry AH 07h AL interleave value (XT only) (01h-10h) CH cylinder number (bits 8,9 in high bits of CL) (00h-03FFh) CL sector number DH head number (0-7) DL drive number (80h-87h, 80h=C, 81h=D,…) ES:BX format buffer, size = 512 bytes the first 2*(sectors/track) bytes contain f,n for each sector f=00h for good sector 80h for bad sector n=sector number return AH status code (see above) Function 08h Read Drive Parameters (XT, CONV, AT, XT/286, PS/2) entry AH 08h DL drive number (0-2) return CF set on error AH status code (see above) BL drive type (see AH=17h below) (AT/PS2 floppies only) CH maximum useable value for cylinder number CL maximum useable value for sector number or cylinder number DH maximum usable value for head number DL number of consecutive acknowledging drives (0-2) ES:DI drive parameter table Function 09h Initialize Two Fixed Disk Base Tables (XT, AT, XT/286, PS/2) (install nonstandard drive) entry AH 09h return CF set on error AH status code (see above) data block definitions: +0 maximum number of cylinders (dw) +2 maximum number of heads (db) +3 starting reduced write current cylinder (dw - XT only) +5 starting write precomp cylinder (dw) +7 maximum ECC data burst length (db - XT only) +8 control byte: Bits 0,1,2 Ä drive option 3,4,5 - always zero 6 - disable ECC retries 7 - disable access retries note 1) Int 41h points to table for drive 0 2) Int 46h points to table for drive 1 3) 41h used by XT, 41h and 46h used by AT Function 0Ah Read Long (Hard disk) (XT, AT, XT/286, PS/2) entry AH 0Ah CH cylinder number (bits 8,9 in high bits of CL) CL sector number DL drive ID DH head number ES:BX pointer to buffer to fill return CF set on error AH status code (see above) AL number of sectors actually transferred note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code 2) Used for diagnostics only on PS/2 systems Function 0Bh Write Long (XT, AT, XT/286, PS/2) entry AH 0Bh CH cylinder (bits 8,9 in high bits of CL) CL sector number DH head number DL drive ID ES:BX pointer to buffer containing data return CF set on error AH status code (see above) AL number of sectors actually transferred note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code 2) Used for diagnostics only on PS/2 systems Function 0Ch Seek To Cylinder (except PC, PCjr) entry AH 0Ch CH cylinder number (bits 8,9 in high bits of CL) DH head number DL drive ID return CF set on error AH status code (see above) note 1) Positions heads over a particular cylinder Function 0Dh Alternate Disk Reset (except PC, PCjr) entry AH 0Dh DL drive ID return CF set on error AH status code (see above) note Not for PS/2 ESDI hard disks Function 0Eh Read Sector Buffer (XT, Portable PS/2) entry AH 0Eh AL number of sectors CH cylinder (bits 8,9 in top two bits of CL) CL sector number DH head number DL drive number ES:BX pointer to buffer return CF set on error AH status code (see above) AL number of sectors actually transferred note 1) Transfers controller's sector buffer. No data is read from the drive 2) Used for diagnostics only on PS/2 systems Function 0Fh Write sector buffer (XT, Portable) entry AH 0Fh AL number of sectors CH cylinder (bits 8,9 in top two bits of CL) CL sector number DH head number DL drive number ES:BX pointer to buffer return CF set if error AH status code AL number of sectors actually transferred note 1) Should be called before formatting to initialize the controller's sector buffer. 2) Used for diagnostics only on PS/2 systems Function 10h Test For Drive Ready entry AH 10h DL drive ID return CF set on error AH status code (see above) Function 11h Recalibrate Drive entry AH 11h DL drive ID return CF set on error AH status code (see above) Function 12h Controller RAM Diagnostic (XT, Portable, PS/2) entry AH 12h return CF set on error AH status code (see AH=1 above) note Used for diagnostics only on PS/2 systems Function 13h Drive Diagnostic (XT, Portable) entry AH 13h return CF set on error AH status code (see above) note Used for diagnostics only on PS/2 systems Function 14h Controller Internal Diagnostic (AT, XT/286) entry AH 14h return CF set on error AH status code (see above) note 1) OEM is Western Digital 1003-WA2 hard/floppy combination controller in AT and XT/286. 2) Used for diagnostics only in PS/2 systems Function 15h Get Disk Type (except PC and XT) entry AH 15h DL drive ID return AH disk type 00h no drive is present 01h diskette, no change detection present 02h diskette, change detection present 03h fixed disk CX:DX number of 512-byte sectors when AH = 03h Function 16h Change of Disk Status (diskette) (except PC and XT) entry AH 16h return AH disk change status 00h no disk change 01h disk changed DL drive that had disk change Function 17h Set Disk Type for Format (diskette) (except PC and XT) entry AH 17h AL 00h no disk 01h 360kb diskette in 360Kb drive 02h 360kb diskette in 1.2M drive 03h 1.2M diskette in 1.2M drive 04h 720kb diskette in 720Kb drive DL drive number return AH status of operation note This function is probably enhanced for the PS/2 series to detect 1.44 in 1.44 and 720k in 1.44. Function 18h Set Media Type For Format (diskette) (AT, XT/286, PS/2) entry AH 18h CH lower 8 bits of number of tracks CL high 2 bits of number of tracks (6,7) sectors per track (bits 0-5) DL drive number return AH 00h if requested combination supported 01h if function not available 0Ch if not suppported or drive type unknown 80h if there is no media in the drive ES:DI pointer to 11-byte parm table Function 19h Park Hard Disk Heads (XT/286, PS/2) entry AH 19h DL drive return CF set on error AH error code Function 1Ah ESDI Hard Disk - Format (PS/2) entry AH 1Ah AL defect table count CL format modifiers bit 0: ignore primary defect map bit 1: ignore secondary defect map bit 2: update secondary defect map bit 3: perform surface analysis bit 4: generate periodic interrupt DL drive ES:BX pointer to defect table return CF set on error AH status (see AH=1 above) note If periodic interrupt selected, int 15h/AH=0Fh is called after each cylinder is formatted Interrupt 14h Initialize and Access Serial Port For Int 14 (0:0050h) the following status is defined: serial status byte: bits 0 delta clear to send 1 delta data set ready 2 trailing edge ring detector 3 delta receive line signal det. 4 clear to send 5 data set ready 6 ring indicator 7 receive line signal detect line status byte: bits 0 data ready 1 overrun error 2 parity error 3 framing error 4 break detect 5 transmit holding reg. empty 6 transmit shift register empty 7 time out note: if bit 7 set then other bits are invalid All routines have AH=function number and DX=RS232 card number (0 based). AL=character to send or received character on exit, unless otherwise noted. entry AH 00h Initialize And Access Serial Communications Port bit pattern: BBBPPSLL BBB = baud rate: 110,150,300,600,1200,2400,4800,9600 PP = parity: 01 = odd, 11 = even S = stop bits: 0 = 1, 1 = 2 LL = word length: 10 = 7-bits, 11 = 8-bits AL parms for initialization: bit pattern: 0 word length 1 word length 2 stop bits 3 parity 4 parity 5 baud rate 6 baud rate 7 baud rate word length 10 7 bits 11 8 bits stop bits 0 1 stop bit 1 2 stop bits parity 00 none 01 odd 11 even baud rate 000 110 baud 001 150 baud 010 300 baud 011 600 baud 100 1200 baud 101 2400 baud 110 4800 baud 111 9600 baud (4800 on PCjr) DX port number return AH line status AL modem status Function 01h Send Character in AL to Comm Port DX (0 or 1) entry AH 01h AL character DX port number (0 or 1) return AH RS232 status code bit 0 data ready 1 overrun error 2 parity error 3 framing error 4 break detected 5 transmission buffer register empty 6 transmission shift register empty 7 timeout AL modem status bit 0 delta clear-to-send 1 delta data-set-ready 2 trailing edge ring detected 3 change, receive line signal detected 4 clear-to-send 5 data-set-ready 6 ring received 7 receive line signal detected Function 02h Wait For A Character From Comm Port DX entry AH 02h return AL character received AH error code (see above)(00h for no error) Function 03h Fetch the Status of Comm Port DX (0 or 1) entry AH 03h return AH set bits (01h) indicate comm-line status bit 7 timeout bit 6 empty transmit shift register bit 5 empty transmit holding register bit 4 break detected ("long-space") bit 3 framing error bit 2 parity error bit 1 overrun error bit 0 data ready AL set bits indicate modem status bit 7 received line signal detect bit 6 ring indicator bit 5 data set ready bit 4 clear to send bit 3 delta receive line signal detect bit 2 trailing edge ring detector bit 1 delta data set ready bit 0 delta clear to send Function 04h Extended Initialize (PC Convertible) entry AH 04h AL break status 01h if break 00h if no break BH parity 00h no parity 01h odd parity 02h even parity 03h stick parity odd 04h stick parity even BL number of stop bits 00h one stop bit 01h 2 stop bits (1« if 5 bit word length) CH word length 00h 5 bits 01h 6 bits 02h 7 bits 03h 8 bits CL baud rate 00h 110 01h 150 02h 300 03h 600 04h 1200 05h 2400 06h 4800 07h 9600 08h 19200 return AL modem status AH line control status Function 05h Extended Communication Port Control (PS/2) entry AH 05h AL 00h read modem control register return BL modem control reg (see AL=1) AL 01h write modem control register BL modem control register: (for AL=00 and AL=01) bits 0 data terminal ready 1 request to send 2 out1 3 out2 4 loop 5,6,7 reserved return AH status Interrupt 15h Cassette I/O (0:0054h) Renamed "System Services" on PS/2 line Function 00h Turn Cassette Motor On (PC, PCjr only) entry AH 00h return AH 86h no cassette present CF set on error note NOP for systems where cassette not supported Function 01h Turn Cassette Motor Off (PC, PCjr only) entry AH 01h return AH 86h no cassette present CF set on error note NOP for systems where cassette not supported Function 02h Read Blocks From Cassette (PC, PCjr only) entry AH 02h CX count of bytes to read ES:BX pointer to data buffer return CF set on error AH error code 01h CRC error 02h bad tape signals 03h no data found on tape 04h no data 80h invalid command 86h no cassette present DX count of bytes actually read ES:BX pointer past last byte written note 1) NOP for systems where cassette not supported 2) Cassette operations normally read 256 byte blocks Function 03h Write Data Blocks to Cassette (PC, PCjr only) entry AH 03h CX count of bytes to write ES:BX pointer to data buffer return CF set on error AH error code (see 02h) CX 0 ES:BX pointer to last byte written+1 note 1) NOP for systems where cassette not supported 2) The last block is padded to 256 bytes with zeroes if needed 3) No errors are returned by this service Function 0Fh ESDI Format Unit Periodic Interrupt (PS/2 50, 60, 80) entry AH 0Fh AL phase code 00h reserved 01h surface analysis 02h formatting return CF clear if formatting should continue set if it should terminate note Called during ESDI drive formatting after each cylinder is completed Function 10h TopView API Function Calls (TopView) entry AX 00h PAUSE Give Up CPU Time return 00h after other processes run 01h GETMEM allocate "system" memory BX number of bytes to allocate return ES:DI pointer to block of memory 02h PUTMEM deallocate "system" memory ES:DI pointer to previously allocated block return block freed 03h PRINTC display character/attribute on screen BH attribute BL character DX segment of object handle for window note BX=0 does not display anything, it only positions the hardware cursor 10h unknown AL 04h thru 12h return TopView - unimplemented in DV 2.0x pops up "Programming error" window in DV 2.0x 11h unknown 12h unknown 13h GETBIT define a 2nd-level interrupt handler ES:DI pointer to FAR service routine return BX bit mask indicating which bit was allocated 0 if no more bits available 14h FREEBIT undefine a 2nd-level interrupt handler BX bit mask from int 15/AH 13h 15h SETBIT schedule one or more 2nd-level interrupts BX bit mask for interrupts to post return indicated routines will be called at next ??? 16h ISOBJ verify object handle ES:DI possible object handle return BX -1 if ES:DI is a valid object handle 0 if ES:DI is not 17h TopView - unimplemented in DV 2.00 return pops up "Programming Error" window in DV 2.00 18h LOCATE Find Window at a Given Screen Location BH column BL row ES segment of object handle for ??? (0 = use default) return ES segment of object handle for window which is visible at the indicated position 19h SOUND Make Tone BX frequency in Hertz CX duration in clock ticks (18.2 ticks/sec) return immediately, tone continues to completion note If another tone is already playing, the new tone does not start until completion of the previous one. In DV 2.00, it is possible to enqueue about 32 tones before the process is blocked until a note completes. In DV 2.00, the lowest tone allowed is 20 Hz 1Ah OSTACK Switch to Task's Internal Stack return stack switched 1Bh BEGINC Begin Critical Region return task-switching temporarily disabled note Will not task-switch until END CRITICAL REGION (AH=1Ch) is called 1Ch ENDC End Critical Region return task-switching enabled 1Dh STOP STOP TASK ES segment of object handle for task to be stopped (== handle of main window for that task) return indicated task will no longer get CPU time note At least in DV 2.00, this function is ignored unless the indicated task is the current task. 1Eh START Start Task ES segment of object handle for task to be started (== handle of main window for that task) return Indicated task is started up again 1Fh DISPEROR Pop-Up Error Window BX bit fields: 0-12 number of characters to display 13,14 which mouse button may be pressed to remove window 00 either 01 left 10 right 11 either 15 beep if 1 DS:DI pointer to text of message CH width of error window (0 = default) CL height of error window (0 = default) DX segment of object handle return BX status: 1 left button pressed 2 right button pressed 27 ESC key pressed note Window remains on-screen until ESC or indicated mouse button is pressed 20h TopView - unimplemented in DV 2.0x return pops up "Programming Error" window in DV 2.0x 21h PGMINT Interrupt Another Task BX segment of object handle for task to interrupt DX:CX address of FAR routine to jump to next time task is run return nothing? note The current ES, DS, SI, DI, and BP are passed to the FAR routine 22h GETVER Get Version BX 00h return BX nonzero, TopView or compatible loaded BH minor version BL major version notes TaskView returns BX = 0001h DESQview 2.0 returns BX = 0A01h 23h POSWIN Position Window BX segment of object handle for parent window within which to position the window (0 = full screen) CH # columns to offset from position in DL CL # rows to offset from position in DL DL bit flags 0,1 horizontal position 00 current 01 center 10 left 11 right 2,3 vertical position 00 current 01 center 10 top 11 bottom 4 don't redraw screen if set 5-7 not used ES segment of object handle for window to be positioned return nothing 24h GETBUF Get Virtual Screen Information BX segment of object handle for window (0=default) return CX size of virtual screen in bytes DL 0 or 1, unknown ES:DI address of virtual screen 25h USTACK Switch Back to User's Stack return stack switched back note Call only after int 15h,fn1Ah 26h thru 2Ah DesQview (TopView?) - unimplemented in DV 2.0x return pops up "Programming Error" window in DV 2.0x 2Bh POSTTASK Awaken Task DesQview 2.0 (TopView?) BX segment of object handle for task return nothing 2Ch Start New Application in New Process DesQview 2.0 (TopView?) ES:DI pointer to contents of .PIF/.DVP file BX size of .PIF/.DVP info return BX segment of object handle for new task 2Dh Keyboard Mouse Control DesQview 2.0 BL subfunction 00h determine whether using keyboard mouse 01h turn keyboard mouse on 02h turn keyboard mouse off return (calling BL was 00h) BL 0 using real mouse 1 using keyboard mouse Function 20h PRINT.COM (DOS internal) (AT, XT-286, PS/2 50+) entry AH 20h AL subfunction 00h unknown (PRINT) 01h unknown (PRINT) 10h sets up SysReq routine on AT, XT/286, PS/2 11h completion of SysReq routine (software only) note 1) AL=0 or 1 sets or resets some flags which affect what PRINT does when it tries to access the disk Function 21h Power-On Self Test (POST) Error Log (PS/2 50+) entry AH 21h AL 00h read POST log 01h write POST log BH device ID BL error code return CF set on error AH status 00h OK 01h list full 80h invalid cmd 86h unsupported if function 00h: BX number of error codes stored ES:DI pointer to error log note: The log is a series of words, the first byte of which identifies the error code and the second the device. Function 40h Read/Modify Profiles (Convertible) entry AH 40h AL 00h read system profile in CX,BX 01h write system profile from CX, BX 02h read internal modem profile in BX 03h write internal modem profile from BX BX profile info return BX internal modem profile (from 02h) CX,BX system profile (from 00h) Function 41h Wait On External Event (Convertible) entry AH 41h AL condition type bits 0-2: condition to wait for 0 any external event 1 compare and return if equal 2 compare and return if not equal 3 test and return if not zero 4 test and return if zero bit 3: reserved bit 4: 1=port address, 0=user byte bits 5-7: reserved BH condition compare or mask value condition codes: 0 any external event 1 compare and return if equal 2 compare and return if not equal 3 test and return if not zero 4 test and return if zero BL timeout value times 55 milliseconds 0 if no time limit DX I/O port address (if AL bit 4 = 1) ES:DI pointer to user byte (if AL bit 4 = 0) Function 42h Request System Power Off (Convertible) entry AH 42h AL 00h to use system profile 01h to force suspend regardless of profile Function 43h Read System Status (Convertible) entry AH 43h return AL status bits: 0 LCD detached 1 reserved 2 RS232/parallel powered on 3 internal modem powered on 4 power activated by alarm 5 standby power lost 6 external power in use 7 battery low Function 44h (De)activate Internal Modem Power (Convertible) entry AH 44h AL 00h to power off 01h to power on Function 4Fh Keyboard Intercept (except PC, PCjr, and XT) entry AH 4Fh AL scan code, CF set return AL scan code, CF set if processing desired note Called by int 9 handler to translate scan codes Function 80h Device Open (AT, XT/286, PS/2) entry AH 80h BX device ID CX process ID return CF set on error AH status Function 81h Device Close (AT, XT/286, PS/2) entry AH 81h BX device ID CX process ID return CF set on error AH status Function 82h Program Termination (AT, XT/286, PS/2) AH 82h BX device ID return: CF set on error AH status note Closes all devices opened with function 80h Function 83h Event Wait (AT, XT/286, Convertible, PS/2) entry AH 83h AL 00h to set interval 10h to cancel CX,DX number of microseconds to wait (granularity is 976 microseconds) ES:BX pointer to memory flag (bit 7 is set when interval expires) (pointer is to caller's memory) return CF set (1) if function already busy Function 84h Read Joystick Input Settings (AT, XT/286, PS/2) entry AH 84h DX 00h to read the current switch settings (return in AL) 01h to read the resistive inputs return AX A(X) value BX A(Y) value CX B(X) value DX B(Y) value AL switch settings (bits 7-4) Function 85h System Request (SysReq) Key Pressed (except PC, PCjr, XT) entry AH 85h return AL 00h key pressed 01h key released note Called by keyboard decode routine Function 86h Elapsed Time Wait (except PC, PCjr, XT) AH 86h CX,DX number of microseconds to wait return CF clear after wait elapses CF set immediately due to error note Only accurate to 977 microseconds Function 87h Extended Memory Block Move (286/386 machines only) AH 87h CX number of words to move ES:SI pointer to Global Descriptor Table (GDT) offset 00h null descriptor 08h uninitialized, will be made into GDT descriptor 10h descriptor for source of move 18h descriptor for destination of move 20h uninitialized, used by BIOS 28h uninitialized, will be made into SS descriptor return CF set on error AH status 00h source copied into destination 01h parity error 02h interrupt error 03h address line 20 gating failed Function 88h Extended Memory Size Determine (AT, XT/286, PS/2) entry AH 88h return AX # of contiguous 1K blocks of memory starting at address 1024k Function 89h Switch Processor to Protected Mode (AT, XT/286, PS/2) entry AH 89h BH interrupt number of IRQ 8 (IRQ 9Fh use next 7 interrupts) BL interrupt number of IRQ 0 (IRQ 17h use next 7 interrupts) CX offset into protected mode CS to jump to DS:SI pointer to Global Descriptor Table for protected mode offset 00h null descriptor 08h GDT descriptor 10h IDT descriptor 18h DS 20h ES 28h SS 30h CS 38h uninitialized, used to build descriptor for BIOS CS return AH 0FFh error enabling address line 20 CF set on error Function 90h Device Busy Loop (except PC, PCjr, XT) entry AH 90h AL type code: 00h disk 01h diskette 02h keyboard 03h PS/2 pointing device 80h network (ES:BX = ncb) 0FCh disk reset 0FDh diskette motor start 0FEh printer ES:BX pointer to request block for type codes 80h through 0BFh return CF 1 (set) if wait time satisfied 0 (clear) if driver must perform wait note Used by NETBIOS Type codes are allocated as follows: 00h-7Fh non-reentrant devices; OS must arbitrate access 80h-BFh reentrant devices; ES:BX points to a unique control block C0h-FFh wait-only calls, no complementary int 15,fn91h call Function 91h Set Flag and Complete Interrupt (except PC, PCjr, XT) entry AH 91h AL type code (see AH=90h above) ES:BX pointer to request block for type codes 80h through 0BFh return AH 0 note Used by NETBIOS Function 0C0h Get System Configuration (XT after 1/10/86, PC Convertible, XT/286, AT, PS/2) entry AH 0C0h return CF 1 if BIOS doesn't support call ES:BX pointer to ROM system descriptor table dword number of bytes following byte ID byte: PC FF XT FE or FB PCjr FD byte secondary ID distingushes between AT and XT/286, etc. byte BIOS revision level, 0 for 1st release, 1 for 2nd, etc. byte feature information 80h DMA channel 3 used by hard disk BIOS 40h 2nd 8259 installed 20h realtime clock installed 10h int 15h,fn 04h called upon int 09h 08h wait for external event supported 04h extended BIOS area allocated at 640k 03h reserved 02h bus is Micro Channel instead of PC 01h reserved 00h reserved word unknown (set to 0) word unknown (set to 0) note Int 15h is also used for the Multitask Hook on PS/2 machines. No register settings availible yet. The 1/10/86 XT BIOS returns an incorrect value for the feature byte. Function 0C1h System - Return Extended-BIOS Data-Area Segment Address (PS/2) entry AH 0C1h return CF set on error ES segment of data area Function 0C2h Pointing Device BIOS Interface (DesQview 2.x) (PS/2) entry AH 0C2h AL 00h enable/disable BH 00h disable 01h reset return BH device ID 02h set sampling rate BH 00h 10/second 01h 20/second 02h 40/second 03h 60/second 04h 80/second 05h 100/second 06h 200/second 03h set resolution BH 00h one count per mm 01h two counts per mm 02h four counts per mm 03h eight counts per mm 04h get type return BH device ID 05h initialize BH data package size (1 - 8 bytes) 06h get/set scaling factor BH 00h return device status return BL status bit 0: right button pressed bit 1: reserved bit 2: left button pressed bit 3: reserved bit 4: 0=1:1 scaling, 1=2:1 scaling bit 5: device enabled bit 6: 0=stream mode, 1=remote mode bit 7: reserved CL resolution (see function 03h) DL sample rate, reports per second 01h set scaling at 1:1 02h set scaling at 2:1 07h set device handler address ES:BX user device handler return AL 00h return CF set on error AH status 00h successful 01h invalid function 02h invalid input 03h interface error 04h need to resend 05h no device handler installed note The values in BH for those functions that take it as input are stored in different locations for each subfunction Function 0C3h Enable/Disable Watchdog Timeout (PS/2 50+) entry AH 0C3h AL 00h disable 01h enable BX timer counter return CF set on error note The watchdog timer generates an NMI Function 0C4h Programmable Option Select (PS/2 50+) entry AH 04Ch AL 00h return base POS register address 01h enable slot BL slot number 02h enable adapter return CF set on error DX base POS register address (if function 00h) Function 0DEh DesQview Services (DesQview) entry AH 0DEh AL 00h Get Program Name return AX offset into DESQVIEW.DVO of current program's record: byte length of name n bytes name 2 bytes keys to invoke program (second = 00h if only one key used) word ? (I see 0 always) byte end flag: 00h for all but last entry, which is 0FFh 01h Update "Open Window" Menu return none note Reads DESQVIEW.DVO, disables Open menu if file not in current directory 02h unimplemented in DV 2.0x return nothing (NOP in DV 2.0x) 03h unimplemented in DV 2.0x return nothing (NOP in DV 2.0x) 04h Get Available Common Memory return BX bytes of common memory available CX largest block available DX total common memory in bytes 05h Get Available Conventional Memory return BX K of memory available CX largest block available DX total conventional memory in K 06h Get Available Expanded Memory return BX K of expanded memory available CX largest block available DX total expanded memory in K 07h APPNUM Get Current Program's Number return AX number of program as it appears on the "Switch Windows" menu 08h GET (unknown) return AX 0 unknown 1 unknown 09h unimplemented in DV 2.00 return nothing (NOP in DV 2.00) 0Ah DBGPOKE Display Character on Status Line BL character return character displayed, next call will display in next position (which wraps back to the start of the line if off the right edge of screen) note 1) Displays character on bottom line of *physical* screen, regardless of current size of window (even entirely hidden) 2) Does not know about graphics display modes, just pokes the characters into display memory 0Bh APILEVEL Define Minimum API Level Required BL API level >2 pops up "You need a newer version" error window in DV 2.00 BH unknown return AX maximum API level? 0Ch GETMEM Allocate "System" Memory BX number of bytes return ES:DI pointer to allocated block 0Dh PUTMEM Deallocate "System" Memory ES:DI pointer to previously allocated block return nothing 0Eh Find Mailbox by Name (DV 2.0+) ES:DI pointer to name to find CX length of name return BX 0 not found 1 found DS:SI object handle 0Fh Enable DesQview Extensions (DV 2.0+) return AX and BX destroyed (seems to be bug, weren't saved & restored) note 1) Sends a manager stream with opcodes AEh, BDh, and BFh to task's window 2) Enables an additional mouse mode 10h PUSHKEY PUT KEY INTO KEYBOARD INPUT STREAM (DV 2.0+) BH scan code BL character return BX unknown (sometimes, but not always, same as BX passed in) note A later read will get the keystroke as if it had been typed by the user 11h ENABLE/DISABLE AUTO JUSTIFICATION OF WINDOW (DV 2.0+) BL 0 viewport will not move automatically nonzero viewport will move to keep cursor visible return none 12h unknown (DV 2.0+) BX 0 clear something? nonzero set something? return none Interrupt 16h Keyboard I/O (0:0058h) access the keyboard Function 00h Get Keyboard Input - read the next character in keyboard buffer, if no key ready, wait for one. entry AH 00h return AH scan code AL ASCII character Function 01h Check Keystroke Buffer - Do Not Clear entry AH 01h return ZF 0 (clear) if character in buffer 1 (set) if no character in buffer AH scan code of character (if ZF=0) AL ASCII character if applicable note Keystroke is not removed from buffer Function 02h Shift Status - fetch bit flags indicating shift status entry AH 02h return AL bit codes (same as [0040:0017]) bit 7 Insert state bit 6 CapsLock state bit 5 NumLock state bit 4 ScrollLock state bit 3 Alt key bit 2 Control key bit 1 Left shift (left caps-shift key) bit 0 Right shift (right caps-shift key) note other codes found at [0040:0018] bit 7 Insert shift (Ins key) bit 6 Caps shift (CapsLock key) bit 5 Num shift (NumLock key) bit 4 Scroll shift (ScrollLock key) bit 3 Hold state (Ctrl-NumLock is in effect) Function 03h Keyboard - Set Repeat Rate (PCjr, AT, XT/286, PS/2) entry AH 03h AL 00h reset typematic (PCjr) 01h increase initial delay (PCjr) 02h increase continuing delay (PCjr) 03h increase both delays (PCjr) 04h turn off typematic (PCjr) 05h set typematic rate (AT, PS/2) BH 00h-03h for delays of 250ms, 500ms, 750ms, or 1s BL 00h-1Fh for typematic rates of 30cps down to 2cps Function 04h Keyboard Click Toggle (PCjr and Convertible) entry AH 04h AL 00h for click off 01h for click on Function 05h Keyboard Buffer Write (AT or PS/2 with enhanced kbd) (XT/286, PS/2, AT with "Enhanced" keyboard) entry AH 05h CH scan code CL ASCII character return AL 01h if buffer full Function 10h Get Enhanced Keystroke And Read (F11, F12 Enhanced Keyb'd) (XT/286, PS/2, AT with "Enhanced" keyboard) entry AH 10h return AH scan code AL ASCII character if applicable Function 11h Check Enhanced Keystroke (F11-F12 on enhanced keyboard) (XT/286, PS/2, AT with "Enhanced" keyboard) entry AH 11h return ZF 0 (clear) if key pressed 1 if buffer empty AH scan code (when ZF=0) AL ASCII character if applicable (when ZF=0) note Keystroke is not removed from buffer Function 12h Extended Get Shift Status (F11, F12 Enhanced keyboard) entry AH 12h return AL bit 0 right Shift key depressed 1 left Shift key depressed 2 Control key depressed 3 Alt key depressed 4 ScrollLock state active 5 NumLock state active 6 CapsLock state active 7 insert state is active AH 0 left Control key pressed 1 left Alt key depressed 2 right Control key pressed 3 right Alt key depressed 4 Scroll Lock key depressed 5 NumLock key depressed 6 CapsLock key depressed 7 SysReq key depressed Function 0F0h Set CPU speed (Compaq 386) entry AH 0F0h set speed return unknown note used by Compaq DOS MODE command. parameters not availible Interrupt 17h Printer (0:005Ch) access the parallel printer(s) AH is changed. All other registers left alone. Function 00h Print Character/send AL to printer DX (0, 1, or 2) entry AH 00h AL character DX printer to be used (0,1,2) return AH status byte bit 0 time out 1 unused 2 unused 3 I/O error 4 selected 5 out of paper 6 acknowledge 7 not busy Function 01h Initialize Printer - set init line low, send 0Ch to printer DX entry AH 01h DX printer port to be initialized (0,1,2) return status as below Function 02h Printer Status - read status of printer DX into AH entry AH 02h DX printer port to be used (0,1,2) return AH bit flags bit 7 0 = printer is busy bit 6 ACKnowledge line state bit 5 out-of-paper line state bit 4 printer selected line state bit 3 I/O error bit 2 unused bit 1 unused bit 0 time-out error Interrupt 18h ROM BASIC (0:0060h) Execute ROM BASIC at address 0F600h:0000h note 1) Often reboots a compatible Interrupt 19h Bootstrap Loader (0:0064h) Reads track 0, sector 1 into address 0000h:7C00h, then transfers control to that address. If no diskette drive available, transfers to ROM-BASIC or displays loader error message. Causes reboot of disk system if invoked while running. (no memory test performed). Interrupt 1Ah Time of Day (0:0068h) access the PC internal clock Function 00h Read System Time Counter entry AH 00h return CX high word of clock count DX low word of clock count AL 00h if clock was read or written (via AH=0,1) within the current 24-hour period. Otherwise, AL > 0 Function 01h Set Clock - set # of 55ms clock ticks in system time counter entry AH 01h CX:DX high word/low word count of timer ticks return none note 1) The clock ticks are incremented by timer interrupt at 18.2065 times per second or 54.9254milliseconds/count. Therefore: counts per second = 18 (12h) counts per minute = 1092 (444h) counts per hour = 65543 (10011h) counts per day = 1573040 (1800B0h) 2) counter is zeroed when system is rebooted 2) IBM and Microsoft recommend using int 21 Fn 4Ch. Using int 20 is officially frowned upon since the introduction of DOS 2.0 Function 02h Read Real Time Clock Time (AT and after) entry AH 02h return CH hours in BCD CL minutes in BCD DH seconds in BCD DL 1 (set) if daylight savings time option CF 1 (set) if clock not operating Function 03h Set Real Time Clock Time (AT and after) entry AH 03h CH hours in BCD CL minutes in BCD DH seconds in BCD DL 0 (clear) if standard time 1 (set) if daylight savings time option return none Function 04h Read Real Time Clock Date (AT and after) entry AH 04h return CH century in BCD (19 or 20) CL year in BCD DH month in BCD DL day in BCD CF 1 (set) if clock not operating Function 05h Set Real Time Clock Date (AT and after) entry AH 05h CH century in BCD (19 or 20) CL year in BCD DH month in BCD DL day in BCD return none Function 06h Set Real Time Clock Alarm (AT and after) entry AH 06h CH hours in BCD CL minutes in BCD DH seconds in BCD return CF set if alarm already set or clock inoperable note Int 4Ah occurs at specified alarm time every 24hrs until reset Function 07h Reset Real Time Clock Alarm (AT and after) entry AH 07h return none Function 08h Set Real Time Clock Activated Power On Mode (Convertible) entry AH 08h CH hours in BCD CL minutes in BCD DH seconds in BCD Function 09h Read Real Time Clock Alarm Time and Status (Convertible and PS/2 Model 30) entry AH 09h return CH hours in BCD CL minutes in BCD DH seconds in BCD DL alarm status: 00h if alarm not enabled 01h if alarm enabled but will not power up system 02h if alarm will power up system Function 0Ah Read System-Timer Day Counter (XT-2 [640k motherboard], PS/2) entry AH 0Ah return CF set on error CX count of days since Jan 1,1980 Function 0Bh Set System-Timer Day Counter (XT-2 [640k motherboard], PS/2) entry AH 0Bh CX count of days since Jan 1,1980 return CF set on error Function 80h Set Up Sound Multiplexor (PCjr) (Tandy 1000?) entry AH 80h AL 00h source is 8253 channel 2 01h source is cassette input 02h source is I/O channel "audio in" 03h source is TI sound generator chip Interrupt 1Bh Control-Break (0:006Ch) This interrupt is called when the keyboard scanner of the IBM machines detects Ctrl and Break pressed at the same time. note 1) If the break occurred while processing an interrupt, one or more end of interrupt commands must be send to the 8259 Programmable Interrupt Controller. 2) All I/O devices should be reset in case an operation was underway at the time. 3) It is normally pointed to an IRET during system initialization so that it does nothing, but some programs change it to return a ctrl-C scan code and thus invoke int 23h. Interrupt 1Ch Timer Tick (0:0070h) note 1) Taken 18.2065 times per second 2) Normally vectors to dummy IRET unless PRINT.COM has been installed. 3) If an application moves the interrupt pointer, it is the responsibility of that application to save and restore all registers that may be modified. Interrupt 1Dh Vector of Video Initialization Parameters. (0:0074h) This doubleword address points to 3 sets of 16-bytes containing data to initialize for video modes for video modes 0 & 1 (40 column), 2 & 3 (80 column), and 4, 5 & 6 (graphics) on the Motorola 6845 CRT controller chip. 6845 registers: R0 horizontal total (horizontal sync in characters) R1 horizontal displayed (characters per line) R2 horizontal sync position (move display left or right) R3 sync width (vertical and horizontal pulse: 4-bits each) R4 vertical total (total character lines) R5 vertical adjust (adjust for 50 or 60 Hz refresh) R6 vertical displayed (lines of chars displayed) R7 vertical sync position (lines shifted up or down) R8 interlace (bits 4 and 5) and skew (bits 6 and 7) R9 max scan line addr (scan lines per character row) R10 cursor start (starting scan line of cursor) R11 cursor stop (ending scan line of cursor) R12 video memory start address high byte (6-bits) R13 video memory start address low byte (8-bits) R14 cursor address high byte (6-bits) R15 cursor address low byte (8-bits) 6845 Video Init Tables: table for modes 0 and 1 \ table for modes 2 and 3 \ each table is 16 bytes long and table for modes 4,5, and 6 / contains values for 6845 registers table for mode 7 / 4 words: size of video RAM for modes 0/1, 2/3, 4/5, and 6/7 8 bytes: number of columns in each mode 8 bytes: video controller mode byte for each mode note 1) There are 4 separate tables, and all 4 must be initialized if all video modes will be used. 2) The power-on initialization code of the computer points this vector to the ROM BIOS video routines. 3) IBM recommends that is this table needs to be modified, it should be copied into RAM and only the nescessary changes made. Interrupt 1Eh Vector of Diskette Controller Parameters (0:0078h) Dword address points to data base table that is used by BIOS. Default location is at 0F000:0EFC7h. 11-byte table format: bytes: 00h 4-bit step rate, 4-bit head unload time 01h 7-bit head load time, 1-bit DMA flag 02h 54.9254 ms counts - delay till motor off (37-38 typ) 03h sector size: 00h 128 bytes 01h 256 bytes 02h 512 bytes 03h 1024 bytes 04h last sector on track (8 or 9 typical) 05h gap between sectors on read/write (42 typical) 06h data length for DMA transfers (0FFh typical) 07h gap length between sectors for format (80 typical) 08h sector fill byte for format (0F6h typical) 09h head settle time (in milliseconds) (15 to 25 typical) DOS 1.0 0 DOS 2.10 15 DOS 3.1 1 10h motor start time (in 1/8 second intervals) (2 to 4 typ.) DOS 2.10 2 note 1) This vector is pointed to the ROM BIOS diskette tables on system initialization 2) IBM recommends that is this table needs to be modified, it should be copied into RAM and only the nescessary changes made. Interrupt 1Fh Pointer to Graphics Character Extensions (Graphics Set 2) (0:007Ch) This is the pointer to data used by the ROM video routines to display characters above ASCII 127 while in CGA medium and high res graphics modes. note 1) Doubleword address points to 1K table composed of 28 8-byte character definition bit-patterns. First byte of each entry is top row, last byte is bottom row. 2) The first 128 character patterns are located in system ROM. 3) This vector is set to 000:0 at system initialization 4) Used by DOS' external GRAFTABL command Interrupt 20h PROGRAM TERMINATE (0:0080h) Issue int 20h to exit from a program. This vector transfers to the logic in DOS to restore the terminate address, the Ctrl-Break address,and the critical error exit address to the values they had on entry to the program. All the file buffers are flushed and all handles are closed. You should close all files changed in length (see function calls 10h and 3Eh) before issuing this interrupt. If the changed file is not closed, its length, time, and date are not recorded correctly in the directory. For a program to pass a completion code or an error code when terminating, it must use either function call 4Ch (Terminate a Process) or 31h (Terminate Process and Stay Resident). These two methods are preferred over using int 20h and the codes returned by them can be interrogated in batch processing. Important: Before you issue an interrupt 20h, your program must ensure that the CS register contains the segment of its program segment prefix. Interrupt 20h DOS - Terminate Program entry no parameters return none Interrupt 20h Minix - Send/Receive Message entry AX process ID of other process BX pointer to message CX 1 send 2 receive 3 send&receive note The message contains the system call number (numbered as in V7 Unix) and the call parameters CHAPTER 4 Programming Technical Reference - IBM Copyright 1988, Dave Williams DOS INTERRUPTS AND FUNCTION CALLS CONTENTS DOS Registers ………………………………………………….. 4- Interrupts …………………………………………………….. 4- 20h Program Terminate ……………………………………. 4- 21h Function Request …………………………………….. 4- Function Calls ………………………………………. 4- Listing of Function Calls …………………………….. 4- 00h Program Terminate …………………………….. 4- 01h Keyboard Input ……………………………….. 4- 02h Display Output ……………………………….. 4- 03h Auxiliary Input ………………………………. 4- 04h Auxiliary Output ……………………………… 4- 05h Printer Output ……………………………….. 4- 06h Direct Console I/O ……………………………. 4- 07h Direct Console Input Without Echo ………………. 4- 08h Console Input Without Echo ………………………4- 09h Print String …………………………………..4- 0Ah Buffered Keyboard Input …………………………4- 0Bh Check Standard Input Status ……………………..4- 0Ch Clear Keyboard Buffer and Invoke a Kbd Function ….. 4- 0Dh Disk Reset …………………………………… 4- 0Eh Select Disk ………………………………….. 4- 0Fh Open File ……………………………………. 4- 10h Close File …………………………………… 4- 11h Search for First Entry ………………………… 4- 12h Search for Next Entry …………………………. 4- 13h Delete File ………………………………….. 4- 14h Sequential Read ………………………………. 4- 15h Sequential Write ……………………………… 4- 16h Create File ………………………………….. 4- 17h Rename File ………………………………….. 4- 18h Unknown …………………………………… 4-

              19h Current Disk ........................................ 4-
              1Ah Set Disk Transfer Address ........................... 4-
              1Bh Allocation Table Information ........................ 4-
              1Ch Allocation Table Information for Specific Device .... 4-
              1Dh ** Unknown .......................................... 4-
              1Eh ** Unknown .......................................... 4-
              1Fh ** Read DOS Disk Block (default drive) .............. 4-
              20h ** Unknown .......................................... 4-
              21h Random Read ......................................... 4-
              22h Random Write ........................................ 4-
              23h File Size ........................................... 4-
              24h Set Relative Record Field ........................... 4-
              25h Set Interrupt Vector ................................ 4-
              26h Create New Program Segment .......................... 4-
              27h Random Block Read ................................... 4-
              28h Random Block Write .................................. 4-
              29h Parse Filename ...................................... 4-
              2Ah Get Date ............................................ 4-
              2Bh Get Date ............................................ 4-
              2Ch Get Time ............................................ 4-
              2Dh Set Time ............................................ 4-
              2Eh Set/Reset Verify Switch ............................. 4-
              2Fh Get Disk Transfer Address (DTA) ..................... 4-
              30h Get DOS Version Number .............................. 4-
              31h Terminate Process and Stay Resident ................. 4-
              32h ** Read DOS Disk Block .............................. 4-
              33h Ctrl-Break Check .................................... 4-
              34h ** Return INDOS Flag ................................ 4-
              35h Get Vector .......................................... 4-
              36h Get Disk Free Space ................................. 4-
              37h ** Get/Set Switch Character (SWITCHAR) .............. 4-
              38h Return Country Dependent Information ................ 4-
              39h Create Subdirectory (MKDIR) ......................... 4-
              3Ah Remove Subdirectory (RMDIR) ......................... 4-
              3Bh Change Durrent Directory (CHDIR) .................... 4-
              3Ch Create a File (CREAT) ............................... 4-
              3Dh Open a File ......................................... 4-
              3Eh Close a File Handle ................................. 4-
              3Fh Read From a File or Device .......................... 4-
              40h Write to a File or Device ........................... 4-
              41h Delete a File from a Specified Directory (UNLINK) ... 4-
              42h Move File Read/Write Pointer (LSEEK) ................ 4-
              43h Change File Mode (CHMOD) ............................ 4-
              44h I/O Control for Devices (IOCTL) ..................... 4-
              45h Duplicate a File Handle (DUP) ....................... 4-
              46h Force a Duplicate of a Handle (FORCDUP) ............. 4-
              47h Get Current Directory ............................... 4-
              48h Allocate Memory ..................................... 4-
              49h Free Allocated Memory ............................... 4-
              4Ah Modify Allocated Memory Blocks (SETBLOCK) ........... 4-
              4Bh Load or Execute a Program (EXEC) .................... 4-
              4Ch Terminate a Process (EXIT) .......................... 4-
              4Dh Get Return Code of a Subprocess (WAIT) .............. 4-
              4Eh Find First Matching File (FIND FIRST) ............... 4-
              4Fh Find Next Matching File (FIND NEXT) ................. 4-
              50h ** Set PSP .......................................... 4-
              51h ** Get PSP .......................................... 4-
              52h ** IN-VARS .......................................... 4-
              53h ** Translate BPB .................................... 4-
              54h Get Verify Setting .................................. 4-
              55h ** Create Child PSP ................................. 4-
              56h Rename a File ....................................... 4-
              57h Get or Set Timestamp of a File ...................... 4-
              58h ** Get/Set Allocation Strategy (DOS 3.x) ............ 4-
              59h Get Extended Error Code ............................. 4-
              5Ah Create Unique Filename .............................. 4-
              5Bh Create a New File ................................... 4-
              5Ch Lock/Unlock File Access ............................. 4-
              5Dh ** Network - Partial ................................ 4-
              5Eh ** Network Printer .................................. 4-
              5Fh ** Network Redirection .............................. 4-
              60h ** Parse Pathname ................................... 4-
              61h ** Unknown .......................................... 4-
              62h Get Program Segment Prefix (PSP) Address ............ 4-
              63h ** Get Lead Byte Table (DOS 2.25) ................... 4-
              64h ** Unknown .......................................... 4-
              65h ** Get Extended Country Information (DOS 3.3) ....... 4-
              66h ** Get/Set Global Code Page Table (DOS 3.3) ......... 4-
              67h ** Set Handle Count (DOS 3.3) ....................... 4-
              68h ** Commit File (DOS 3.3) ............................ 4-
          Calling the DOS Services .................................... 4-
      22h Terminate Address ........................................... 4-
      23h Ctrl-Break Exit Address ..................................... 4-
      24h Critical Error Handler Vector ............................... 4-
      25h Absolute Disk Read .......................................... 4-
      26h Absolute Disk Write ......................................... 4-
      27h Terminate and Stay Resident ................................. 4-
      28h ** DOS Idle Interrupt ....................................... 4-
      29h ** Quick Screen Output ...................................... 4-
      2Ah Microsoft Networks Session Layer Interrupt .................. 4-
      2Bh ** Unknown .................................................. 4-
      2Ch ** Unknown .................................................. 4-
      2Dh ** Unknown .................................................. 4-
      2Eh ** Alternate EXEC ........................................... 4-
      2Fh Multiplex Interrupt ......................................... 4-

DOS REGISTERS

DOS uses the following registers, pointers, and flags when it executes interrupts and function calls:

GENERAL REGISTERS register definition

                      AX      accumulator (16 bit)
                      AH      accumulator high-order byte (8 bit)
                      AL      accumulator low order byte (8 bit)
                      BX      base (16 bit)
                      BH      base high-order byte (8 bit)
                      BL      base low-order byte (8 bit)
                      CX      count (16 bit)
                      CH      count high order byte (8 bit)
                      CL      count low order byte (8 bit)
                      DX      data (16 bit)
                      DH      date high order byte (8 bit)
                      DL      data low order byte (8 bit)

FLAGS AF, CF, DF, IF, OF, PF, SF, TF, ZF

POINTERS register definition

                      SP      stack pointer (16 bit)
                      BP      base pointer (16 bit)
                      IP      instruction pointer (16 bit)

SEGMENT REGISTERS register definition

                      CS      code  segment (16 bit)
                      DS      data  segment (16 bit)
                      SS      stack segment (16 bit)
                      ES      extra segment (16 bit)

INDEX REGISTERS register definition

                      DI      destination index (16 bit)
                      SI      stack       index (16 bit)

INTERRUPTS

Microsoft recommends that a program wishing to examine or set the contents of any interrupt vector use the DOS function calls 35h and 25h provided for those purposes and avoid referencing the interrupt vector locations directly. DOS reserves interrupt numbers 20h to 3Fh for its own use. This means absolute memory locations 80h to 0FFh are reserved by DOS. The defined interrupts are as follows with all values in hexadecimal.

Interrupt 21h FUNCTION CALL REQUEST (0:0084h) DOS provides a wide variety of function calls for character device I/O, file management, memory management, date and time functions,execution of other programs, and more. They are grouped as follows:

        call              description
      00h             program terminate
      01h-0Ch         character device I/O, CP/M compatibility format
      0Dh-24h         file management,      CP/M compatibility format
      25h-26h         nondevice functions,  CP/M compatibility format
      27h-29h         file management,      CP/M compatibility format
      2Ah-2Eh         nondevice functions,  CP/M compatibility format
      2Fh-38h         extended functions
      39h-3Bh         directory group
      3Ch-46h         extended file management
      47h             directory group
      48h-4Bh         extended memory management
      54h-57h         extended functions
      5Eh-5Fh         networking
      60h-62h         extended functions
      63h-66h         enhanced foreign language support

List of DOS services: * = undocumented

      00h     terminate program
      01h     get keyboard input
      02h     display character to STDIO
      03h     get character from STDAUX
      04h     output character to STDAUX
      05h     output character to STDPRN
      06h     direct console I/O - keyboard to screen
      07h     get char from std I/O without echo
      08h     get char from std I/O without echo, checks for ^C
      09h     display a string to STDOUT
      0Ah     buffered keyboard input
      0Bh     check STDIN status
      0Ch     clear keyboard buffer and invoke keyboard function
      0Dh     flush all disk buffers
      0Eh     select disk
      0Fh     open file with File Control Block
      10h     close file opened with File Control Block
      11h     search for first matching file entry
      12h     search for next matching file entry
      13h     delete file specified by File Control Block
      14h     sequential read from file specified by File Control Block
      15h     sequential write to file specified by File Control Block
      16h     find or create firectory entry for file
      17h     rename file specified by file control block
      18h*    unknown
      19h     return current disk drive
      1Ah     set disk transfer area (DTA)
      1Bh     get current disk drive FAT
      1Ch     get disk FAT for any drive
      1Dh*    unknown
      1Eh*    unknown
      1Fh*    read DOS disk block, default drive
      20h*    unknown
      21h     random read from file specified by FCB
      22h     random write to file specified by FCB
      23h     return number of records in file specified by FCB
      24h     set relative file record size field for file specified by FCB
      25h     set interrupt vector
      26h     create new Program Segment Prefix (PSP)
      27h     random file block read from file specified by FCB
      28h     random file block write to file specified by FCB
      29h     parse the command line for file name
      2Ah     get the system date
      2Bh     set the system date
      2Ch     get the system time
      2Dh     set the system time
      2Eh     set/clear disk write VERIFY
      2Fh     get the Disk Transfer Address (DTA)
      30h     get DOS version number
      31h     TSR, files opened remain open
      32h*    read DOS Disk Block
      33h     get or set Ctrl-Break
      34h*    INDOS  Critical Section Flag
      35h     get segment and offset address for an interrupt
      36h     get free disk space
      37h*    get/set option marking character (SWITCHAR)
      38h     return country-dependent information
      39h     create subdirectory
      3Ah     remove subdirectory
      3Bh     change current directory
      3Ch     create and return file handle
      3Dh     open file and return file handle
      3Eh     close file referenced by file handle
      3Fh     read from file referenced by file handle
      40h     write to file referenced by file handle
      41h     delete file
      42h     move file pointer (move read-write pointer for file)
      43h     set/return file attributes
      44h     device IOCTL (I/O control) info
      45h     duplicate file handle
      46h     force a diuplicate file handle
      47h     get current directory
      48h     allocate memory
      49h     release allocated memory
      4Ah     modify allocated memory
      4Bh     load or execute a program
      4Ch     terminate prog and return to DOS
      4Dh     get return code of subprocess created by 4Bh
      4Eh     find first matching file
      4Fh     fine next matching file
      50h*    set new current Program Segment Prefix (PSP)
      51h*    puts current PSP into BX
      52h*    pointer to the DOS list of lists
      53h*    translates BPB (Bios Parameter Block, see below)
      54h     get disk verification status (VERIFY)
      55h*    create PSP: similar to function 26h
      56h     rename a file
      57h     get/set file date and time
      58h     get/set allocation strategy             (DOS 3.x)
      59h     get extended error information
      5Ah     create a unique filename
      5Bh     create a DOS file
      5Ch     lock/unlock file contents
      5Dh*    network
      5Eh*    network printer
      5Fh*    network redirection
      60h*    parse pathname
      61h*    unknown
      62h     get program segment prefix (PSP)
      63h*    get lead byte table                     (DOS 2.25)
      64h*    unknown
      65h*    get extended country information        (DOS 3.3)
      66h*    get/set global code page table          (DOS 3.3)
      67h*    set handle count                        (DOS 3.3)
      68h*    commit file                             (DOS 3.3)

CALLING THE DOS SERVICES

The DOS services are invoked by placing the number of the desired function in register AH, subfunction in AL, setting the other registers to any specific requirements of the function, and invoking int 21h.

On return, the requested service will be performed if possible. Most codes will return an error; some return more information. Details are contained in the listings for the individual functions. Extended error return may be obtained by calling function 59h (see 59h).

Register settings listed are the ones used by DOS. Some functions will return with garbage values in unused registers. Do not test for values in unspecified registers; your program may exhibit odd behavior.

DS:DX pointers are the data segment register (DS) indexed to the DH and DL registers (DX). DX always contains the offset address, DS contains the segment address.

The File Control Block services (FCB services) were part of DOS 1.0. Since the release of DOS 2.0, Microsoft has recommended that these services not be used. A set of considerably more enhanced services (handle services) were introduced with DOS 2.0. The handle services provide support for wildcards and subdirectories, and enhanced error detection via function 59h.

The data for the following calls was compiled from various Intel, Microsoft, IBM, and other publications. There are many subtle differences between MSDOS and PCDOS and between the individual versions. Differences between the versions are noted as they occur.

There are various ways of calling the DOS functions. For all methods, the function number is loaded into register AH, subfunctions and/or parameters are loaded into AL or other registers, and call int 21 by one of the following methods: A) call interrupt 21h directly B) perform a long call to offset 50h in the program's PSP.

  1) This method will not work under DOS 1.x

C) place the function number in CL and perform an intrasegment call to

  location 05h in the current code segment. This location contains a long
  call to the DOS function dispatcher.
  1) IBM recommends this method be used only when using existing programs
     written for different calling conventions. This method should be avoided
     unless you some specific use for it
  2) AX is always destroyed by this method
  3) This method is valid only for functions 00h-24h.

INT 21H DOS services

        Function (hex)

* Indicates Functions not documented in the IBM DOS Technical Reference. Note some functions have been documented in other Microsoft or licensed OEM documentation.

00h Terminate Program

    Ends program, updates, FAT, flushes buffers, restores registers

entry AH 00h

      CS      segment address of PSP

return none note 1) Program must place the segment address of the PSP control block in CS

      before calling this function.
   2) The terminate, ctrl-break,and critical error exit addresses (0Ah, 0Eh,
      12h) are restored to the values they had on entry to the terminating
      program, from the values saved in the program segment prefix at
      locations PSP:000Ah, PSP:000Eh, and PSP:0012h.
   3) All file buffers are flushed and the handles opened by the process are
      closed.
   4) Any files that have changed in length and are not closed are not
      recorded properly in the directory.
   5) Control transfers to the terminate address.
   6) This call performs exactly the same function as int 20h.
   7) All memory used by the program is returned to DOS.

01h Get Keyboard Input

      Waits for char at STDIN (if nescessary), echoes to STDOUT

entry AH 01h return AL char from STDIN (8 bits) note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes int 23h.

   2) For function call 06h, extended ASCII codes require two function calls.
      The first call returns 00h as an indicator that the next call will be an
      extended ASCII code.
   3) Input and output are redirectable. If redirected, there is no way to
      detect EOF.

02h Display Output

    Outputs char in DL to STDOUT

entry AH 02h

      DL      8 bit data (usually ASCII character)

return none note 1) If char is 08 (backspace) the cursor is moved 1 char to the left

      (nondestructive backspace).
   2) If Ctrl-C is detected after input, int 23h is executed.
   3) Input and output are redirectable. If redirected, there is no way to
      detect disk full.

03h Auxiliary Input

    Get (or wait until) character from STDAUX

entry AH 03h return AL char from auxiliary device note 1) AUX, COM1, COM2 is unbuffered and not interrupt driven

   2) This function call does not return status or error codes. For greater
      control it is recommended that you use ROM BIOS routine (int 14h) or
      write an AUX device driver and use IOCTL.
   3) At startup, PC-DOS initializes the first auxiliary port (COM1) to 2400
      baud, no parity, one stop bit, and an 8-bit word. MSDOS may differ.
   4) If Ctrl-C is has been entered from STDIN, int 23h is executed.

04h Auxiliary Output

    Write character to STDAUX

entry AH 04h

      DL      char to send to AUX

return none note 1) This function call does not return status or error codes. For greater

      control it is recommended that you use ROM BIOS routine (int 14h) or
      write an AUX device driver and use IOCTL.
   2) If Ctrl-C is has been entered from STDIN, int 23h is executed.
   3) Default is COM1 unless redirected by DOS.
   4) If the device is busy, this function will wait until it is ready.

05h Printer Output

    Write character to STDPRN

entry AL 05h

      DL      character to send

return none note 1) If Ctrl-C is has been entered from STDIN, int 23h is executed.

   2) Default is PRN or LPT1 unless redirected with the MODE command.
   3) If the printer is busy, this function will wait until it is ready.

06h Direct Console I/O

    Get character from STDIN; echo character to STDOUT

entry AH 06h

      DL      0FFh for console input, or 00h-0FEh for console output

return ZF zero flag set (1) = no character

              zero flag clear (0) = character recieved
      AL      character

note 1) Extended ASCII codes require two function calls. The first call returns

      00h to indicate the next call will return an extended code.
   2) If DL is not 0FFh, DL is assumed to have a valid character that is
      output to STDOUT.
   3) This function does not check for Ctrl-C or Ctrl-PrtSc.
   4) Does not echo input to screen
   5) If I/O is redirected, EOF or disk full cannot be detected.

07h Direct Console Input Without Echo (does not check BREAK)

    Get or wait for char at STDIN, returns char in AL

entry AH 07h return AL character from standard input device note 1) Extended ASCII codes require two function calls. The first call returns

      00h to indicate the next call will return an extended code.
   2) No checking for Ctrl-C or Ctrl-PrtSc is done.
   3) Input is redirectable.

08h Console Input Without Echo (checks BREAK)

    Get or Wait for char at STDIN, return char in AL

entry AH 08h return AL char from standard input device note 1) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.

   2) For function call 08h, extended ASCII characters require two function
      calls. The first call returns 00h to signify an extended ASCII code.
      The next call returns the actual code.
   3) Input is redirectable. If redirected, there is no way to check EOF.

09h Print String

    Outputs Characters in the Print String to the STDOUT

entry AH 09h

      DS:DX   pointer to the Character String to be displayed

return none note 1) The character string in memory must be terminated by a $ (24h)

      The $ is not displayed.
   2) Output to STDOUT is the same as function call 02h.

0Ah Buffered Keyboard Input

    Reads characters from STDIN and places them in the buffer beginning
    at the third byte.

entry AH 0Ah

      DS:DX   pointer to an input buffer

return none note 1) Min buffer size = 1, max = 255

   2) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.
   3) Format of buffer DX:
      byte       contents
       1      Maximum number of chars the buffer will take, including CR.
              Reading STDIN and filling the buffer continues until a carriage
              return (<Enter> or 0Dh) is read. If the buffer fills to one less
              than the maximum number the buffer can hold, each additional
              number read is ignored and ASCII 7 (BEL) is output to the
              display until a carriage return is read. (you must set this
              value)
       2      Actual number of characters received, excluding the carriage
              return, which is always the last character. (the function sets
              this value)
       3-n    Characters received are placed into the buffer starting here.
              Buffer must be at least as long as the number in byte 1.
   4) Input is redirectable. If redirected, there is no way to check EOF.
   5) The string may be edited with the standard DOS editing commands as it
      is being entered.
   6) Extended ASCII characters are stored as 2 bytes, the first byte being
      zero.

0Bh Check Standard Input (STDIN) status

    Checks for character availible at STDIN

entry AH 0Bh return AL 0FFh if a character is availible from STDIN

              00h     if no character is availible from STDIN

note 1) Checks for Ctrl-C. If Ctrl-C is detected, int 23h is executed

   2) Input can be redirected.
   3) Checks for character only, it is not read into the application

0Ch Clear Keyboard Buffer & Invoke a Keyboard Function (FCB)

    Dumps buffer, executes function in AL (01h,06h,07h,08h,0Ah only)

entry AH 0Ch

      AL      function number (must be 01h, 06h, 07h, 08h, or 0Ah)

return AL 00h buffer was flushed, no other processing performed

              other   any other value has no meaning

note 1) Forces system to wait until a character is typed.

   2) Flushes all typeahead input, then executes function specified by AL (by
      moving it to AH and repeating the int 21 call).
   3) If AL contains a value not in the list above, the keyboard buffer is
      flushed and no other action is taken.

0Dh Disk Reset

    Flushes all currently open file buffers to disk

entry AH 0Dh return none note 1) Does not close files. Does not update directory entries; files changed

      in size but not closed are not properly recorded in the directory
   2) Sets DTA address to DS:0080h
   3) Should be used before a disk change, Ctrl-C handlers, and to flush
      the buffers to disk.

0Eh Select Disk

    Sets the drive specified in DL (if valid) as the default drive

entry AL 0Eh

      DL      new default drive number (0=A:,1=B:,2=C:,etc.)

return AL total number of logical drives (not nescessarily physical) note 1) For DOS 1.x and 2.x, the minimum value for AL is 2.

   2) For DOS 3.x, the minimum value for AL is 5.
   3) The drive number returned is not nescessarily a valid drive.
   4) For DOS 1.x: 16 logical drives are availible. A-P.
      For DOS 2.x: 63 logical drives are availible. (Letters are only used for
                   the first 26 drives. If more than 26 logical drives are
                   used, further drive letters will be other ASCII characters
                   ie {,], etc.
      For DOS 3.x: 26 logical drives are availible. A-Z.

0Fh Open Disk File (FCB)

    Searches current directory for specified filename and opens it

entry AH 0Fh

      DS:DX   pointer to an unopened FCB

return AL 00h if file found

              0FFh    if file not not found

note 1) If the drive code was 0 (default drive) it is changed to the actual

      drive used (1=A:,2=B:,3=C:, etc). This allows changing the default drive
      without interfering with subsequent operations on this file.
   2) The current block field (FCB bytes C-D, offset 0Ch) is set to zero.
   3) The size of the record to be worked with (FCB bytes E-F, offset 0Eh) is
      set to the system default of 80h. The size of the file (offset 10h) and
      the date (offset 14h) are set from information obtained in the root
      directory. You can change the default value for the record size (FCB
      bytes E-F) or set the random record size and/or current record field.
      Perform these actions after the open but before any disk operations.
   4) The file is opened in compatibility mode.
   5) Microsoft recommends handle function call 3Dh be used instead.
   6) This call is also used by the APPEND command in DOS 3.2+
   7) Before performing a sequential disk operation on the file, you must
      set the Current Record field (offset 20h). Before performing a random
      disk operation on the file, you must set the Relative Record field
      (offset 21h). If the default record size of 128 bytes is incorrect, set
      it to the correct value.

10h Close File (FCB)

   Closes a File After a File Write

entry AH 10h

      DS:DX   pointer to an opened FCB

return AL 00h if the file is found and closed

              0FFh    if the file is not found in the current directory

note 1) This function call must be done on open files that are no longer needed,

      and after file writes to insure all directory information is updated.
   2) If the file is not found in its correct position in the current
      directory, it is assumed that the diskette was changed and AL returns
      0FFh. This error return is reportedly not completely reliable with DOS
      version 2.x.
   3) If found, the directory is updated to reflect the status in the FCB, the
      buffers to that file are flushed, and AL returns 00h.

11h Search For First Matching Entry (FCB)

    Searches current disk & directory for first matching filename

entry AH 11h

      DS:DX   pointer to address of FCB

return AL 00h successful match

              0FFh    no matching filename found

note 1) The FCB may contain the wildcard character ? under Dos 2.x, and ? or *

      under 3.x.
   2) The original FCB at DS:DX contains information to continue the search
      with function 12h, and should not be modified.
   3) If a matching filename is found, AL returns 00h and the locations at the
      Disk Transfer Address are set as follows:
      a) If the FCB provided for searching was an extended FCB, then the first
         byte at the disk transfer address is set to 0FFh followed by 5 bytes
         of zeroes, then the attribute byte from the search FCB, then the
         drive number used (1=A, 2=B, etc) then the 32 bytes of the directory
         entry. Thus, the disk transfer address contains a valid unopened FCB
         with the same search attributes as the search FCB.
      b) If the FCB provided for searching was a standard FCB, then the first
         byte is set to the drive number used (1=A,2=b,etc), and the next 32
         bytes contain the matching directory entry. Thus, the disk transfer
         address contains a valid unopened normal FCB.
   4) If an extended FCB is used, the following search pattern is used:
      a) If the FCB attribute byte is zero, only normal file entries are
         found. Entries for volume label, subdirectories, hidden or system
         files, are not returned.
      b) If the attribute byte is set for hidden or system files, or
         subdirectory entries, it is to be considered as an inclusive search.
         All normal file entries plus all entries matching the specified
         attributes are returned. To look at all directory entries except the
         volume label, the attribute byte may be set to hidden + system +
         directory (all 3 bits on).
      c) If the attribute field is set for the volume label, it is considered
         an exclusive search, and ONLY the volume label entry is returned.
   5) This call is also used by the APPEND command in DOS 3.2+

12h Search For Next Entry Using FCB (FCB)

    Search for next matching filename

entry AH 12h

      DS:DX   pointer to the unopened FCB specified from the previous Search
              First (11h) or Search Next (12h)

return AL 00h if matching filename found

              0FFh    if matching filename was not found

note 1) After a matching filename has been found using function call 11h,

      function 12h may be called to find the next match to an ambiguous
      request. For DOS 2.x, ?'s are allowed in the filename. For DOS 3.x,
      global (*) filename characters are allowed.
   2) The DTA contains info from the previous Search First or Search Next.
   3) All of the FCB except for the name/extension field is used to keep
      information nescessary for continuing the search, so no disk operations
      may be performed with this FCB between a previous function 11h or 12h
      call and this one.
   4) If the file is found, an FCB is created at the DTA address and set up to
      open or delete it.

13h Delete File Via FCB (FCB)

    Deletes file specified in FCB from current directory

entry AH 13h

      DS:DX   pointer to address of FCB

return AL 00h file deleted

              0FFh    if file not found or was read-only

note 1) All matching current directory entries are deleted. The global filename

      character "?" is allowed in the filename.
   2) Will not delete files with read-only attribute set
   3) Close open files before deleting them.
   4) Requires Network Access Rights

14h Sequential Disk File Read (FCB)

    Reads record sequentially from disk via FCB

entry AH 14h

      DS:DX   pointer to an opened FCB

return AL 00h successful read

              01h     end of file (no data read)
              02h     Data Transfer Area too small for record size specified
                      or segment overflow
              03h     partial record read, EOF found

note 1) The record size is set to the value at offset 0Eh in the FCB.

   2) The record pointed to by the Current Block (offset 0Ch) and the Current
      Record (offset 20h) fields is loaded at the DTA, then the Current Block
      and Current Record fields are incremented.
   3) The record is read into memory at the current DTA address as specified
      by the most recent call to function 1Ah. If the size of the record and
      location of the DTA are such that a segment overflow or wraparound would
      occur, the error return is set to AL=02h
   4) If a partial record is read at the end of the file, it is passed to the
      requested size with zeroes and the error return is set to AL=03h.

15h Sequential Disk Write (FCB)

    Writes record specified by FCB sequentially to disk

entry AH 15h

      DS:DX   pointer to address of FCB

return AL 00h successful write

              01h     diskette full, write canceled
              02h     disk transfer area (DTA) too small or segment wrap

note 1) The data to write is obtained from the disk transfer area

   2) The record size is set to the value at offset 0Eh in the FCB.
   3) This service cannot write to files set as read-only
   4) The record pointed to by the Current Block (offset 0Ch) and the Current
      Record (offset 20h) fields is loaded at the DTA, then the Current Block
      and Current Record fields are incremented.
   5) If the record size is less than a sector, the data in the DTA is written
      to a buffer; the buffer is written to disk when it contains a full
      sector of data, the file is closed, or a Reset Disk (function 0Dh) is
      issued.
   6) The record is written to disk at the current DTA address as specified
      by the most recent call to function 1Ah. If the size of the record and
      location of the DTA are such that a segment overflow or wraparound would
      occur, the error return is set to AL=02h

16h Create A Disk File (FCB)

    Search and open or create directory entry for file

entry AH 16h

      DS:DX   pointer to an FCB

return AL 00h successful creation

              0FFh    no room in directory

note 1) If a matching directory entry is found, the file is truncated to zero

      bytes.
   2) If there is no matching filename, a filename is created.
   3) This function calls function 0Fh (Open File) after creating or
      truncating a file.
   4) A hidden file can be created by using an extended FCB with the attribute
      byte (offset FCB-1) set to 2.

17h Rename File Specified by File Control Block (FCB) (FCB)

    Renames file in current directory

entry AH 17h

      DS:DX   pointer to an FCB (see note 4)

return AL 00h successfully renamed

              0FFh    file not found or filename already exists

note 1) This service cannot rename read-only files

   2) The "?" wildcard may be used.
   3) If the "?" wildcard is used in the second filename, the corresponding
      letters in the filename of the directory entry are not changed.
   4) The FCB must have a drive number, filename, and extension in the usual
      position, and a second filename starting 6 bytes after the first, at
      offset 11h.
   5) The two filenames cannot have the same name.
   6) FCB contains new name starting at byte 17h.

18h Internal to DOS * Unknown entry AH 18h return AL 0

19h Get Current Disk Drive

    Return designation of current default disk drive

entry AH 19h return AL current default drive (0=A, 1=B,etc.) note Some other DOS functions use 0 for default, 1=A, 2=B, etc.

1Ah Set Disk Transfer Area Address (DTA)

    Sets DTA address to the address specified in DS:DX

entry AH 1Ah

      DS:DX   pointer to buffer

return none note 1) The default DTA is 128 bytes at offset 80h in the PSP. DOS uses the

      DTA for all file I/O.
   2) Registers are unchanged.
   3) No error codes are returned.
   2) Disk transfers cannot wrap around from the end of the segment to the
      beginning or overflow into another segment.

1Bh Get Current Drive File Allocation Table Information

    Returns information from the FAT on the current drive

entry AH 1Bh exit AL number of sectors per allocation unit (cluster)

      DS:BX   address of the current drive's media descriptor byte
      CX      number of bytes per sector
      DX      number of allocation units (clusters) for default drive

note 1) Save DS before calling this function.

   2) This call returned a pointer to the FAT in DOS 1.x. Beginning with
      DOS 2.00, it returns a pointer only to the table's ID byte.
   3) IBM recommends programmers avoid this call and use int 25h instead.

1Ch Get File Allocation Table Information for Specific Device

    Returns information on specified drive

entry AH 1Ch

      DL      drive number (1=A, 2=B, 3=C, etc)

return AL number of sectors per allocation unit (cluster)

      DS:BX   address of media descriptor byte for drive in DL
      CX      sector size in bytes
      DX      number of allocation units (clusters)

note 1) DL = 0 for default.

   2) Save DS before calling this function.
   3) Format of media-descriptor byte:
      bits:   0       0   (clear)   not double sided
                      1   (set)     double sided
              1       0   (clear)   not 8 sector
                      1   (set)     8 sector
              2       0   (clear)   nonremovable device
                      1   (set)     removable device
              3-7     always set (1)
   4) This call returned a pointer to the FAT in DOS 1.x. Beginning with
      DOS 2.00, it returns a pointer only to the table's ID byte.
   5) IBM recommends programmers avoid this call and use int 25h instead.

1Dh Not Documented by Microsoft * Unknown entry AH 1Dh return AL 0

1Eh Not Documented by Microsoft * Unknown entry AH 1Eh return AL 0 note Apparently does nothing

1Fh Get Default Drive Parameter Block * Same as function call 32h (below), except that the table is accessed from

  the default drive

entry AH 1Fh

      other registers unknown

return AL 00h no error

              0FFh    error
      DS:BX   points to DOS Disk Parameter Block for default drive.

note 1) Unknown vector returned in ES:BX.

   2) For DOS 2.x and 3.x, this just invokes function 32h (undocumented,
      Read DOS Disk Block) with DL=0

20h Unknown * Internal - does nothing? entry AH 20h return AL 0

21h Random Read from File Specified by File Control Block (FCB)

   Reads one record as specified in the FCB into the current DTA.

entry AH 21h

      DS:DX   address of the opened FCB

return AL 00h successful read operation

              01h     end of file (EOF), no data read
              02h     DTA too small for the record size specified
              03h     end of file (EOF), partial data read

note 1) The current block and current record fields are set to agree with the

      random record field. Then the record addressed by these fields is read
      into memory at the current Disk Transfer Address.
   2) The current file pointers are NOT incremented this function.
   3) If the DTA is larger than the file, the file is padded to the requested
      length with zeroes.

22h Random Write to File Specified by FCB (FCB)

   Writes one record as specified in the FCB to the current DTA

entry AH 22h

      DS:DX   address of the opened FCB

return AL 00h successful write operation

              01h     disk full; no data written (write was canceled)
              02h     DTA too small for the record size specified (write was
                      canceled)

note 1) This service cannot write to read-only files.

   2) The record pointed to by the Current Block (offset 0Ch) and the Current
      Record (offset 20h) fields is loaded at the DTA, then the Current Block
      and Current Record fields are incremented.
   3) If the record size is less than a sector, the data in the DTA is written
      to a buffer; the buffer is written to disk when it contains a full
      sector of data, the file is closed, or a Reset Disk (function 0Dh) is
      issued.
   4) The current file pointers are NOT incremented this function.
   5) The record is written to disk at the current DTA address as specified
      by the most recent call to function 1Ah. If the size of the record and
      location of the DTA are such that a segment overflow or wraparound would
      occur, the error return is set to AL=02h

23h Get File Size (FCB)

   Searches current subdirectory for matching file, returns size in FCB

entry AH 23h

      DS:DX   address of an unopened FCB

return AL 00h file found

              0FFh file not found

note 1) Record size field (offset 0Eh) must be set before invoking this function

   2) The disk directory is searched for the matching entry. If a matching
      entry is found, the random record field is set to the number of records
      in the file. If the value of the Record Size field is not an even
      divisor of the file size, the value set in the relative record field is
      rounded up. This gives a returned value larger than the actual file size
   3) This call is used by the APPEND command in DOS 3.2+

24h Set Relative Record Field (FCB)

   Set random record field specified by an FCB

entry AH 24h

      DS:DX   address of an opened FCB

return Random Record Field of FCB is set to be same as Current Block

      and Current Record.

note 1) You must invoke this function before performing random file access.

   2) The relative record field of FCB (offset 21h) is set to be same as the
      Current Block (offset 0Ch) and Current Record (offset 20h).
   3) No error codes are returned.
   4) The FCB must already be opened.

25h Set Interrupt Vector

   Sets the address of the code DOS is to perform each time the specified
   interrupt is invoked.

entry AH 25h

      AL      int number to reassign the handler to
      DS:DX   address of new interrupt vector

return none note 1) Registers are unchanged.

   2) No error codes are returned.
   3) The interrupt vector table for the interrupt number specified in AL
      is set to the address contained in DS:DX. Use function 35h (Get Vector)
      to get the contents of the interrupt vector and save it for later use.
   4) When you use function 25 to set an interrupt vector, DOS 3.2 doesn't
      point the actual interrupt vector to what you requested. Instead, it
      sets the interrupt vector to point to a routine inside DOS, which does
      this:
              1. Save old stack pointer
              2. Switch to new stack pointer allocated from DOS's stack pool
              3. Call your routine
              4. Restore old stack pointer
      The purpose for this was to avoid possible stack overflows when there
      are a large number of active interrupts. IBM was concerned (this was an
      IBM change, not Microsoft) that on a Token Ring network there would be
      a lot of interrupts going on, and applications that hadn't allocated
      very much stack space would get clobbered.

26h Create New Program Segment Prefix (PSP)

   This service copies the current program-segment prefix to a new memory
   location for the creation of a new program or overlay. Once the new PSP is
   in place, a DOS program can read a DOS COM or overlay file into the memory
   location immediately following the new PSP and pass control to it.

entry AH 26h

      DX      segment number for the new PSP

return none note 1) Microsoft recommends you use the newer DOS service 4Bh (EXEC) instead.

   2) The entire 100h area at location 0 in the current PSP is copied into
      location 0 of the new PSP. The memory size information at location 6
      in the new segment is updated and the current termination, ctrl-break,
      and critical error addresses from interrupt vector table entries for
      ints 22h, 23h, and 24 are saved in the new program segment starting at
      0Ah. They are restored from this area when the program terminates.
   3) Current PSP is copied to specified segment

27h Random Block Read From File Specified by FCB (FCB)

   Similar to 21h (Random Read) except allows multiple files to be read.

entry AH 27h

      CX      number of records to be read
      DS:DX   address of an opened FCB

return AL 00h successful read

              01h     end of file, no data read
              02h     DTA too small for record size specified (read canceled)
              03h     end of file
      CX      actual number of records read (includes partial if AL=03h)

note 1) The record size is specified in the FCB. The service updates the Current

      Block (offset 0Ch) and Current Record (offset 20h) fields to the next
      record not read.
   2) If CX contained 0 on entry, this is a NOP.
   3) If the DTA is larger than the file, the file is padded to the requested
      length with zeroes.
   4) This function assumes that the FCB record size field (0Eh) is correctly
      set. If not set by the user, the default is 128 bytes.
   5) The record is written to disk at the current DTA address as specified
      by the most recent call to function 1Ah. If the size of the record and
      location of the DTA are such that a segment overflow or wraparound would
      occur, the error return is set to AL=02h

28h Random Block Write to File Specified in FCB (FCB)

   Similar to 27h (Random Write) except allows multiple files to be read.

entry AH 28h

      CX      number of records to write
      DS:DX   address of an opened FCB

return AL 00h successful write

              01h     disk full, no data written
              02h     DTA too small for record size specified (write canceled)
      CX      number of records written

note 1) The record size is specified in the FCB.

   2) This service allocates disk clusters as required.
   3) This function assumes that the FCB Record Size field (offset 0Eh) is
      correctly set. If not set by the user, the default is 128 bytes.
   4) The record size is specified in the FCB. The service updates the Current
      Block (offset 0Ch) and Current Record (offset 20h) fields to the next
      record not read.
   5) The record is written to disk at the current DTA address as specified
      by the most recent call to function 1Ah. If the size of the record and
      location of the DTA are such that a segment overflow or wraparound would
      occur, the error return is set to AL=02h
   6) If called with CX=0, no records are written, but the FCB's File Size
      entry (offset 1Ch) is set to the size specified by the FCB's Relative
      Record field (offset 21h).

29h Parse the Command Line for Filename (FCB)

   Parses a text string into the fields of a File Control Block

entry AH 29h

      DS:SI   pointer to string to parse
      ES:DI   pointer to memory buffer to fill with unopened FCB
      AL      bit mask to control parsing
              bit 0 = 0: parsing stops if file seperator found
                      1: causes service to scan past leading chars such as
                         blanks. Otherwise assumes the filename begins in
                         the first byte
                  1 = 0: drive number in FCB set to default (0) if string
                         contains no drive number
                      1: drive number in FCB not changed
                  2 = 0: filename in FCB set to 8 blanks if no filename in
                         string
                      1: filename in FCB not changed if string does not
                         contain a filename
                  3 = 0: extension in FCB set to 3 blanks if no extension in
                         string
                      1: extension left unchanged
                  4-7    must be zero

return AL 00h no wildcards in name or extension

              01h     wildcards appeared in name or extension
              0FFh    invalid drive specifier
      DS:SI   pointer to the first character after the parsed string
      ES:DI   pointer to the unopened FCB

note 1) If the * wildcard characters are found in the command line, this service

      will replace all subsequent chars in the FCB with question marks.
   2) This service uses the characters as filename separators
      DOS 1       : ; . , + / [ ] = " TAB SPACE
      DOS 2,3     : ; . , + = TAB SPACE
   3) This service uses the characters
      : ; . , + < > | / \ [ ] = " TAB SPACE
      or any control characters as valid filename separators
   4) A filename cannot contain a filename terminator. If one is encountered,
      all processing stops. The handle functions will allow use of some of
      these characters.
   5) If no valid filename was found on the command line, ES:DI +1 points
      to a blank (ASCII 32).
   6) This function cannot be used with filespecs which include a path
   7) Parsing is in the form D:FILENAME.EXT. If one is found, a corresponding
      unopened FCB is built at ES:DI

2Ah Get Date

   Returns day of the week, year, month, and date

entry AH 2Ah return CX year (1980-2099)

      DH      month   (1-12)
      DL      day     (1-31)
      AL      weekday 00h     Sunday
                      01h     Monday
                      02h     Tuesday
                      03h     Wednesday
                      04h     Thursday
                      05h     Friday
                      06h     Saturday

note 1) Date is adjusted automatically if clock rolls over to the next day,

      and takes leap years and number of days in each month into account.
   2) Although DOS cannot set an invalid date, it can read one, such as
      1/32/80, etc.
   3) DesQview also accepts CX = 4445h and DX = 5351h, i.e. 'DESQ' as valid
   4) DOS will accept CH=0 (midnight) as a valid time, but if a file's time
      is set to exactly midnight the time will not be displayed by the DIR
      command.

2Bh Set Date

   set current system date

entry AH 2Bh

      CX      year    (1980-2099)
      DH      month   (1-12)
      DL      day     (1-31)

return AL 00h no error (valid date)

              0FFh    invalid date specified

note 1) On entry, CX:DX must have a valid date in the same format as returned

      by function call 2Ah
   2) DOS 3.3 also sets CMOS clock

2Ch Get Time

   Get current system time from CLOCK$ driver

entry AH 2Ch return CH hours (0-23)

      CL      minutes (0-59)
      DH      seconds (0-59)
      DL      hundredths of a second (0-99)

note 1) Time is updated every 5/100 second.

   2) The date and time are in binary format

2Dh Set Time

   Sets current system time

entry AH 2Dh

      CH      hours   (0-23)
      CL      minutes (0-59)
      DH      seconds (0-59)
      DL      hundredths of seconds (0-99)

return AL 00h if no error

              0FFh    if bad value sent to routine

note 1) DOS 3.3 also sets CMOS clock

   2) CX and DX must contain a valid time in binary

2Eh Set/Reset Verify Switch

   Set verify flag

entry AH 2Eh

      AL      00      to turn verify off (default)
              01      to turn verify on

return none note 1) This is the call invoked by the DOS VERIFY command

   2) Setting of the verify switch can be obtained by calling call 54h
   3) This call is not supported on network drives
   4) DOS checks this flag each time it accesses a disk

2Fh Get Disk Transfer Address (DTA)

   Returns current disk transfer address used by all DOS read/write operations

entry AH 2Fh return ES:BX address of DTA note 1) The DTA is set by function call 1Ah

   2) Default DTA address is a 128 byte buffer at offset 80h in that program's
      Program Segment Prefix

30h Get DOS Version Number

   Return DOS version and/or user number

entry AH 30h return AH minor version number (i.e., DOS 2.10 returns AX = 0A02h)

      AL      major version number
      BH      OEM ID number
              00h     IBM
              16h     DEC
      BL:CX   24-bit user serial number

note 1) If AL returns a major version number of zero, the DOS version is

      below 1.28 for MSDOS and below 2.00 for PCDOS.
   2) IBM PC-DOS always returns 0000h in BX and CX.

31h Terminate Process and Stay Resident

   KEEP, or TSR

entry AH 31h

      AL      exit code
      DX      program memory requirement in 16 byte paragraphs

return AX return code (retrieveable by function 4Dh) note 1) Files opened by the application are not closed when this call is made

   2) Memory can be used more efficiently if the block containing the copy of
      the DOS environment is deallocated before terminating. This can be done
      by loading ES with the segment contained in 2Ch of the PSP and issuing
      function call 49h (Free Allocated Memory).
   3) Unlike int 27h, more than 64k may be made resident with this call

32h Read DOS Disk Block * Retrieve the pointer to the drive parameter block for a drive entry AH 32h

      DL      drive (0=default, 1=A:, etc.).

return AL 00h if drive is valid

              0FFh    if drive is not valid
      DS:BX   points to DOS Drive Parameter Table.  Format of block:
              Bytes   Type        Value
              00h     byte    Drive: 0=A:, 1=B:, etc.
              01h     byte    Unit within drive (0, 1, 2, etc.)
              02h-03h word    Bytes per sector
              04h     byte    Sectors per cluster - 1
              05h     byte    Cluster to sector shift (i.e., how far to shift-
                              left the bytes/sector to get bytes/cluster)
              06h-07h word    Number of reserved (boot) sectors
              08h     byte    Number of FATs
              09h-0Ah word    Number of root directory entries
              0Bh-0Ch word    Sector # of 1st data. Should be same as # of
                              sectors/track.
              0Dh-0Eh word    # of clusters + 1 (=last cluster #)
              0Fh     byte    Sectors for FAT
              10h-11h word    First sector of root directory
              12h-15h dword   Address of device driver header for this drive
              16h     byte    Media Descriptor Byte for this drive
              17h     byte    Zero if disk has been accessed
              18h-1Bh dword   address of next DOS Disk Block (0FFFFh means
                              last in chain)
              22h     byte    Current Working Directory (2.0 only) (64 bytes)

note 1) Use [BX+0D] to find no. of clusters (>1000H, 16-bit FAT; if not, 12-bit

      (exact dividing line is probably a little below 1000h to allow for
      bad sectors, EOF markers, etc.)
   2) Short article by C.Petzold, PC Magazine  Vol.5,no.8, and the article
      "Finding Disk Parameters" in the May 1986 issue of PC Tech Journal.
   3) This call is mostly supported in OS/2 1.0's DOS Compatibility Box. The
      dword at 12h will not return the address of the next device driver when
      in the Compatibility Box.

33h Control-Break Check

   Get or set control-break checking at CON

entry AH 33h

      AL      00h     to test for break checking
              01h     to set break checking
                      DL      00h     to disable break checking
                              01h     to enable break checking
              02h     internal, called by PRINT.COM (DOS 3.1)

return DL 00h if break=off

              01h     if break=on
      AL      0FFh    error

34h Return INDOS Flag * Returns ES:BX pointing to Critical Section Flag, byte indicating whether

   it is safe to interrupt DOS.

entry AH 34h return ES:BX points to DOS "critical section flag" note 1) If byte is 0, it is safe to interrupt DOS. This was mentioned in some

      documentation by Microsoft on a TSR standard, and PC Magazine reports
      it functions reliably under DOS versions 2.0 through 3.3. Chris
      Dunford (of CED fame) and a number of anonymous messages on the BBSs
      indicate it may not be totally reliable.
   2) The byte at ES:BX+1 is used by the Print program for this same purpose,
      so it's probably safer to check the WORD at ES:BX.
   3) Reportedly, examination of DOS 2.10 code in this area indicates that the
      byte immediately following this "critical section flag" must be 00h to
      permit the PRINT.COM interrupt to be called. For DOS 3.0 and 3.1 (except
      Compaq DOS 3.0), the byte before the "critical section flag" must be
      zero; for Compaq DOS 3.0, the byte 01AAh before it must be zero.
   4) In DOS 3.10 this reportedly changed to word value, with preceding byte.
   5) This call is supported in OS/2 1.0's DOS Compatibility Box
   6) Gordon Letwin of Microsoft discussed this call on ARPAnet in 1984. He
      stated:
      a) this is not supported under any version of the DOS
      b) it usually works under DOS 2, but there may be circumstances
         when it doesn't (general disclaimer, don't know of a specific
         circumstance)
      c) it will usually not work under DOS 3 and DOS 3.1; the DOS is
         considerably restructured and this flag takes on additional
         meanings and uses
      d) it will fail catastrophically under DOS 4.0 and forward.
      Obviously this information is incorrect since the call works fine
      through DOS 3.3. Microsoft glasnost?

35h Get Vector

   Get interrupt vector

entry AH 35h

      AL      interrupt number (hexadecimal)

return ES:BX address of interrupt vector note Use function call 25h to set the interrupt vectors

36h Get Disk Free Space

   get information on specified drive

entry AH 36h

      DL      drive number (0=default, 1=A:, 2=B:, etc)

return AX number of sectors per cluster

              0FFFFh means drive specified in DL is invalid
      BX      number of availible clusters
      CX      bytes per sector
      DX      clusters per drive

note 1) Mult AX * CX * BX for free space on disk

   2) Mult AX * CX * DX for total disk space
   3) Function 36h returns an incorrect value after an ASSIGN command. Prior
      to ASSIGN, the DX register contains 0943h on return, which is the free
      space in clusters on the HC diskette. After ASSIGN, even with no
      parameters, 0901h is returned in the DX register; this is an incorrect
      value. Similar results occur with DD diskettes on a PC-XT or a PC-AT.
      This occurs only when the disk is not the default drive. Results are as
      expected when the drive is the default drive. Therefore, the
      circumvention is to make the desired drive the default drive prior to
      issuing this function call.
   4) Int 21h, function call 36h returns an incorrect value after an ASSIGN
      command. Prior to ASSIGN, the DX register contains 0943h on return,
      which is the free space in clusters on the HC diskette. After ASSIGN,
      even with no parameters, 0901h is returned in the DX register; this is
      an incorrect value. Similar results occur with DD diskettes on a PC-XT
      or a PC-AT. This occurs only when the disk is not the default drive.
      Results are as expected when the drive is the default drive. Therefore,
      the circumvention is to make the desired drive the default drive prior
      to issuing this function call.
   5) This function supercedes functions 1Bh and 1Ch.

37h SWITCHAR / AVAILDEV * Get/set option marking character (is usually "/"), and device type entry AH 37h

      AL      00h     read switch character (returns current character in DL)
              01h     set character in DL as new switch character
    (DOS 2.x) 02h     read device availability (as set by function AL=3) into
                      DL. A 0 means devices that devices must be accessed in
                      file I/O calls by /dev/device. A non-zero value means
                      that devices are accessible at every level of the
                      directory tree (e.g., PRN is the printer and not a file
                      PRN).
                      AL=2 to return flag in DL, AL=3 to set from DL (0 = set,
                      1 = not set).
    (DOS 2.x) 03h     get device availability, where:
      DL      00h     means /dev/ must precede device names
              01h     means /dev/ need not precede device names

return DL switch character (if AL=0 or 1)

              device availability flag (if AL=2 or 3)
      AL      0FFh    the value in AL was not in the range 0-3.

note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x.

   2) It is documented on page 4.324 of the MS-DOS (version 2) Programmer's
      Utility Pack (Microsoft - published by Zenith).
   3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1.
   4) The SWITCHAR is the character used for "switches" in DOS command
      arguments (defaults to '/', as in "DIR/P"). '-' is popular to make a
      system look more like UNIX; if the SWITCHAR is anything other than '/',
      then '/' may be used instead of '\' for pathnames
   5) Ignored by XCOPY, PKARC, LIST
   6) SWITCHAR may not be set to any character used in a filename
   7) In DOS 3.x you can still read the "AVAILDEV" byte with subfunction 02h
      but it always returns 0FFh even if you try to change it to 0 with
      subfunction 03h.
   8) AVAILDEV=0 means that devices must be referenced in an imaginary
      subdirectory "\dev" (similar to UNIX's /dev/*); a filename "PRN.DAT"
      can be created on disk and manipulated like any other. If AVAILDEV != 0
      then device names are recognized anywhere (this is the default):
      "PRN.DAT" is synonymous with "PRN:".
   9) These functions reportedly are not supported in the same fashion in
      various implementations of DOS.

38h Return Country Dependent Information (PCDOS 2.0, 2.1, MSDOS 2.00 only)

   get country-dependent information

entry AH 38h

      AL      function code  (must be 0 in DOS 2.x)
      DS:DX   pointer to 32 byte memory area

return AX error code if CF set

      DS:DX   country data if CF not set
              word    date/time format
                      0 = USA standard       H:M:S   M/D/Y
                      1 = European standard  H:M:S   D/M/Y
                      2 = Japanese standard  H:M:S   D:M:Y
              byte    ASCIIZ string currency symbol followed by byte of zeroes
              byte    ASCIIZ string thousands separator followed by byte of
                      zeroes
              byte    ASCIIZ string decimal separator followed by byte of
                      zeroes
           24 bytes   reserved

38h Get Country Dependent Information (PCDOS 3.x+, MSDOS 2.01+)

   get country-dependent information

entry AH 38h

      AL      function code
              00h     to get current country information
              code    country code to get information for, for countries
                      with codes less than 255
              0FFh    to get country information for countries with a code
                      greater than 255
      BX      16 bit country code if AL=0FFh
      DS:DX   pointer to the memory buffer where the data will be returned

return CF 0 (clear) function completed

              1 (set) error
                 AX   error code if CF set
                      2  invalid country code (no table for it)
      BX      country code (usually international telephone code)
      DS:DX   country data if CF not set
              word    date/time format
                      0 = USA standard       H:M:S   M/D/Y
                      1 = European standard  H:M:S   D/M/Y
                      2 = Japanese standard  H:M:S   D:M:Y
            5 bytes   currency symbol null terminated
            2 bytes   thousands separator null terminated
            2 bytes   decimal separator null terminated
            2 bytes   date separator null terminated
            2 bytes   time separator null terminated
              byte    bit field currency format
                      bit 0 = 0  if currency symbol precedes the value
                              1  if currency symbol is after the value
                      bit 1 = 0  no spaces between value and currency symbol
                              1  one space between value and currency symbol
                      bits 2-7   not defined by Microsoft
              byte    number of significant decimal digits in currency
                      (number of places to right of decimal point)
              byte    time format
                      bit 0 = 0  12 hour clock
                      bit 0 = 1  24 hour clock
            2 words   case map call address
                      entry   AL  ASCII code of character to be converted to
                                  uppercase
                      return  AL  ASCII code of the uppercase input character
            2 bytes   data list separator null terminated
            5 words   reserved

note 1) When an alternate keyboard handler is invoked, the keyboard routine is

      loaded into user memory starting at the lowest portion of availible
      user memory. The BIOS interrupt vector that services the keyboard is
      redirected to the memory area where the new routine resides. Each new
      routine takes up about 1.6K of memory and has lookup tables that return
      values unique to each language. (KEYBxx in the DOS book)
       Once the keyboard interrupt vector is changed by the DOS keyboard
      routine, the new routine services all calls unless the system is
      returned to the US format by the ctrl-alt-F1 keystroke combination. This
      does not change the interrupt vector back to the BIOS location; it
      merely passes the table lookup to the ROM locations.
   2) Ctrl-Alt-F1 will only change systems with US ROMS to the US layout.
      Some systems are delivered with non-US keyboard handler routines in ROM
   3) Case mapping call: the segment/offset of a FAR procedure that performs
      country-specific lower-to-upper case mapping on ASCII characters 80h to
      0FFh. It is called with the character to be mapped in AL. If there is
      an uppercase code for the letter, it is returned in AL, if there is no
      code or the function was called with a value of less than 80h AL is
      returned unchanged.

38h Set Country Dependent Information

   set country-dependent information

entry AH 38h

      AL      code    country code to set information for, for countries
                      with codes less than 255
              0FFh    to set country information for countries with a code
                      greater than 255
      BX      16 bit country code if AL=0FFh
      DX      0FFFFh

return CF clear successful

              set     if error
                      AX      error code if CF flag set

39h Create Subdirectory (MKDIR)

   Makes a subdirectory along the indicated path

entry AH 39h

      DS:DX   address of ASCIIZ pathname string

return flag CF 0 successful

              1       error
                      AX      error code if any  (3, 5)

note 1) The ASCIIZ string may contain drive and subdirectory.

   2) Drive may be any valid drive (not nescessarily current drive)
   3) The pathname cannot exceed 64 characters

3Ah Remove Subdirectory (RMDIR)

   remove a directory entry

entry AH 3Ah

      DS:DX   address of ASCIIZ pathname string

return CF clear successful

              set       AX      error code if any  (3, 5, 16)

note 1) The ASCIIZ string may contain drive and subdirectory.

   2) Drive may be any valid drive (not nescessarily current drive)
   3) The pathname cannot exceed 64 characters

3Bh Change Current Directory

   (CHDIR)

entry AH 3Bh

      DS:DX   address of ASCIIZ string

return flag CF 0 successful

              1       error
      AX      error code if any (3)

note 1) The pathname cannot exceed 64 characters

   2) The ASCIIZ string may contain drive and subdirectory.
   3) Drive may be any valid drive (not nescessarily current drive)

3Ch Create A File (CREAT)

   create a file with handle

entry AH 3Ch

      CX      attributes for file
              00h     normal
              01h     read only
              02h     hidden
              03h     system
      DS:DX   address of ASCIIZ filename string

return flag CF 0 successful creation

              1       error
      AX      16 bit file handle
              or error code  (3, 4, 5)

note 1) The ASCIIZ string may contain drive and subdirectory.

   2) Drive may be any valid drive (not nescessarily current drive)
   3) If the volume label or subdirectory bits are set in CX, they are ignored
   4) The file is opened in read/write mode
   5) If the file does not exist, it is created. If one of the same name
      exists, it is truncated to a length of 0.

3Dh Open A File

   Open disk file with handle

entry AH 3Dh

      AL      access code byte

(DOS 2.x) bits 0-2 file attribute

              000     read only
              001     write only
              010     read/write
              bits 3-7 should be set to zero

(DOS 3.x) bits 0-2 file attribute

              000     read only
              001     write only
              010     read/write
              bit 3   reserved
              0       should be set to zero
              bits 4-6 sharing mode (network)
              000     compatibility mode (the way FCBs open files)
              001     read/write access denied (exclusive)
              010     write access denied
              011     read access denied
              100     full access permitted
              bit 7   inheritance flag
              0       file inherited by child process
              1       file private to child process
      DS:DX   address of ASCIIZ pathname string

return flag CF set on error

              AX      error code
              1       error
      AX      16 bit file handle
              or error code (1, 2, 4, 5, 0Ch)

note 1) Opens any normal, system, or hidden file

   2) Files that end in a colon are not opened
   3) The rear/write pointer is set at the first byte of the file and the
      record size of the file is 1 byte (the read/write pointer can be changed
      through function call 42h). The returned file handle must be used for
      all subsequent input and output to the file.
   4) If the file handle was inherited from a parent process or was
      duplicated by DUP or FORCEDUP, all sharing and access restrictions are
      also inherited.
   5) A file sharing error (error 1) causes an int 24h to execute with an
      error code of 2

3Eh Close A File Handle

   Close a file and release handle for reuse

entry AH 3Eh

      BX      file handle

return flag CF 0 successful close

              1       error
      AX      error code if error (6)

note 1) When executed, the file is closed, the directory is updated, and all

      buffers for that file are flushed. If the file was changed, the time
      and date stamps are changed to current
   2) If called with the handle 00000, it will close STDIN (normally the
      keyboard).

3Fh Read From A File Or Device

   Read from file with handle

entry AH 3Fh

      BX      file handle
      CX      number of bytes to read
      DS:DX   address of buffer

return flag CF 0 successful read

              1       error
      AX      0       pointer was already at end of file
                      or number of bytes read
                      or error code (5, 6)

note 1) This function attempts to transfer the number of bytes specified in CX

      to a buffer location. It is not guaranteed that all bytes will be read.
      If AX < CX a partial record was read.
   2) If performed from STDIN (file handle 0000), the input can be redirected
   3) If used to read the keyboard, it will only read to the first CR
   4) The file pointer is incremented to the last byte read.

40h Write To A File Or Device

   Write to file with handle

entry AH 40h

      BX      file handle
      CX      number of bytes to write
      DS:DX   address of buffer

return flag CF 0 successful write

              1       error
      AX      number of bytes written
              or error code  (5, 6)

note 1) This call attempts to transfer the number of bytes indicated in CX

      from a buffer to a file. If CX and AX do not match after the write,
      an error has taken place; however no error code will be returned for
      this problem. This is usually caused by a full disk.
   2) If the write is performed to STDOUT (handle 0001), it may be redirected
   3) To truncate the file at the current position of the file pointer, set
      the number of bytes in CX to zero before calling int 21h. The pointer
      can be moved to any desired position with function 42h.
   4) This function will not write to a file or device marked read-only.

41h Delete A File From A Specified Subdirectory

   (UNLINK)

entry AH 41h

      DS:DX   pointer to ASCIIZ filespec to delete

return CF 0 successful

              1       error
              AX      error code if any  (2, 5)

note 1) This function will not work on a file marked read-only

   2) Wildcards are not accepted

42h Move a File Read/Write Pointer

   (LSEEK)

entry AH 42h

      AL      method code
              00h     offset from beginning of file
              01h     offset from present location
              02h     offset from end of file
      BX      file handle
      CX      most significant half of offset
      DX      least significant half of offset

return AX low offset of new file pointer

      DX      high offset of new file pointer
      CF      0       successful move
              1       error
              AX      error code (1, 6)

note 1) If pointer is at end of file, reflects file size in bytes.

   2) The value in DX:AX is the absolute 32 bit byte offset from the beginning
      of the file

43h Get/Set file attributes

   (CHMOD)

entry AH 43h

      AL      00h     get file attributes
              01h     set file attributes
              CX      file attributes to set
                      bit 0       read only
                      1       hidden file
                      2       system file
                      3       volume label
                      4       subdirectory
                      5       written since backup
      DS:DX   pointer to full ASCIIZ file name

return CF set if error

      AX      error code  (1, 2, 3, 5)
      CX      file attributes on get
              attributes:
              01h     read only
              02h     hidden
              04h     system
              0FFh    archive

44h I/O Control for Devices (IOCTL)

   Get or set device information

entry AH 44h

      AL      00h     get device information (from DX)
                      BX      file or device handle
                      return  DX      device info
                                      If bit 7 set: (character device)
                                         bit 0: console input device
                                             1: console output device
                                             2: NUL device
                                             3: CLOCK$ device
                                             4: device is special
                                             5: binary (raw) mode
                                             6: Not EOF
                                            12: network device (DOS 3.x)
                                            14: can process IOCTL control
                                                strings (func 2-5)
                                       If bit 7 clear: (file)
                                       bits 0-5 are block device number
                                              6: file has not been written
                                             12: Network device (DOS 3.x)
                                          15: file is remote (DOS 3.x)
              01h     set device information (DH must be zero for this call)
                      DX bits:
                      0    1  console input device
                      1    1  console output device
                      2    1  null device
                      3    1  clock device
                      4    1  reserved
                      5    0  binary mode - don't check for control chars
                           1  cooked mode - check for control chars
                      6    0  EOF - End Of File on input
                      7       device is character device if set, if not, EOF
                              is 0 if channel has been written, bits 0-5 are
                              block device number
                      12      network device
                      14   1  can process control strings (AL 2-5, can only be
                              read, cannot be set)
                      15   n  reserved
              02h     read CX bytes to device in DS:DX from BX control chan
              03h     Write Device Control String
                      BX      device handle
                      CX      number of bytes to write
                      DS:DX   pointer to buffer
                      return  AX      number of bytes written
              04h     read from block device (drive number in BL)
                      BL      drive number (0=default)
                      CX      number of bytes to read
                      DS:DX   pointer to buffer
                      return  AX      number of bytes read
              05h     write to block device  (drive number in BL)
                      AX      number of bytes transfered
              06h     get input handle status
              07h     get output handle status
                      AX      0FFh    for ready
                              00h     for not ready
              08h     removable media bit (DOS 3.x)
                      return  AX      00h     device is removable
                                      01h     device is nonremovable
                                      0Fh     invalid drive specification
              09h     test local or network device in BL (DOS 3.x)
                      BL      drive number (0=default)
                      return  DX      attribute word, bit 12 set if device is
                                      remote
              0Ah     is handle in BX local or remote? (DOS 3.x)
                      BX     file handle
                      return DX (attribute word) bit 15 set if file is remote
              0Bh     change sharing retry count to DX (default=3), (DOS 3.x)
                      CX     delay (default 1)
                      DX     retry count (default 3)
              0Ch     general IOCTL (DOS 3.3 [3.2?]) allows a device driver to
                      prepare, select, refresh, and query Code Pages
              0Dh     Block Device Request (DOS 3.3+)
                      BL      drive number (0=default)
                      CH      major subfunction
                      CL      minor subfunction
                              40h set device parameters
                              41h write logical device track
                              42h format and verify logical device track
                              60h get device parameters
                              61h read logical device track
                              62h verify logical device track
                      DS:DX   pointer to parameter block
              0Eh     GET LOGICAL DEVICE (DOS 3.3+)
                      BL      drive number (0=default)
                      return  AL=0 block device has only one logical drive
                              assigned 1..n the last letter used to reference
                              the device (1=A:,etc)
              0Fh     Set Logical Device (DOS 3.3+)
      BL      drive number:  0=default, 1=A:, 2=B:, etc.
      BX      file handle
      CX      number of bytes to read or write
      DS:DX   data or buffer
      DX      data

return AX number of bytes transferred

              or error code (call function 59h for extended error codes)
              or status
                      00h     not ready
                      0FFh    ready
      CF      set if error

45h Duplicate a File Handle (DUP)

   Create duplicate handle

entry AH 45h

      BX      file handle to duplicate

return CF clear AX duplicate handle

              set     AX      error code  (4, 6)

note 1) If you move the pointed of one handle, the pointer of the other will

      also be moved.
   2) The handle in BX must be open

46h Force Duplicate of a Handle (FORCEDUP or CDUP)

   forces handle in CX to refer to the same file at the same position as BX

entry AH 46h

      BX      existing file handle
      CX      new file handle

return CF clear both handles now refer to existing file

              set     error
              AX      error code (4, 6)

note 1) If CX was an open file, it is closed first

   2) If you move the read/write pointer of either file, both will move
   3) The handle in BX must be open

47h Get Current Directory

   places full pathname of current directory/drive into a buffer

entry AH 47h

      DL      drive (0=default, 1=A:, etc.)
      DS:SI   points to 64-byte buffer area

return CF clear DS:DI pointer to ASCIIZ pathname of current directory

              set     AX      error code (0Fh)

note String does not begin with a drive identifier or a backslash

48h Allocate Memory

   allocates requested number of 16-byte paragraphs of memory

entry AH 48h

      BX      number of 16-byte paragraphs desired

return CF clear AX segment address of allocated space

                      BX      maximum number paragraphs available
              set     AX      error code (7, 8)

note BX indicates maximum memory availible only if allocation fails

49h Free Allocated Memory

   frees specified memory blocks

entry AH 49h

      ES      segment address of area to be freed

return CF clear successful

              set     AX      error code (7, 9)

note 1) This call is only valid when freeing memory obtained by function 48h.

   2) A program should not try to release memory not belonging to it.

4Ah Modify Allocated Memory Blocks (SETBLOCK)

   expand or shrink memory for a program

entry AH 4AH

      BX      new size in 16 byte paragraphs
      ES      segment address of block to change

return CF clear nothing

              set     AX      error code (7, 8, 9)
                  or  BX      max number paragraphs available

note 1) Max number paragraphs availible is returned only if the call fails

   2) Memory can be expanded only if there is memory availible

4Bh Load or Execute a Program

   (EXEC)

entry AH 4Bh

      AL      00h     load and execute program. A PSP is built for the program
                      the ctrl-break and terminate addresses are set to the
                      new PSP.
             *01h     load but don't execute  (note 1)
             *02h     load (internal) but do not execute
              03h     load overlay (do not create PSP, do not begin execution)
      DS:DX   points to the ASCIIZ string with the drive, path, and filename
              to be loaded
      ES:BX   points to a parameter block for the load
             (AL=00h) word    segment address of environment string to be
                              passed
                      dword   pointer to the command line to be placed at
                              PSP+80h
                      dword   pointer to default FCB to be passed at PSP+5Ch
                      dword   pointer to default FCB to be passed at PSP+5Ch
             (AL=03h) word    segment address where file will be loaded
                      word    relocation factor to be applied to the image

return CF clear successful

              set     error
              AX      error code (1, 2, 8, 0Ah, 0Bh)

note 1) If you make this call with AL=1 the program will be loaded as if you

      made the call with AL=0 except that the program will not be executed.
      Additionally, with AL=1 the stack segment and pointer along with the
      program's CS:IP entry point are returned to the program which made the
      4B01h call. These values are put in the four words at ES:BX+0eh. On
      entry to the call ES:BX points to the environment address, the command
      line and the two default FCBs. This form of EXEC is used by DEBUG.COM.
   2) Application programs may invoke a secondary copy of the command
      processor (normally COMMAND.COM) by using the EXEC function.  Your
      program may pass a DOS command as a parameter that the secondary
      command processor will execute as though it had been entered from the
      standard input device.
      The procedure is:
       A. Assure that adequate free memory (17k for 2.x and 3.0, 23k for 3.1
          up) exists to contain the second copy of the command processor and
          the command it is to execute. This is accomplished by executing
          function call 4Ah to shrink memory allocated to that of your current
          requirements. Next, execute function call 48h with BX=0FFFFh. This
          returns the amount of memory availible.
      B. Build a parameter string for the secondary command processor in the
         form:
                       1 byte   length of parameter string
                      xx bytes  parameter string
                       1 byte   0Dh (carriage return)
         For example, the assembly language statement below would build the
         string to cause execution of the command FOO.EXE:
                            DB 19, "/C C:FOO" , 13
      C. Use the EXEC function call (4Bh), function value 0 to cause execution
         of the secondary copy of the command processor. (The drive,
         directory, and name of the command processor can be gotten from the
         COMSPEC variable in the DOS environment passed to you at PSP+2Ch.)
      D. Remember to set offset 2 of the EXEC control block to point to the
         string built above.
   3) All open files of a process are duplicated in the newly created
      process after an EXEC, except for files originally opened with the
      inheritance bit set to 1.
   4) The environment is a copy of the original command processor's
      environment. Changes to the EXECed environment are not passed back to
      the original. The environment is followed by a copy of the DS:DX
      filename passed to the child process. A zero value will cause the
      child process to inherit the environment of the calling process. The
      segment address of the environment is placed at offset 2Ch of the
      PSP of the program being invoked.
   5) This function uses the same resident part of COMMAND.COM, but makes a
      duplicate of the transient part.
   6) How EXEC knows where to return to:  Basically the vector for int 22h
      holds the terminate address for the current process.  When a process
      gets started, the previous contents of int 22h get tucked away in the
      PSP for that process, then int 22h gets modified.  So if Process A
      EXECs process B, while Process B is running, the vector for int 22h
      holds the address to return to in Process A, while the save location in
      Process B's PSP holds the address that process A will return to when
      *it* terminates.  When Process B terminates by one of the usual legal
      means, the contents of int 22h are (surmising) shoved onto the stack,
      the old terminate vector contents are copied back to int 22h vector from
      Process B's PSP, then a RETF or equivalent is executed to return control
      to process A.

4Ch Terminate a Process (EXIT)

   Quit with exit code

entry AH 4Ch

      AL      exit code in AL when called, if any, is passed to next process

return none note 1) Control passes to DOS or calling program

   2) return code from AL can be retrieved by ERRORLEVEL or function 4Dh
   3) all files opened by this process are closed, buffers are flushed, and
      the disk directory is updated
   4) Restores Terminate vector from PSP:000Ah
               Ctrl-C vector from PSP:000Eh
               Critical Error vector from PSP:0012h

4Dh Get Return Code of a Subprocess (WAIT)

   gets return code from functions 31h and 4Dh  (ERRORLEVEL)

entry AH 4Dh return AL exit code of subprogram (functions 31h or 4Ch)

      AH      circumstance which caused termination
              00h     normal termination
              01h     control-break
              02h     critical device error
              03h     terminate and stay resident (function 31h)

note The exit code is only returned once

4Eh Find First Matching File (FIND FIRST)

   Find first ASCIIZ

entry AH 4Eh

      CX      search attributes
      DS:DX   pointer to ASCIIZ filename (with attributes)

return CF set AX error code (2, 12h)

              clear   data block written at current DTA
                      format of block is:
documented by Micro-  |00h   1 byte   attribute byte of search
soft as "reserved for |01h   1 byte   drive used in search
DOS' use on subsquent |02h   11 bytes the search name used
Find Next calls"      |0Ch   2 bytes  word value of last entry
function 4Fh          |0Fh   4 bytes  dword pointer to this DTA
                      |13h   2 bytes  word directory start
                       15h   1 byte   file attribute
                       16h   2 bytes  file time
                       18h   2 bytes  file date
                       1Ah   2 bytes  low word of file size
                       1Ch   2 bytes  high word of file size
                       1Eh  13 bytes  name and extension of file found, plus
                                      1 byte of 0s. All blanks are removed
                                      from the name and extension, and if an
                                      extension is present it is preceded by a
                                      period.

note 1) Will not find volume label

   2) This function does not support network operations
   3) Wildcards are allowed in the filespec
   4) If the attribute is zero, only ordinary files are found. If the volume
      label bit is set, only volume labels will be found. Any other attribute
      will return that attribute and all normal files together.
   5) To look for everything except the volume label, set the hidden, system,
      and subdirectory bits all to 1

4Fh Find Next Matching File (FIND NEXT)

   Find next ASCIIZ

entry AH 4Fh return CF clear data block written at current DTA

              set     AX      error code (2, 12h)

note 1) If file found, DTA is formatted as in call 4Eh

   2) Volume label searches using 4Eh/4Fh reportedly aren't 100% reliable
      under DOS 2.x. The calls sometime report there's a volume label and
      point to a garbage DTA, and if the volume label is the only item they
      often won't find it
   3) This function does not support network operations
   4) Use of this call assumes that the original filespec contained wildcards

50h Set PSP * Set new Program Segment Prefix; current process ID entry AH 50h

      BX      segment address of new PSP

return none - swaps PSP's regarded as current by DOS note 1) By putting the PSP segment value into BX and issuing call 50h DOS stores

      that value into a variable and uses that value whenever a file call is
      made.
   2) Note that in the PSP (or PDB) is a table of 20 (decimal) open file
      handles. The table starts at offset 18h into the PSP. If there is an
      0FFh in a byte then that handle is not in use. A number in one of the
      bytes is an index into an internal FB table for that handle. For
      instance the byte at offset 18h is for handle 0, at offset 19h handle
      1, etc. up to 13h. If the high bit is set then the file associated by
      the handle is not shared by child processes EXEC'd with call 4Bh.
   3) Function 50h is dangerous in background operations prior to DOS 3.x as
      it uses the wrong stack for saving registers.  (same as functions
      0..0Ch in DOS 2.x)
   4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler
      without setting the Critical Error flag
   5) Open File information, etc. is stored in the PSP DOS views as current.
      If a program (eg. a resident program) creates a need for a second PSP,
      then the second PSP should be set as current to make sure DOS closes
      that as opposed to the first when the second application finishes.
   6) See PC Mag Vol.5, No 9, p.314 for discussion.

51h Get Program Segment Prefix * Returns the PSP address of currently executing program entry AH 51h return BX address of currently executing program note 1) Used in DOS 2.x, 3.x uses 62h

   2) Function 51h is dangerous in background operations prior to DOS 3.x as
      it uses the wrong stack for saving registers.  (same as functions
      0..0Ch in DOS 2.x)
   3) 50h and 51h might be used if you have more than one process in a PC.
      For instance if you have a resident program that needs to open a file
      you could first call 51h to save the current id and then call 50h to set
      the ID to your PSP.
   4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler

52h IN-VARS * returns a pointer to a set of DOS data variables MCB chain, pointer to

      first device driver and a pointer to disk parameter blocks (first one)

entry AH 52h return ES:BX pointer to the DOS list of lists, for disk information. Does not

              access the disk, so information in tables might be incorrect if
              disk has been changed. Returns a pointer to the following array
              of longword pointers:
              Bytes   Value
              -2h,-1h segment of first memory control block
              0h-3h   pointer to first DOS disk block (see function 36h)
              4h-7h   partially unknown. Pointer to a device driver. Maybe
                      first resident driver?
              8h-0Bh  pointer to CLOCK$ device driver, whether installable or
                      resident
              0Ch-0Fh pointer to actual CON: device driver, whether
                      installable or resident
      (DOS 2.x)
              10      Number of logical drives in system
              11-12   Maximum bytes/block of any block device
              13-16   unknown
              17      Beginning (not a pointer. The real beginning!) of NUL
                      device driver. This is the first device on DOS's linked
                      list of device drivers.
      (DOS 3.x)
              10h-11h maximum bytes/block of any block device (0200h)
              12h-15h unknown. Pointer to current directory block?
              16h-19h partially undefined: Pointer to array of drive info:
                      51h bytes per drive, starting with A: ...
                      00h-3Fh current path as ASCIIZ, starting with 'x:\'
                      40h-43h unknown    zeros always
                      44h     unknown    flags? Usually 40h, except for
                              entry after last valid entry = 00h
                      45h-48h pointer to DOS disk block for this drive
                      49h-4Ah unknown. Current track or block?
                              -1 if never accessed
                      4Bh-4Eh unknown  -1 always
                      4Fh-52h unknown   2 always
              1Ah-1Dh unknown. Pointer to data area, maybe including cluster
                      allocation table?
              1Eh-1Fh unknown. Zero always
              20h     Number of block devices
              21h     value of LASTDRIVE command in CONFIG.SYS (default 5)
              22h     Beginning (not a pointer. The real beginning!) of NUL
                      device driver. This is the first device on DOS's linked
                      list of device drivers.

note This call is not supported in OS/2 1.0's DOS Compatibility Box

53h Translate BPB * Translates BPB (BIOS Parameter Block, see below) into a DOS Disk Block (see

   function call 32h).

entry AH 53h

      DS:SI   pointer to BPB
      ES:BP   pointer to area for DOS Disk Block.
              Layout of Disk Block:
              bytes   value
              00h-01h bytes per sector, get from DDB bytes 02h-03h.
              02h     sectors per cluster, get from (DDB byte 4) + 1
              03h-04h reserved sectors, get from DDB bytes 06h-07h
              05h     number of FATs, get from DDB byte 08h
              06h-07h number of root dir entries, get from DDB bytes 09h-0Ah
              08h-09h total number of sectors, get from:
                      ((DDB bytes 0Dh-0Eh) - 1) * (sectors per cluster (BPB
                      byte 2)) + (DDB bytes 0Bh-0Ch)
              0Ah     media descriptor byte, get from DDB byte 16h
              0Bh-0Ch number of sectors per FAT, get from DDB byte 0Fh

return unknown

54h Get Verify Setting

   Get verify flag status

entry AH 54h return AL 00h if flag off

              01h if flag on

note Flag can be set with function 2Eh

55h Create "Child" PSP * Create PSP: similar to function 26h (which creates a new Program Segment

   Prefix at segment in DX) except creates a "child" PSP rather than copying
   the existing one.

entry AH 55h

      DX      segment number at which to create new PSP.

return unknown note 1) This call is similar to call 26h which creates a PSP except that unlike

      call 26h the segment address of the parent process is obtained from the
      current process ID rather than from the CS value on the stack (from the
      INT 21h call). DX has the new PSP value and SI contains the value to be
      placed into PSP:2 (top of memory).
   2) Function 55 is merely a substitute for function 26h. It will copy the
      current PSP to the segment address DX with the addition that SI is
      assumed to hold the new memory top segment. This means that function
      26h sets SI to the segment found in the current PSP and then calls
      function 55h.

56h Rename a File

   if the first file exists, it is renamed to the name in ES:DI

entry AH 56h

      DS:DX   pointer to ASCIIZ old pathname
      ES:DI   pointer to ASCIIZ new pathname

return CF clear successful rename

              set     AX      error code (2, 3, 5, 11h)

note 1) Works with files in same drive only

   2) Global characters not allowed in filename
   3) The name of a file is its full pathname. The file's full pathname can
      be changed, while leaving the actual FILENAME.EXT unchanged. Changing
      the pathname allows the file to be "moved" from subdirectory to
      subdirectory on a logical drive without actually copying the file.
   4) DOS 3.x allows renaming of directories

57h Get/Set a File's Date and Time

   read or modify time and date stamp on a file's directory entry

entry AH 57h

      AL      function code
              00h     get date and time
              01h     set date and time
                      CX      time to be set
                      DX      date to be set
      BX      file handle

return CF clear CX time of last write (if AL = 0)

                      DX      date of last write (if AL = 0)
             set      AX      error code (1, 6)

note Date/time formats are:

      CX bits 0Bh-0Fh hours (0-23)    DX bits 09h-0Fh year (relative to 1980)
              05h-0Ah minutes (0-59)          05h-08h month (0-12)
              00h-04h #2 sec. incr. (0-29)   00h-04h day of the month (0-31)

58h Get/Set Allocation Strategy

    DOS 3.x

entry AH 58h

      AL      0       set current strategy
              1       set new current strategy
      BX      new strategy if AH=1
              0       first fit - chooses the lowest block in memory which
                      will fit (this is the default). (use first memory block
                      large enough)
              1       best fit - chooses the smallest block which will fill
                      the request.
              2       last fit - chooses the highest block which will fit.

return CF clear (0) successful

              set     (1)     error (1)
                              AX      error code
      AX      strategy code (CF=0)

note 1) Documented in Zenith DOS version 3.1, some in Advanced MSDOS

   2) The set subfunction accepts any value in BL; 2 or greater means last
      fit. The get subfunction returns the last value set, so programs should
      check whether the value is greater than or equal to 2.

59h Get Extended Error Code (DOS 3.x)

   returns additional error information when requested
    The Get Extended Error function call (59h) is intended to provide a common
   set of error codes and to supply more extensive information about the error
   to the application. The information returned from function call 59h, in
   addition to the error code, is the error class, the locus, and the
   recommended action. The error class provides information about the error
   type (hardware, internal, system, etc.). The locus provides information
   about the area involved in the failure (serial device, block device,
   network, or memory). The recommended action provides a default action for
   programs that do not understand the specific error code.
     Newly written programs should use the extended error support both from
   interrupt 24h hard error handlers and after any int 21h function calls. FCB
   function calls report an error by returning 0FFh in AL. Handle function
   calls report an error by setting the carry flag and returning the error
   code in AX. Int 21h handle function calls for DOS 2.x continue to return
   error codes 0-18. Int 24h handle function calls continue to return error
   codes 0-12. But the application can obtain any of the error codes used in
   the extended error codes table by issuing function call 59h. Handle
   function calls for DOS 3.x can return any of the error codes. However, it
   is recommended that the function call be followed by function call 59h to
   obtain the error class, the locus, and the recommended action.
     The Get Extended Error function (59h) can always be called, regardless of
   whether the previous DOS call was old style (error code in AL) or new style
   (carry bit). It can also be used inside an int 24h handler.
    You can either check AL or the carry bit to see if there was no error,
   and call function 59h only if there was an error, or take the simple
   approach of always calling 59h and letting it tell you if there was an
   error or not. When you call function 59h it will return with AX=0 if the
   previous DOS call was successful.

entry AH 59h

      BX      version code (0000 for DOS 3.0 and 3.1)

return AX extended error code:

              01h     Invalid function number
              02h     File not found
              03h     Path not found
              04h     Too many open files, no file handles left
              05h     Access denied
              06h     Invalid handle
              07h     Memory control blocks destroyed
              08h     Insufficient memory
              09h     Invalid memory block address
              0Ah     Invalid environment
              0Bh     Invalid format
              0Ch     Invalid access code
              0Dh     Invalid data
              0Eh     Reserved
              0Fh     Invalid drive was specified
              10h     Attempt to remove the current directory
              11h     Not same device
              12h     No more files
              13h     Attempt to write on write-protected diskette
              14h     Unknown unit
              15h     Drive not ready
              16h     Unknown command
              17h     Bad CRC check
              18h     Bad request structure length
              19h     Seek error
              1Ah     Unknown media type
              1Bh     Sector not found
              1Ch     Printer out of paper
              1Dh     Write fault
              1Eh     Read fault
              1Fh     General Failure
              20h     Sharing violation
              21h     Lock violation
              22h     Invalid disk change
              23h     FCB unavailible
              24h     Sharing buffer overflow
              25h     Reserved
              26h        "
              27h        "
              28h        "
              29h        "
              2Ah        "
              2Bh        "
              2Ch        "
              2Dh        "
              2Eh        "
              2Fh        "
              30h        "
              31h     Reserved
              32h     Network: request not supported (DOS 3.1 + MS Networks)
              33h     Remote computer not listening
              34h     Duplicate name on network
              35h     Network: name not found
              36h     Network: busy
              37h     Network: device no longer exists
              38h     NETBIOS command limit exceeded
              39h     Network: adapter hardware error
              3Ah     Incorrect response from network
              3Bh     Unexpected network error
              3Ch     Incompatible remote adapter
              3Dh     Print queue full
              3Eh     Not enough space for print file
              3Fh     Print file was deleted
              40h     Network: name was deleted
              41h     Network: Access denied
              42h     Network: device type incorrect
              43h     Network: name not found
              44h     Network: name limit exceeded
              45h     NETBIOS session limit exceeded
              46h     Temporarily paused
              47h     Network: request not accepted
              48h     Print or disk redirection paused (DOS 3.1 + MS Networks)
              49h     Reserved
              4Ah        "
              4Bh        "
              4Ch        "
              4Dh        "
              4Eh        "
              4Fh     Reserved
              50h     File exists
              51h     Reserved
              52h     Cannot make directory entry
              53h     Fail on interrupt 24h
              54h     Too many redirections
              55h     Duplicate redirection
              56h     Invalid password
              57h     Invalid parameter
              58h     Network: device fault
      BH      class of error:
              01h     Out of resource
              02h     Temporary situation
              03h     Authorization (denied access)
              04h     Internal
              05h     Hardware failure
              06h     System failure
              07h     Application program error
              08h     Not found
              09h     Bad format
              0Ah     Locked
              0Bh     Media error (wrong volume ID, disk failure)
              0Ch     Already exists
              0Dh     Unknown
      BL      suggested action code:
              01h     Retry
              02h     Delayed retry
              03h     Prompt user
              04h     Abort after cleanup
              05h     Immediate abort
              06h     Ignore
              07h     Retry after user intervention
      CH      locus (where error occurred):
              01h     Unknown or not appropriate
              02h     Block device
              03h     Network related
              04h     Serial device
              05h     Memory related

note 1) Not all DOS functions use the carry flag to indicate an error. Carry

      should be tested only on those functions which are documented to use it.
   2) None of the DOS functions which existed before 2.0 use the carry
      indicator.  Many of them use register AL as an error indication instead,
      usually by putting 0FFh in AL on an error. Most, but not all, the "new"
      (2.x, 3.x) functions do use carry, and most, but not all, of the "old"
      (1.x) functions use AL.
   3) On return, CL, DI, DS, DX, ES, BP, and SI are destroyed - save before
      calling this function if required.
   4) DOS 2.x Error Codes:  If you are using function calls 38h-57h with DOS
      2.x, to check if an error has occurred, check for the following error
      codes in the AX register:
      call    error code      call    error code      call    error code
      38h     2               41h     2,3,5           4Ah     7,8,9
      39h     3,5             42h     1,6             4Bh     1,2,3,5,8,10,11
      3Ah     3,5,15          43h     1,2,3,5         4Eh     2,3,18
      3Bh     3               44h     1,3,5,6         4Fh     18
      3Ch     3,4,5           45h     4,6             56h     2,3,5,17
      3Dh     2,3,4,5,12      46h     4,6             57h     1,6
      3Eh     6               47h     15
      3Fh     5,6             48h     7,8
      40h     5,6             49h     7,9
   5) note that extended error codes 13h through 1Fh correspond to error
      codes 00h through 0Ch returned by int 24h

5Ah Create Temporary File

   Create unique filename (for temporary use) (DOS 3.x)

entry AH 5Ah

      DS:DX   pointer to ASCIIZ directory pathname ending with a backslash (\)
      CX      file attribute

return CF clear DS:DX new ASCIIZ path name

                      AX      handle
              set     AX      error code (3 or 5)

note 1) The file created is not truly "temporary". It must be removed by the

      user.
   2) If the filename created already exists in the current directory, this
      function will call itself again with another unique filename until
      a unique filename is found

5Bh Create a New File

   (DOS 3.x)

entry AH 5Bh

      DS:DX   pointer to directory ASCIIZ path name
      CX      file attribute

return CF clear AX file handle

                      DS:DX   new ASCIIZ path name
              set     AX      error code (3, 4, 5, 50h)

note 1) Unlike function 3Ch, function 5Bh will fail if the file already exists.

   2) The new file is opened in read/write mode

5Ch Lock/Unlock File Access

   (DOS 3.x)

entry AH 5Ch

      AL      00h     to lock file
              01h     to unlock file
      BX      file handle
      CX:DX   starting offset of region to lock
      SI:DI   size of region  to lock

return CF clear successful

              set     AX      error code (1, 6, 21h)

note 1) Close all files before exiting or undefined results may occur

   2) Programs spawned with EXEC inherit all the parent's file handles but
      not the file locks

5Dh Set Extended Error Information * DOS Internal - partial (DOS 3.x) entry AH 5dh

      AL      subfunction
              06h     get address of critical error flag
                      return  DS:SI   pointer to critical error flag
              08h     (unknown - used by command.com)
              09h     (unknown - used by command.com)
              0Ah     set error info (Error, Class, Action, and Locus)
                      DS:DX   address of 11-word error information
                              words 0 to 7: values of AX,BX,CX,DX,SI,DI,DS,
                                            ES that function 59h will return
                              words 8 to 10: zero (reserved)

return: CX unknown

      DX      unknown
      DS:SI   (for 06h) pointer to critical error flag

note 1) This call seems to have many different functions

   2) Function 0Ah; DOS 3.1+
   3) Function 06h; setting CritErr flag allows use of functions 50h/51h from
      int 28h under DOS 2.x by forcing use of correct stack

5Eh Network Printer (Partially documented by Microsoft)

      DOS 3.1+ with Networks software

entry AH 5Eh

      AL      00      Get Machine Name
                      DS:DX   pointer to buffer for ASCIIZ name
                      return  CH      0       if name not defined
                              CL      NETBIOS name number if CH <> 0
                              DS:DX   pointer to identifier if CH <> 0
                      note    the ASCIIZ name is a 15 byte string padded
                              to length with zeroes
              01      Set Machine Name
                      DS:DX   pointer to ASCIIZ name
                      CH      unknown
                      CL      name number
              02      Set Printer Control String
                      BX      redirection list index
                      CX      length of setup string (max 64 bytes)
                      DS:SI   pointer to string buffer
              03      Get Printer Control String
                      BX      redirection list index
                      ES:DI   pointer to string buffer
                      return  CX      length of setup string (max 64 bytes)

return CF clear successful

              set     error
                      AX      error code (1 for all listed subfunctions)

note 1) Used in IBM's & Microsoft's Network programs

   2) Partial documentation in Fall 1985 Byte
   3) These services require that the network software be installed
   4) Partial documentation in Advanced MS-DOS
   5) SHARE must be loaded or results can be unpredictable on 00h, or fail
      with 02h or 03h

5Fh Network Redirection

      (DOS 3.1 + Microsoft Networks)

entry AH 5Fh

      AL     *00h     Unknown
             *01h     Unknown
              02h     Get Redirection List Entry
                      BX      redirection list index
                      DS:SI   pointer to 16 byte buffer for local device name
                      ES:DI   pointer to 128 byte buffer for network name
                      return  BH      device status flag (bit 0 = 0 if valid)
                                                        (bit 0 = 1 if invalid)
                              BL      device type
                                      03      printer device
                                      04      drive device
                              CX      stored parameter value
                              DS:SI   pointer to 16 byte local device name
                              ES:DI   pointer to 128 byte network name
                      note    DX and BP are destroyed by this call!
              03h     Redirect Device
                      BL      device type
                              03      printer device
                              04      file device
                      CX      stored parameter value
                      DS:SI   pointer to source device name
                      ES:DI   pointer to destination ASCIIZ network path +
                              ASCIIZ password
              04h     Cancel Redirection
                      DS:SI   pointer to ASCIIZ device name or network path

return CF clear successful

              set     if error
                      AX      error code

return as above note 1) Used in IBM's Network program

   2) Partial documentation in Fall 1985 Byte
   3) These services require that the network software be installed
   4) Partial documentation in Advanced MS-DOS
   5) SHARE must be loaded or the call will fail
   6) The network device name requires a password

60h Parse pathname (DOS 3.x) * Translate - perform name processing on a string (internal to DOS) entry AH 60h

      DS:SI   pointer to source string (null terminated)
      ES:DI   pointer to destination string buffer.

return ES:DI buffer filled with qualified name

      CF      0       no error
              1       error
                      AX      error code

note 1) Documented in Zenith 3.05 Tech Ref

   2) All name processing is performed on the input string: string
      substitution is performed on the components, current drive/directories
      are prepended, .  and ..  are removed.
   3) Example: If current drive/directory is c:\test,  myfile.x is translated
      to c:\test\myfile.x; ..\source\sample.asm is tranlated to c:\source\
      sample.asm
   4) It is the caller's responsibility to make sure DS:SI does not point to
      a null string. If it does, SI is incremented, a null byte is stored at
      ES:DI, and the routine returns.

61h No Information Availible (DOS 3.x) * internal to DOS - parameters not known entry AH 61h return AL 0 note Supposedly documented in Zenith DOS 3.05 Tech Ref

62h Get Program Segment Prefix (PSP)

   Get PSP address (DOS 3.x)

entry AH 62h return BX segment address of PSP

63h Get Lead Byte Table (MS-DOS 2.25 only)

   added in MS-DOS version 2.25 for additional foreign character set support.

entry AH 63h

      AL      subfunction
              00h     get system lead byte table address
              01h     set/clear interim console flag
                      DL      0 to clear interim console flag
                              1 to set interim console flag
              02h     get interim console flag

return DS:SI pointer to lead byte table (AL = 00h)

      DL      interim console flag (AL = 02h)

note Function 63h destroys all registers on return.

64h Internal

   unknown (DOS 3.3+)

entry AH 64h

65h Get Extended Country Information (DOS 3.3+)

   returns information about the selected country formats, code pages, and
   conversion tables

entry AH 65h

      AL      info ID (1 - 6)
      BX      code page (-1 = global code page)
      CX      size of buffer
      DX      country ID (-1 = current country)
      ES:DI   pointer to country information buffer

return AX error code if carry set, otherwise

      CX      size of country information returned
      CF      set on error
      ES:DI   pointer to country information:
              1 byte info ID
              if info ID <> 1
                  dword  pointer to information
              if info ID = 1
                  word   size
                  word   country ID
                  word   code page
               34 bytes  (see function 38h)

66h Get/Set Global Code Page Table (DOS 3.3+)

    query/reset code page defaults

entry AH 66h

      AL      00h     Get Global Code Page
              01h     Set Global Page
                      BX      active code page
                      DX      system code page (active page at boot time)

return CF clear successful

              set    AX       error code
      if 00h         BX       active code page
                     DX       system code page (active page at boot time)

note BX = active code page: 437 = US, 860 = Portugal, 863 = Canada (French)

                             865 = Norway/Denmark

67h Set Handle Count (DOS 3.3+)

   supports more than 20 open files per process

entry AH 67h

      BX      desired number of handles (max 255)

return CF clear if OK

      CF      set if error
              AX      error code

68h Commit File (DOS 3.3+)

   Write all buffered data to disk

entry AH 68h

      BX      file handle

return CF set AX error code

              clear   successful

note Faster and more secure method of closing a file in a network than

      current close commands

69h Disk Serial Number DOS 4.0 (US)

      Places and reads "Volume Serial Number" on disks formatted with 4.0+

entry unknown return unknown note A call for DOS function 69h (AL=0, possibly a subfunction) uses DS:DX

      as a pointer to a table. On return, the table is filled in as follows:
      word            unknown (zeroes on my system)
      dword           disk serial number (binary)
      char[11]        volume label or "NO NAME    " if none
      char[8]         FAT type
      The FAT type field returns "FAT16   " on hard disk formatted with DOS
      3.3 and "FAT12   " on a 360K floppy.

6Ah unknown (DOS 4.0?)

6Bh unknown (DOS 4.0?)

6Ch Extended Open/Create DOS 4.0 (US)

      Combines functions available with Open, Create, Create New, and Commit

entry AH 6Ch

      AL      00h  reserved  [which means there might be other subfunctions?]
      BX      mode    format  0WF0 0000 ISSS 0AAA
                              AAA is access code (read, write, read/write)
                              SSS is sharing mode
                              I       0       pass handle to child
                                      1       no inherit [interesting!]
                              F       0       use int 24h for errors
                                      1       disable int 24h for all
                                              I/O on this handle; use own
                                              error routine
                              W       0       no commit
                                      1       auto commit on all writes
      CX      create attribute
      DL      action if file exists/does not exists
              bits 7-4 action if file does not exist
                       0000   fail
                       0001   create
              bits 3-0 action if file exists
                       0000    fail
                       0001    open
                       0010    replace/open
      DH      0
      DS:SI   pointer to ASCIIZ file name

return CF set on error

              AX      error code
              clear
              AX      file handle
              CX      action taken
                      01h     file opened
                      02h     created/opened
                      03h     replaced/opened

89h DOS_Sleep * not documented by Microsoft entry AH 89h return unknown note 1) Function included in Microsoft C 4.0 startup code MSDOS.INC

   2) Debugging shows that the first instruction on entry to DOS compares AH
      with 64h (at least in DOS 3.2) and aborts the call if AH > 64.
   3) Possibly used in European MSDOS 4.0?

Aftermarket Application Installed Function Calls:

0B6h, 0B8h, 0BBh, 0BCh, B0Eh, 0BFh, 0C0h, 0C1h, 0C2h, 0C3h, 0C4h, 0C5h, 0C6h, 0C7h, 0C8h, 0C9h, 0CAh, 0CBh, 0CCh, 0CDh, 0CEh, 0CFh, 0D0h, 0D1h, 0D2h, 0D3h, 0D4h, 0D5h, 0D6h, 0D7h, 0DAh, 0DBh

      Used by Novell NetWare

0DCh Novell NetWare

      Get Station Number

entry AH 0DCh return AL station number

              00h     if NetWare not loaded or this machine is a non-
                      dedicated server

0DDh Novell NetWare

0DEh Novell NetWare

      Set Broadcast Mode

0DFh Novell NetWare

0E0h Novell NetWare

0E1h Novell NetWare

      Broadcast Messages

entry AH E1h

      AL      00h     send broadcast message
              01h     get broadcase message
              02h-09h unknown

0E2h Novell NetWare

0E3h Novell NetWare

      Connection Control

entry AH E3h

      AL      00h-14h unknown
              15h     get object connection numbers
              16h     get connection information
              32h-47h unknown

E4h DoubleDOS

      check status

entry AX 00h return AL <> 0 if DoubleDOS is active

0E4h Novell NetWare

0E5h, 0E6h, 0E7h, 0E8h, 0E9h

      Novell NetWare

0EAh DoubleDOS

      turn off task switching

entry AX EAh return task switching turned off

0EAh Novell NetWare

0EBh DoubleDOS

      turn on task switching

entry AH EBh return Task switching turned on

0EBh Novell NetWare

0ECh DoubleDOS

      get virtual screen address

entry AH ECh return ES segment of virtual screen note Screen address can change if task switching is on!

0ECh Novell NetWare

0EDh Novell NetWare

0EEh DoubleDOS

      give away time to other tasks

entry AH EEh

      AL      number of 55ms time slices to give away

return Returns after giving away time slices

0EEh Novell NetWare

      Get Node Address

entry AH EEh return CX:BX:AX = six-byte address

0EFh, 0F0h, 0F1h, 0F2h, 0F3h Reportedly used by Novell NetWare.

      No parameters known

0FFh CED (CJ Dunford's DOS macro and command-line editor)

      CED installable commands

entry AH 0FFh

      AL      00h     add installable command
              01h     remove installable command
              02h     reserved, may be used to test for CED installation
      BL      mode    bit 0 = 1 callable from DOS prompt
                      bit 1 = 1 callable from application
      DS:SI   pointer to cr-terminated command name
      ES:DI   pointer to far routine entry point

return CF set on error

      AX      01h     invalid function
              02h     command not found (subfunction 1 only)
              08h     insufficient memory (subfunction 0 only)
              0Eh     bad data (subfunction 0 only)
      AH      0FFh    if CED not installed

Chapter 5

Programming Technical Reference - IBM Copyright 1988, Dave Williams

                      Interrupts 22h Through 86h

Interrupt 22h Terminate Address (0:0088h) This interrupt transfers control to the far (dword) address at this interrupt location when an application program terminates. The default address for this interrupt is 0:0088h through 0:008Bh. This address is copied into the program's Program Segment Prefix at bytes 0Ah through 0Dh at the time the segment is created and is restored from the PSP when the program terminates. The calling program is normally COMMAND.COM or an application. Do not issue this interrupt directly, as the EXEC function call does this for you. If an application spawns a child process, it must set the Terminate Address prior to issuing the EXEC function call, otherwise when the second program terminated it would return to the calling program's Terminate Address rather than its own. This address may be set with int 21, function 25h.

Interrupt 23h Ctrl-Break Exit Address (0:008Ch) If the user enters a Ctrl-Break during STDIN, STDOUT, STDPRN, or STDAUX, int 23h is executed. If BREAK is on, int 23h is checked on MOST function calls (notably 06h). If the user written Ctrl-Break routine saves all registers, it may end with a return-from-interrupt instruction (IRET) to continue program execution. If the user-written interrupt program returns with a long return, the carry flag is used to determine whether the program will be aborted. If the carry flag is set, the program is aborted, otherwise execution continues (as with a return by IRET). If the user-written Ctrl-Break interrupt uses function calls 09h or 0Ah, then ctrl-C/CR/LF are output. If execution is continued with an IRET, I/O continues from the start of the line. When the interrupt occurs, all registers are set to the value they had when the original function call to DOS was made. There are no restrictions on what the Ctrl-Break handler is allowed to do, including DOS function calls, as long as the registers are unchanged if an IRET is used. If the program creates a new segment and loads a second program which itself changes the Ctrl-Break address, the termination of the second program and return to the first causes the Ctrl-Break address to be restored from the PSP to the value it had before execution of the second program.

Interrupt 24h Critical Error Handler (0:0090h) When a critical error occurs within DOS, control is transferred to an error handler with an int 24h. This may be the standard DOS error handler (ABORT, RETRY, IGNORE) or a user-written routine. On entry to the error handler, AH will have its bit 7=0 (high order bit) if the error was a disk error (probably the most common error), bit 7=1 if not. BP:SI contains the address of a Device Header Control Block from which additional information can be retrieved (see below). The register is set up for a retry operation and an error code is in the lower half of the DI register with the upper half undefined. These are the error codes:

The user stack is in effect and contains the following from top to bottom:

      IP      DOS registers from issuing int 24h
      CS      int 24h
      flags
      AX      user registers at time of signal
      BX      int 21h request
      CX
      DX
      SI
      DI
      BP
      DS
      ES
      IP      from original int 21h
      CS
      flags

To reroute the critical error handler to a user-writen critical error handler, the following should be done:

Before an int 24h occurs: 1) The user application initialization code should save the int 24h vector and

 replace the vector with one pointing to the user error routine.

When the int 24h occurs: 2) When the user error routine received control it should push the flag

 registers onto the stack and execute a far call to the original int 24h
 vector saved in step 1.

3) DOS gives the appropriate prompt, and waits for user input (Abort, Retry,

 Ignore, Fail). After the user input, DOS returns control to the user error
 routine instruction following the far call.

4) The user error routine can now do any tasks nescessary. To return to the

 original application at the point the error occurred, the error routine needs
 to execute an IRET instruction. Otherwise, the user error routine should
 remove the IP, CS, and flag registers from the stack. Control can then be
 passed to the desired point.

Int 24h provides the following values in registers on entry to interrupt handler:

entry AH status byte

          bit 7       0       disk I/O hard error
                      1       other error - if block device, bad FAT
                              - if char device, code in DI
              6       unused
              5       0       if IGNORE is not allowed
                      1       if IGNORE is allowed
              4       0       if RETRY  is not allowed
                      1       if RETRY  is allowed
              3       0       if FAIL   is not allowed
                      1       if FAIL   is allowed
              2 \     disk area of error  00 = DOS area  01 = FAT
              1 /                         10 = root dir  11 = data area
              0       0       if read operation
                      1       if write operation
      AL      drive number if AH bit 7 = 1, otherwise undefined
              If it is as hard error on disk (AH bit 7=0), register AL
              contains the failing drive number (0=A:, 1=B:, etc.).
      BP:SI   address of a Device Header Control Block for which error
              occurred block device if high bit of BP:SI+4 = 1

low byte of DI: error code (note: high byte is undefined)

             error code      description
              00h             attempt to write on write-protected diskette
              01h             unknown unit
              02h             drive not ready
              03h             unknown command
              04h             data error (bad CRC)
              05h             bad request structure length
              06h             seek error
              07h             unknown media type
              08h             sector not found
              09h             printer out of paper
              0Ah             write fault
              0Bh             read fault
              0Ch             general failure
              0Fh             invalid disk change (DOS 3.x)

handler must return

The registers are set such that if an IRET is executed, DOS responds according to (AL) as follows: AL 00h ignore the error

      01h  retry the operation
      02h  terminate via int 22h
      03h  fail the system call that is in progress (DOS 3.2+)

note 1) Be careful when choosing to ignore a response because this causes DOS to

      beleive that an operation has completed successfully when it may not
      have.
   2) If the error was a character device, the contents of AL are invalid.

OTHER ERRORS

If AH bit 7=1, the error occurred on a character device, or was the result of a bad memory image of the FAT. The device header passed in BP:SI can be examined to determine which case exists. If the attribute byte high-order bit indicates a block device, then the error was a bad FAT. Otherwise, the error is on a character device. If a character device is involved, the contents of AL are unpredictable, the error code is in DI as above.

Notes: 1. Before giving this routine control for disk errors, DOS performs several

  retries. The number of retries varies according to the DOS version.

2. For disk errors, this exit is taken only for errors occurring during an

  int 21h function call. It is not used for errors during an int 25h or 26h.

3. This routine is entered in a disabled state. 4. All registers must be preserved. 5. This interrupt handler should refrain from using DOS function calls. If

  necessary, it may use calls 01h through 12h. Use of any other call destroys
  the DOS stack and leaves DOS in an unpredictable state.

6. The interrupt handler must not change the contents of the device header. 7. If the interrupt handler handles errors itself rather than returning to DOS,

  it should restore the application program's registers from the stack,
  remove all but the last three words on the stack, then issue an IRET. This
  will return to the program immediately after the int 21h that experienced
  the error. Note that if this is done DOS will be in an unstable state until
  a function call higher than 12h is issued, therefore not recommended.

8. For DOS 3.x, IGNORE requests (AL=0) are converted to FAIL for critical

  errors that occur on FAT or DIR sectors.

9. For DOS 3.10 up, IGNORE requests (AL=0) are converted to FAIL requests

  for network critical errors (50-79).

The device header pointed to by BP:SI is as follows:

DWORD Pointer to next device (0FFFFh if last device)

WORD Attributes:

Bit 15 1 if character device.

                      If bit 15 is 1:
                      Bit 0 = 1 if current standard input
                      Bit 1 = 1 if current standard output
                      Bit 2 = 1 if current NULL device
                      Bit 3 = 1 if current CLOCK device
              0       if block device

Bit 14 is the IOCTL bit WORD pointer to device driver strategy entry point WORD pointer to device driver interrupt entry point 8-BYTE character device named field for block devices. The first byte is the number of units. To tell if the error occurred on a block or character device, look at bit 15 in the attribute field (WORD at BP:SI+4). If the name of the character device is desired, look at the eight bytes starting at BP:SI+10.

HANDLING OF INVALID RESPONSES (DOS 3.x)

      A) If IGNORE (AL=0) is specified by the user and IGNORE is not allowed
         (bit 5=0), make the response FAIL (AL=3).
      B) If RETRY (AL=1) is specified by the user and RETRY is not allowed
         (bit 4=0), make the response FAIL (AL=3).
      C) If FAIL (AL=3) is specified by the user and FAIL is not allowed (bit
         3=0), make the response ABORT. (AL=2)

Interrupt 25h Absolute Disk Read Interrupt 26h Absolute Disk Write (0:0094h, 0:0098h)

These transfer control directly to the device driver. On return, the original

flags are still on the stack (put there by the INT instruction). This is necessary because return information is passed back in the current flags.

The number of sectors specified is transferred between the given drive and the

transfer address. Logical sector numbers are obtained by numbering each sector sequentially starting from track 0, head 0, sector 1 (logical sector 0) and continuing along the same head, then to the next head until the last sector on the last head of the track is counted. Thus, logical sector 1 is track 0, head 0, sector 2; logical sector 2 is track 0, head 0, sector 3; and so on. Numbering then continues wih sector 1 on head 0 of the next track. Note that although the sectors are sequentially numbered (for example, sectors 2 and 3 on track 0 in the example above), they may not be physically adjacent on disk, due to interleaving. Note that the mapping is different from that used by DOS 1.10 for dual-sided diskettes.

The request is as follows:

int 25 for Absolute Disk Read, int 26 for Absolute Disk Write entry AL drive number (0=A:, 1=B:, etc)

      CX      number of sectors to read
      DS:BX   disk transfer address (buffer)
      DX      first relative sector to read - beginning logical sector number

return CF set if error

      AL      error code issued to int 24h in low half of DI
      AH      01h     bad command
              02h     bad address mark
              03h     write-protected disk
              04h     requested sector not found
              08h     DMA failure
              10h     data error (bad CRC)
              20h     controller failed
              40h     seek operation failed
              80h     attachment failed to respond

note 1) Original flags on stack! Be sure to pop the stack to prevent

      uncontrolled growth
   2) Ints 25 and 26 will try rereading a disk if they get an error the first
      time.
   3) All registers except the segment registers are destroyed by these calls

Interrupt 27h Terminate And Stay Resident (0:009Ch) (obsolete) This vector is used by programs that are to remain resident when COMMAND.COM regains control. After initializing itself, the program must set DX to its last address plus one relative to the program's initial DS or ES value (the offset at which other programs can be loaded), then execute interrupt 27h. DOS then considers the program as an extension of itself, so the program is not overlaid when other programs are executed. This is useful for loading programs such as utilities and interrupt handlers that must remain resident.

entry CS current program segment

      DX      last program byte + 1

return none note 1) This interrupt must not be used by .EXE programs that are loaded into

      the high end of memory.
   2) This interrupt restores the interrupt 22h, 23h, and 24h vectors in the
      same manner as interrupt 20h.  Therefore, it cannot be used to install
      permanently resident Ctrl-Break or critical error handler routines.
   3) The maximum size of memory that can be made resident by this method is
      64K.
   4) Memory can be more efficiently used if the block containing a copy of
      the environment is deallocated before terminating. This can be done by
      loading ES with the segment contained in 2Ch of the PSP, and issuing
      function call 49h (Free Allocated Memory).
   5) DOS function call 4Ch allows a program to pass a completion code to DOS,
      which can be interpreted with processing (see function call 31h).
   6) Terminate and stay resident programs do not close files.
   7) Int 21, function 31h is the preferred method to cause a program to
      remain resident because this allows return information to be passed and
      allows a program larger than 64K to remain resident.

Interrupt 28h (not documented by Microsoft)

  • DOS Idle Interrupt

This interrupt is continuously called by DOS itself whenever it is in a wait state (i.e., when it is waiting for keyboard input) during a function call of 01h through 0Ch. DOS uses 3 separate internal stacks: one for calls 01h through 0Ch; another for calls 0Dh and above; and a third for calls 01h through 0Ch when a Critical Error is in progress. When int 28h is called, any calls above 0Ch can be executed without destroying the internal stack used by DOS at the time. It is used primarily by the PRINT.COM routines, but any number of other routines can be chained to it by saving the original vector and calling it with a FAR call (or just JMPing to it) at the end of the new routine. Int 28h is being issued it is usually safe to do DOS calls. You won't get int 28hs if a program is running that doesn't do its keyboard input through DOS. You should rely on the timer interrupt for these. Int 28h is not called at all when any non-trivial foreground task is running. As soon as a foreground program has a file open, INT28 no longer gets called. Could make a good driver for for abackground program that really works as long as there is nothing else going on in the machine.

entry no parameters availible return none note 1) The int 28h handler may invoke any int 21h function except functions

      00h through 0Ch (and 50h/51h under DOS 2.x).
   2) Apparently int 28h is also called during screen writes
   3) Until some program installs its own routine, this interrupt vector
      simply points to an IRET opcode.
   4) Supported in OS/2 1.0's DOS Compatibility Box

Interrupt 29h (not documented by Microsoft)

  • Internal - Quick Screen Output

This method is extremely fast (much faster than DOS 21h subfunctions 2 and 9, for example), and it is portable, even to "non-compatible" MS-DOS computers.

entry AL character to output to screen return unknown note 1) Documented by Digital Research's DOS Reference as provided with the

      DEC Rainbow
   2) If ANSI.SYS is installed, character output is filtered through it.
   3) Works on the IBM PC and compatibles, Wang PC, HP-150 and Vectra, DEC
      Rainbow, NEC APC, Texas Instruments PC and others
   4) This interrupt is called from the DOS's output routines if output is
      going to a device rather than a file, and the device driver's attribute
      word has bit 3 (04h) set to "1".
   5) This call has been tested with MSDOS 2.11, PCDOS 2.1, PCDOS 3.1, PCDOS
      3.2, and PCDOS 3.3.
   6) Used in IBMBIO.COM as a vector to int 10, function 0Eh (write TTY)
      followed by an IRET.

Interrupt 2Ah Microsoft Networks - Session Layer Interrupt

  • (not documented by Microsoft)

entry AH 00h check to see if network BIOS installed

                      return: AH      <> 0 if installed
              01h     execute NETBIOS request
              02h     set net printer mode
              03h     get shared-device status (check direct I/O)
                      AL      00h
                      DS:SI   pointer to ASCIIZ disk device name
                      return  CF      0 if allowed
              04h     execute NETBIOS
                      AL      0 for error retry
                              1 for no retry
                      ES:BX   pointer to ncb
                      return  AX      0 for no error
                              AH      1 if error
                              AL      error code
              05h     get network resource information
                      AL      00h
                      return  AX      reserved
                              BX      number of network names
                              CX      number of commands
                              DX      number of sessions
              82h     unknown
                      return  ??

note called by the int 21h function dispatcher in DOS 3.10

Interrupt 2Bh (not documented by Microsoft)

  • Unknown - Internal Routine for DOS (IRET)

Interrupt 2Ch (not documented by Microsoft)

  • Unknown - Internal Routine for DOS (IRET)

Interrupt 2Dh (not documented by Microsoft)

  • Unknown - Internal Routine for DOS (IRET)

Interrupt 2Eh (undocumented by Microsoft)

  • Internal Routine for DOS (Alternate EXEC)
This interrupt passes a command line addressed by DS:SI to COMMAND.COM. The

command line must be formatted just like the unformatted parameter area of a Program Segment Prefix. That is, the first byte must be a count of characters, and the second and subsequent bytes must be a command line with parameters, terminated by a carriage return character.

When executed, int 2Eh will reload the transient part of the command

interpreter if it is not currently in memory. If called from a program that was called from a batch file, it will abort the batch file. If executed from a program which has been spawned by the EXEC function, it will abort the whole chain and probably lock up the computer. Int 2Eh also destroys all registers including the stack pointer.

Int 2Eh is called from the transient portion of the program to reset the DOS

PSP pointers using the above Functions #81 & #80, and then reenters the resident program.

When called with a valid command line, the command will be carried out by

COMMAND.COM just as though you had typed it in at the DOS prompt. Note that the count does not include the carriage return. This is an elegant way to perform a SET from an application program against the master environment block for example.

entry DS:SI pointer to an ASCIIZ command line in the form:

                      count byte
                      ASCII string
                      carriage return
                      null byte

note 1) Destroys all registers including stack pointer

   2) Seems to work OK in both DOS 2.x and 3.x
   3) It is reportedly not used by DOS.
   4) As far as known, int 2Eh is not used by DOS 3.1, although it was called
      by COMMAND.COM of PCDOS 3.0, so it appears to be in 3.1 only for the
      sake of compatibility.

Interrupt 2Fh Multiplex Interrupt

Interrupt 2Fh is the multiplex interrupt. A general interface is defined between two processes. It is up to the specific application using interrupt 2Fh to define specific functions and parameters. Every multiplex interrupt handler is assigned a specific multiplex number. The multiplex number is specified in the AH register; the AH value tells which program your request is directed toward. The specific function that the handler is to perform is placed in the AL register. Other parameters are places in the other registers as needed. The handlers are chained into the 2Fh interrupt vector and the multiplex number is checked to see if any other application is using the same multiplex number. There is no predefined method for assigning a multiplex number to a handler. You must just pick one. To avoid a conflict if two applications choose the same multiplex number, the multiplex numbers used by an application should be patchable. In order to check for a previous installation of the current application, you can search memory for a unique string included in your program. If the value you wanted in AH is taken but you don't find the string, then another application has grabbed that location. Int 2Fh was not documented under DOS 2.x. There is no reason not to use int 2Fh as the multiplex interrupt in DOS 2.x. The only problem is that DOS does not initialize the int 2Fh vector, so when you try to chain to it like you are supposed to, it will crash. But if your program checks the vector for being zero and doesn't chain in that case, it will work for you in 2.x just the same as 3.x. Int 2Fh doesn't require any support from DOS itself for it to be used in application programs. It's not handled by DOS, but by the programs themselves. The only support DOS has to provide is to initialize the vector to an IRET. DOS 3.2 does itself contain some int 2Fh handlers - it uses values of 08h, 13h, and 0F8h. There may be more.

entry AH 01h PRINT.COM

              AL      00h     PRINT  Get Installed State
                      This call must be defined by all int 2Fh handlers. It
                      is used by the caller of the handler to determine if
                      the handler is present. On entry, AL=0. On return, AL
                      contains the installed state as follows:
              return  AL      0FFh    installed
                              01h     not installed, not OK to install
                              00h     not installed, OK to install
                      01h     PRINT  Submit File
                      DS:DX   pointer to submit packet
                              format  BYTE    level
                                      DWORD   pointer to ASCIIZ filename
              return  CF      set if error
                              AX      error code
              note 1) A submit packet contains the level (BYTE) and a pointer
                      to the ASCIIZ string (DWORD in offset:segment form).
                      The ASCIIZ string must contain the drive, path, and
                      filename of the file you want to print. The filename
                      cannot contain global filename characters.
              return  CF      set if error
                              AX      error code
                      02h     PRINT Cancel File
                      On entry, AL=2 and DS:DX points to the ASCIIZ string for
                      the print file you want to cancel. Global filename
                      characters are allowed in the filename.
              DS:DX   pointer to ASCIIZ file name to cancel (wildcards OK)
              return  CF      set if error
                              AX      error code
                      03h     PRINT remove all files
              return  CF      set if error
                              AX      error code
                      04h     PRINT hold queue/get status
                      This call holds the jobs in the print queue so that you
                      can scan the queue. Issuing any other code releases the
                      jobs. On entry, AL=4. On return, DX contains the error
                      count. DS:SI points to the print queue. The print queue
                      consists of a series of filename entries. Each entry is
                      64 bytes long. The first entry in the queue is the file
                      currently being printed. The end of the queue is marked
                      by the entry having a null as the first character.
             return   DX      error count
                      DS:SI   pointer to print queue (null-string terminated
                              list of 64-byte ASCIIZ filenames)
                      CF      set if error
                             AX       error code
                                      01h     function invalid
                                      02h     file not found
                                      03h     path not found
                                      04h     too many open files
                                      05h     access denied
                                      08h     queue full
                                      09h     spooler busy
                                      0Ch     name too long
                                      0Fh     drive invalid
                      05h     PRINT restart queue
              return  CF      set if error
                              AX      error code
      AH      05h     DOS 3.x critical error handler
              AL      00h     installation check
                      return  AL      00h not installed, OK to install
                                      01h not installed, can't install
                                      0FFh installed
                      note    This set of functions allows a user program to
                              partially or completely override the default
                              critical error handler in COMMAND.COM
              AL      01h     handle error - nonzero error code in AL
                      return  CF      clear
                                      ES:DI   pointer to ASCIIZ error message
                              CF      set     use default error handler
                              AL      (?)
      AH      06h     ASSIGN
                      00h     installation check
                      return  AH <> 0 if installed
                      01h     get memory segment
                      return  ES      segment of ASSIGN work area
      AH      10h     SHARE
                      00h     installation check
                      return  AL      00h    not installed, OK to install
                                      01h    not installed, not OK to install
                                      0FFh   installed
      AH      11h     multiplex - network redirection
                      00h     installation check
                      return  AL      00h    not installed, OK to install
                                      01h    not installed, not OK to install
                                      0FFh   installed
                      01h     unknown
                      02h     unknown
                      03h     unknown
                      04h     unknown
                      05h     unknown
                      06h     close remote file
                      07h     unknown
                      08h     unknown
                      09h     unknown
                      0Ah     unknown
                              STACK: WORD (?)
                              return  CF      set on error
                      0Bh     unknown
                              STACK: WORD (?)
                              return  CF      set on error(?)
                      0Ch     unknown
                      0Dh     unknown
                      0Eh     unknown
                              STACK: WORD (?)
                              return  (?)
                      0Fh     unknown
                      11h     unknown
                      13h     unknown
                      16h     unknown
                      17h     unknown
                              STACK: WORD (?)
                              return  (?)
                      18h     unknown
                              STACK: WORD (?)
                              return  (?)
                      19h     unknown
                      1Bh     unknown
                      1Ch     unknown
                      1Dh     unknown
                      1Eh     do redirection
                              STACK: WORD function to execute
                              return  CF      set on error
                      1Fh     printer setup
                              STACK: WORD function(?)
                              return  CF      set on error(?)
                      20h     unknown
                      21h     unknown
                      22h     unknown
                      23h     unknown
                      24h     unknown
                      25h     unknown
                              STACK: WORD (?)
                      26h     unknown
      AH      12h     multiplex, DOS 3.x internal services
                      00h     installation check
                      return  AL      0FFh    for compatibility with other
                                              int 2Fh functions
                      01h     close file (?)
                              stack   word value - unknown
                              return  BX      unknown
                                      CX      unknown
                                      ES:DI   pointer to unknown value
                              note    Can be called only from within DOS
                      02h     get interrupt address
                              stack: word vector number
                              return  ES:BX pointer to interrupt vector
                                      Stack unchanged
                      03h     get DOS data segment
                              return  DS      segment of IBMDOS
                      04h     normalize path separator
                              stack: word character to normalize
                              return  AL      normalized character (forward
                                              slash turned to backslash)
                                      Stack unchanged
                      05h     output character
                              stack: word character to output
                              return  Stack unchanged
                              note    Can be called only from within DOS
                      06h     invoke critical error
                              return  AL      0-3 for Abort, Retry, Ignore,
                                              Fail
                              note    Can be called only from within DOS
                      07h     move disk buffer (?)
                              DS:DI   pointer to disk buffer
                              return  buffer moved to end of buffer list
                              note    Can be called only from within DOS
                      08h     decrement word
                              ES:DI   pointer to word to decrement
                              return  AX      new value of word
                              note    Word pointed to by ES:DI decremented,
                                      skipping zero
                      09h     unknown
                              DS:DI   pointer to disk buffer(?)
                              return  (?)
                              note    Can be called only from within DOS
                      0Ah     unknown
                              note    Can be called only from within DOS
                      0Bh     unknown
                              ES:DI   pointer to system file table entry(?)
                              return  AX      (?)
                              note    Can be called only from within DOS
                      0Ch     unknown
                              note    Can be called only from within DOS
                      0Dh     get date and time
                              return  AX      current date in packed format
                                      DX      current time in packed format
                              note    Can be called only from within DOS
                      0Eh     do something to all disk buffers (?)
                              return  DS:DI   pointer to first disk buffer
                              note    can be called only from within DOS
                      0Fh     unknown
                              DS:DI   pointer to (?)
                              return  DS:DI pointer to (?)
                              note 1) Can be called only from within DOS
                                   2) Calls on function 1207h
                      10h     find dirty/clean(?) buffer
                              DS:DI   pointer to first disk buffer
                              return  DS:DI   pointer to first disk buffer
                                              which has (?) flag clear
                                      ZF      clear if found
                                              set if not found
                      11h     normalize ASCIIZ filename
                              DS:SI   pointer to ASCIZ filename to normalize
                              ES:DI   ptr to buffer for normalized filename
                              return  destination buffer filled with upper-
                                      case filename, with slashes turned to
                                      backslashes
                      12h     get length of ASCIIZ string
                              ES:DI   pointer to ASCIZ string
                              return  CX      length of string
                      13h     uppercase character
                              stack: word character to convert to uppercase
                              return  AL      uppercase character
                                      Stack unchanged
                      14h     compare far pointers
                              DS:SI   first pointer
                              ES:DI   second pointer
                              return  ZF      set if pointers are equal
                                      ZF      clear if not equal
                      15h     unknown
                              DS:DI   pointer to disk buffer
                              stack: word (?)
                              return  Stack unchanged
                              note    Can be called only from within DOS
                      16h     get address of system FCB
                              BX      system file table entry number
                              return  ES:DI pointer to system file table entry
                      17h     set default drive (?)
                              stack: word drive (0 = A:, 1 = B:, etc)
                              return  DS:SI   pointer to drive data block for
                                              specified drive
                                      Stack unchanged
                              note    Can be called only from within DOS
                      18h     get something (?)
                              return  DS:SI pointer to (?)
                      19h     unknown
                              stack: word drive (0 = default, 1 = A:, etc)
                              return  (?)
                                      Stack unchanged
                              note 1) Can be called only from within DOS
                                   2) Calls function 1217h
                      1Ah     get file's drive
                              DS:SI   pointer to filename
                              return  AL      drive
                                      (0=default, 1=A:, etc, 0FFh=invalid)
                      1Bh     set something (?)
                              CL      unknown
                              return  AL      (?)
                              note    Can be called only from within DOS
                      1Ch     checksum memory
                              DS:SI   pointer to start of memory to checksum
                              CX      number of bytes
                              DX      initial checksum
                              return  DX      checksum
                              note    Can be called only from within DOS
                      1Dh     unknown
                              DS:SI   pointer to (?)
                              CX      (?)
                              DX      (?)
                              return  AX      (?)
                                      CX      (?)
                                      DX    = (?)
                      1Eh     compare filenames
                              DS:SI   pointer to first ASCIIZ filename
                              ES:DI   pointer to second ASCIIZ filename
                              return  ZF      set     if filenames equivalent
                                              clear   if not
                      1Fh     build drive info block
                              stack: word drive letter
                              return  ES:DI pointer to drive info block
                                            (will be overwritten by next call)
                                      Stack unchanged
                              note    Can be called only from within DOS
                      20h     get system file table number
                              BX      file handle
                              return  CF set on error
                                      AL      6 (invalid file handle)
                                      CF      clear if successful
                                      byte ES:[DI] = system file table entry
                                          number for file handle
                      21h     unknown
                              DS:SI   pointer to (?)
                              return  (?)
                              note    Can be called only from within DOS
                      22h     unknown
                              SS:SI   pointer to (?)
                              return  nothing(?)
                              note    Can be called only from within DOS
                      23h     check if character device (?)
                              return  DS:SI   pointer to device driver with
                                              same name as (?)
                              note    Can be called only from within DOS
                      24h     delay
                              return  after delay of (?) ms
                              note    Can be called only from within DOS
                      25h     get length of ASCIIZ string
                              DS:SI   pointer to ASCIIZ string
                              return  CX      length of string
      AH      43h     Microsoft Extended Memory Specification (XMS)
      AH      64h     SCRNSAV2.COM
              AL      00h     installation check
                      return  AL      00h     not installed
                                      0FFh    installed
                      note    SCRNSAV2.COM is a screen saver for PS/2's with
                              VGA by Alan Ballard
      AH      7Ah     Novell NetWare
              AL      00h     installation check
                      note    Returns address of entry point for IPX and SPX
      AH      0AAh    VIDCLOCK.COM
              AL      00h     installation check
                      return  AL      00h     not installed
                                      0FFh    installed
                      note    VIDCLOCK.COM is a memory-resident clock by
                              Thomas G. Hanlin III
      AH      0B7h    APPEND
              AL      00h     APPEND installation check
                              return  AH <> 0 if installed
                      01h     APPEND - unknown
                      02h     APPEND - version check
      AH      0B8h    Microsoft Networks
              AL      00h     network program installation check
                      return  AH <> 0 if installed
                              BX      installed component flags (test in this
                                      order!)
                              bit 6   server
                              bit 2   messenger
                              bit 7   receiver
                              bit 3   redirector
                      01h     unknown
                      02h     unknown
                      03h     get current POST address
                      return  ES:BX   POST address
                      04h     set new POST address
                      ES:BX   new POST address
                      09h     version check
      AH      0BBh    Network functions
              AL      00h     net command installation check
                      03h     get server POST address
                      04h     get server POST address
      AH      0F7h    AUTOPARK.COM  (PD TSR hard disk parking utility)
              AL      00h     installation check
                      return  AL      00h     not installed
                                      0FFh    installed
                      note    AUTOPARK is a TSR HD parker by Alan D. Jones
                      01h     set parking delay
                      BX:CX   32 bit count of 55ms timer ticks

return AX Error

              Codes       Description
              01h     invalid function number
              02h     file not found
              03h     path not found
              04h     too many open files
              05h     access denied
              06h     invalid handle
              08h     queue full
              09h     busy
              0Ch     name too long
              0Fh     invalid drive was specified
      CF      clear (0) if OK
              set (1) if error - error returned in AX

note 1) The multiplex numbers AH=0h through AH=7Fh are reserved for DOS.

      Applications should use multiplex numbers 80h through 0FFh.
   2) When in the chain for int 2Fh, if your code calls DOS or if you execute
      with interrupts enabled, your code must be reentrant/recursive.

Interrupt 30h (not a vector!) far jump instruction for CP/M-style calls

Interrupt 31h Unknown note The CALL 5 entry point does a FAR jump to here

Interrupt 32h Unknown

Interrupt 33h Used by Microsoft Mouse Driver

              Function Calls
      00h     Reset Driver and Read Status
      entry   AH      00h
      return  AH      status
                      0  hardware/driver not installed
                      -1 hardware/driver installed
              BX      number of buttons
                      -1      two buttons
                      0       other than two
                      3       Mouse Systems mouse
      01h     Show Mouse Cursor
      entry   AH      01h
      return  unknown
      02h     Hide Mouse Cursor
      entry   AH      02h
      return  unknown
      note    multiple calls to hide the cursor will require multiple calls
              to function 01h to unhide it.
      03h     Return Position and Button Status
      entry   AH      03h
      return  BX      button status
                      bit 0   left button pressed if 1
                      bit 1   right button pressed if 1
                      bit 2   middle button pressed if 1 (Mouse Systems mouse)
              CX      column
              DX      row
      04h     Position Mouse Cursor
      entry   AH      04h
              CX      column
              DX      row
              return  unknown
      05h     Return Button Press Data
      entry   AH      05h
              BX      button
                      0 left
                      1 right
                      2 middle (Mouse Systems mouse)
      return  AH      button states
                      bit 0   left button pressed if 1
                      bit 1   right button pressed if 1
                      bit 2   middle button pressed if 1 (Mouse Systems mouse)
              BX      no. of times specified button pressed since last call
              CX      column at time specified button was last pressed
              DX      row at time specified button was last pressed
      06h     Return Button Release Data
      entry   AH      06h
              BX      button
                      0       left
                      1       right
                      2       middle (Mouse Systems mouse)
      return  AH      button states
                      bit 0   left button pressed if 1
                      bit 1   right button pressed if 1
                      bit 2   middle button pressed if 1 (Mouse Systems mouse)
              BX      no. of times specified button released since last call
              CX      column at time specified button was last released
              DX      row at time specified button was last released
      07h     Define Horizontal Curos Range
      entry   AH      0007h
              CX      minimum column
              DX      maximum column
      return  unknown
      08h     Define Vertical Cursor Range
      entry   AH      08h
              CX      minimum row
              DX      maximum row
              return  unknown
      09h     Define Graphics Cursor
      entry   AH      09h
              BX      column of cursor hot spot in bitmap (-16 to 16)
              CX      row of cursor hot spot  (-16 to 16)
              ES:DX   pointer to bitmap
                      16 words screen mask
                      16 words cursor mask
              return  unknown
              note    Each word defines the sixteen pixels of a row, low bit
                      rightmost
      0Ah     Define Text Cursor
      entry   AH      0Ah
              BX      hardware/software text cursor
                      00h     software
                              CX      screen mask
                              DX      cursor mask
                      01h     hardware
                              CX      start scan line
                              DX      end scan line
      return  unknown
      note    When the software cursor is selected, the char/attribute data
              at the current screen position is ANDed with the screen mask
              and the with the cursor mask
      0BH     Read Motion Counters
      entry   AH      0Bh
      return  CX      number of mickeys mouse moved horiz. since last call
              DX      number of mickeys mouse moved vertically
      note 1) A mickey is the smallest increment the mouse can sense.
              Positive values indicate up/right
      0Ch     Define Interrupt Subroutine Parameters
      entry   AH      0Ch
              CX      call mask bit
                      bit 0   call if mouse moves
                      bit 1   call if left button pressed
                      bit 2   call if left button released
                      bit 3   call if right button pressed
                      bit 4   call if right button released
                      bit 5   call if middle button pressed (Mouse Systems)
                      bit 6   call if middle button released (Mouse Systems)
              ES:DX  address of FAR routine
      return  unknown
      note    when the subroutine is called, it is passed these values:
              AH      condition mask (same bit assignments as call mask)
              BX      button state
              CX      cursor column
              DX      cursor row
              DI      horizontal mickey count
              SI      vertical mickey count
      0Dh     Light Pen Emulation On
      entry   AH      0Dh
      return  unknown
      0Eh     Light Pen Emulation Off
      entry   AH      0Eh
      return  unknown
      0Fh     Define Mickey/Pixel Ratio
      entry   AH      0Fh
              CX      number of mickeys per 8 pixels horizontally
              DX      number of mickeys per 8 pixels vertically
      return  unknown
      10h     Define Screen Region for Updating
      entry   AH      10h
              CX,DX   X,Y coordinates of upper left corner
              SI,DI   X,Y coordinates of lower right corner
      return  unknown
      note    Mouse cursor is hidden during updating, and needs to be
              explicitly turned on again
      11h     not documented by Microsoft
      12h     Set Large Graphics Cursor Block
              AH      12h
              BH      cursor width in words
              CH      rows in cursor
              BL      horizontal hot spot (-16 to 16)
              CL      vertical hot spot (-16 to 16)
              ES:DX   pointer to bit map of screen and cursor maps
      return  AH      -1 if successful
      note    PC Mouse. Not dodcumented by Microsoft
      13h     Define Double-Speed Threshold
      entry   AH      13h
              DX      threshold speed in mickeys/second,
                      0 = default of 64/second
      return  unknown
      note    If speed exceeds threshold, the cursor's on-screen motion
              is doubled
      14h     Exchange Interrupt Subroutines
      entry   AH      14h
      return  unknown
      15h     Return Drive Storage Requirements
      entry   AH      15h
      return  BX      size of buffer needed to store driver state
      16h     Save Driver State
      entry   AH      16h
              ES:DX   pointer to buffer
      return  unknown
      17h     Restore Driver State
      entry   AH      17h
              ES:DX   pointer to buffer containing saved state
      return  unknown
      18h     not documented by Microsoft
      19h     not documented by Microsoft
      1Ah     not documented by Microsoft
      1Bh     not documented by Microsoft
      1Ch     not documented by Microsoft
      1Dh     Define Display Page Number
      entry   AH      1Dh
      1Eh     Return Display Page Number
      entry   AH      1Eh
      return  unknown
      42h     PCMouse - Get MSmouse Storage Requirements
              AH      42h
              return  AX      0FFFFh successful
              BX      buffer size in bytes for functions 50h and 52h
                      00h     MSmouse not installed
                      42h     functions 42h, 50h, and 52h not supported
      52h     PCMouse - Save MSmouse State
      entry   AH      50h
              BX      buffer size
              ES:DX   pointer to buffer
              return  AX      0FFFFh if successful
      52h     PCMouse - restore MSmouse state
      entry   AH      52h
              BX      buffer size
              ES:DX   pointer to buffer
              return  AX      0FFFFh if successful

Int 33: In addition, the following functions are appended to BIOS int 10h and

      implemented as the EGA Register Interface Library:
      0F0h    read one register
      0F1h    write one register
      0F2h    read consecutive register range
      0F3h    write consecutive register range
      0F4h    read non-consecutive register set
      0F5h    write non-consecutive register set
      0F6h    revert to default register values
      0F7h    define default register values
      0FAh    get driver status

Interrupt 34h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0D8h

Interrupt 35h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0D9h

Interrupt 36h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DAh

Interrupt 37h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DBh

Interrupt 38h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DCh

Interrupt 39h Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DDh

Interrupt 3Ah Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DEh

Interrupt 3Bh Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates opcode 0DFh

Interrupt 3Ch Turbo C/Microsoft languages - Floating Point emulation

              This int emulates instructions with an ES segment override

Interrupt 3Dh Turbo C/Microsoft languages - Floating Point emulation

              This interrupt emulates a standalone FWAIT instruction

Interrupt 3Eh Turbo C/Microsoft languages - Floating Point emulation

Interrupt 3Fh Overlay manager interrupt (Microsoft LINK.EXE)

Interrupt 40h Hard Disk BIOS

              Pointer to disk BIOS entry when a hard disk controller is
              installed. The BIOS routines use int 30h to revector the
              diskette handler (original int 13h) here so int 40 may be used
              for hard disk control

Interrupt 41h Hard Disk Parameters (XT,AT,XT2,XT286,PS except ESDI disks)

              Pointer to first Hard Disk Parameter Block, normally located
              in the controller card's ROM. This table may be copied to RAM
              and changed, and this pointer revectored to the new table.

note 1) format of parameter table is:

      dw      cylinders
      db      heads
      dw      starting reduced write current cylinder (XT only, 0 for others)
      db      maximum ECC burst length
      db      control byte
                bits 0-2 drive option (XT only, 0 for others)
                bit 3    set if more than 8 heads
                bit 4    always 0
                bit 5    set if manufacturer's defect map on max cylinder+1
                bit 6    disable ECC retries
                bit 7    disable access retries
      db      standard timeout (XT only, 0 for others)
      db      formatting timeout (XT only, 0 for others)
      db      timeout for checking drive (XT only, 0 for others)
      dw      landing zone    (AT, PS/2)
      db      sectors/track   (AT, PS/2)
      db      0
   2) normally vectored to ROM table when system is initialized.

Interrupt 42h Pointer to screen BIOS entry (EGA, VGA, PS/2)

              Relocated (by EGA, etc.) video handler (original int 10h).
              Revectors int 10 calls to EGA BIOS.

Interrupt 43h Pointer to EGA initialization parameter table. The POST

              initializes this vector pointing to the default table located
              in the EGA ROM BIOS. (PC-2 and up). Not initialized if EGA not
              present.

Interrupt 44h Pointer to EGA graphics character table (also PCjr). This (0:0110h) table contains the dot patterns for the first 128 characters

              in video modes 4,5, and 6, and all 256 characters in all
              additional graphics modes. Not initialized if EGA not present.
           2) EGA/VGA/CONV/PS - EGA/PCjr fonts, characters 00h to 7Fh
           3) Novell NetWare - High-Level Language API

Interrupt 45h Reserved by IBM (not initialized)

Interrupt 46h Pointer to second hard disk, parameter block (AT, XT/286, PS/2)

              (see int 41h) (except ESDI hard disks) (not initialized unless
              specific user software calls for it)

Interrupt 47h Reserved by IBM (not initialized)

Interrupt 48h Cordless Keyboard Translation (PCjr, XT [never delivered]) (0:0120h) This vector points to code to translate the cordless keyboard

              scancodes into normal 83-key values. The translated scancodes
              are then passed to int 9. (not initialized on PC or AT)

Interrupt 49h Non-keyboard Scan Code Translation Table Address (PCjr) (0:0124h) This interrupt has the address of a table used to translate

              non-keyboard scancodes (greater than 85 excepting 255). This
              interrupt can be revectored by a user application. IBM
              recommends that the default table be stored at the beginning
              of an application that required revectoring this interrupt,
              and that the default table be restored when the application
              terminates. (not initialized on PC or AT)

Interrupt 4Ah Real-Time Clock Alarm (Convertible, PS/2)

              (not initialized on PC or AT)
              Invoked by BIOS when real-time clock alarm occurs

Interrupt 4Bh Reserved by IBM (not initialized)

Interrupt 4Ch Reserved by IBM (not initialized)

Interrupt 4Dh Reserved by IBM (not initialized)

Interrupt 4Eh Reserved by IBM (not initialized)

Interrupt 4Fh Reserved by IBM (not initialized)

Interrupt 50-57 IRQ0-IRQ7 relocated by DesQview

              (normally not initialized)

Interrupt 58h Reserved by IBM (not initialized)

Interrupt 59h Reserved by IBM (not initialized)

              GSS Computer Graphics Interface (GSS*CGI)
              DS:DX   Pointer to block of 5 array pointers
              return  CF      0
                      AX      return code
                      CF      1
                      AX      error code
              note 1) Int 59 is the means by which GSS*CGI language bindings
                      communicate with GSS*CGI device drivers and the GSS*CGI
                      device driver controller.
                   2) Also used by the IBM Graphic Development Toolkit

Interrupt 5Ah Reserved by IBM (not initialized)

Interrupt 5Bh Reserved by IBM (not initialized)

Interrupt 5Ah Cluster Adapter BIOS entry address

              (normally not initialized)

Interrupt 5Bh Reserved by IBM (not initialized) (cluster adapter?)

Interrupt 5Ch NETBIOS interface entry port

              ES:BX   pointer to network control block

note 1) When the NETBIOS is installed, interrupts 13 and 17 are interrupted by

      the NETBIOS; interrupt 18 is moved to int 86 and one of int 2 or 3 is
      used by NETBIOS. Also, NETBIOS extends the int 15 function 90 and 91h
      functions (scheduler functions)
   2) Normally not initialized.
   3) TOPS network card uses DMA 1, 3 or none.

Interrupt 5Dh Reserved by IBM (not initialized)

Interrupt 5Eh Reserved by IBM (not initialized)

Interrupt 5Fh Reserved by IBM (not initialized)

Interrupt 60h-67h User Program Interrupts (availible for general use)

Interrupt 67h Used by Lotus-Intel-Microsoft Expanded Memory Specification

      user    and Ashton-Tate/Quadram/AST Enhanced Expanded Memory
              specification (See Chapter 10)

Interrupt 68h Not Used (not initialized)

Interrupt 69h Not Used (not initialized)

Interrupt 6Ah Not Used (not initialized)

Interrupt 6Bh Not Used (not initialized)

Interrupt 6Ch System Resume Vector (Convertible) (not initialized on PC)

Interrupt 6Dh Not Used (not initialized)

Interrupt 6Fh Not Used (not initialized)

Interrupt 70h IRQ 8, Real Time Clock Interrupt (AT, XT/286, PS/2)

Interrupt 71h IRQ 9, Redirected to IRQ 8 (AT, XT/286, PS/2)

              LAN Adapter 1 (rerouted to int 0Ah by BIOS)

Interrupt 72h IRQ 10 (AT, XT/286, PS/2) Reserved

Interrupt 73h IRQ 11 (AT, XT/286, PS/2) Reserved

Interrupt 74h IRQ 12 Mouse Interrupt (AT, XT/286, PS/2)

Interrupt 75h IRQ 13, Coprocessor Error, BIOS Redirect to int 2 (NMI) (AT)

Interrupt 76h IRQ 14, Hard Disk Controller (AT, XT/286, PS/2)

Interrupt 77h IRQ 15 (AT, XT/286, PS/2) Reserved

Interrupt 78h Not Used

Interrupt 79h Not Used

Interrupt 7Ah Novell NetWare - LOW-LEVEL API

Interrupt 7Bh-7Fh Not Used

Interrupt 80h-85h Reserved by BASIC

note interrupts 80h through ECh are apparently unused and not initialized.

Interrupt 86h Relocated by NETBIOS int 18

Interrupt 86h-0F0h Used by BASIC when BASIC interpreter is running

Intrerrupt 0E0h CP/M-86 function calls

Interrupt 0E4h Logitech Modula-2 v2.0 MONITOR entry AX 05h monitor entry

              06h     monitor exit
      BX      priority

Interrupts 0F1h-0FFh (absolute addresses 3C4-3FF)

                    Location of Interprocess Communications Area

Interrupt 0F8h Set Shell Interrupt (OEM)

              Set OEM handler for int 21h calls from 0F9h through 0FFh

entry AH 0F8h

      DS:DX   pointer to handler for Functions 0F9h thru 0FFh

note 1) To reset these calls, pass DS and DX with 0FFFFh. DOS is set up to

      allow ONE handler for all 7 of these calls. Any call to these handlers
      will result in the carry bit being set and AX will contain 1 if they are
      not initialized. The handling routine is passed all registers just as
      the user set them. The OEM handler routine should be exited through an
      IRET.
   2) 10 ms interval timer (Tandy?)

Interrupt 0F9h First of 8 SHELL service codes, reserved for OEM shell (WINDOW);

              use like HP Vectra user interface?

Interrupt 0FAh USART ready (RS-232C)

Interrupt 0FBh USART RS ready (keyboard)

Interrupt 0FCh Unknown

Interrupt 0FDh reserved for user interrupt

Interrupt 0FEh AT/XT286/PS50+ - destroyed by return from protected mode

Interrupt 0FFh AT/XT286/PS50+ - destroyed by return from protected mode

CHAPTER 6

Programming Technical Reference - IBM Copyright 1988, Dave Williams

                   DOS CONTROL BLOCKS AND WORK AREAS

When DOS loads a program, it first sets aside a section of memory for the program called the program segment, or code segment. Then it constructs a control block called the program segment prefix, or PSP, in the first 256 (100h) bytes. Usually, the program is loaded directly after the PSP at 100h. The PSP contains various information used by DOS to help run the program. The PSP is always located at offset 0 within the code segment. When a program recieves control certain registers are set to point to the PSP. For a COM file, all registers are set to point to the beginning of the PSP and the program begins at 100h. For the more complex EXE file structures, only DS and ES registers are set to point to the PSP. The linker passes the settings for the DS, IP, SS, and SP registers and may set the starting location in CS:IP to a location other than 100h.

IBMBIO provides an IRET instruction at absolute address 847h for use as a dummy routine for interrupts that are not used by DOS. This lets the interrupts do nothing until their vectors are rerouted to ttheir appropriate handlers.

A storage block is used by DOS to record the amount and location of allocated memory withion the machine's address spacd. A storage block, a Program Segment Prefix, and an environment area are built by FDOS for each program currently resident in the address space. The storage block is used by DOS to record the address range of memory allocated to a program. IOt us used by DOs to find th enext availible area to load a program and to determine if there is a\enough memory to run that porogram. When a memory area is in use, it is said to be allocated. Then the program ends, or releases memory, it is said to bne deallocated. A storage block contains a pointer ro rhe Program Segment Prefix assoiciated with each program. This control block is constructed by IBMDOS for the purpose opf providing stanfdardized areas for DOS/program communication., Within ghr PSP are arsas which are used to save interrupt vectors, pass parameters to the program, record disk directory information, and to buffer disk reads and writes. This control block is 100h bytes in lengrth and is followed by the program mopdule loaded by DOS. The PSP contains a pointer to the environment area for that program. This area contains a copy of the current DOS SET, PROMPT, COMSPEC, and PATH values as well as any user-set variables. The program may examine and modify this information as desired. Each storage block is 10h bytes long, although only 5 bytes are currently used by DOS. The first byte contains 4Dh (a capital M) to indicate that it contains a pointer to the next storage block. A 5Ah (a capital Z) in the first byte of a storage block indicatres there are no more storage blocks following this one (it is the end of the chain). The identifier byte is followeed by a 2 byte segment number for the associated PSP for that program. The next 2 bytes contain the number of segments what are allocated to the program. If this is not the last storage block, then another storage block follows the allocated memory area. When thge storage block contains zero for the nuymber of allocated segments, then no storage is allocated to thius block and the next storage block immediately follows this one. This can ha-p[en whjen memory is allocated and then deallocated repeatedly. IBMDOS constructs a storage block and PSP before loading the command interpreter (default is COMMAND.COM).

If the copy of COMMAND.COM is a secondary copy, it will lack an environment address as PSP+2Ch.

The Disk Transfer Area (DTA)

DOS uses an area in memory to contain the data for all file reads and writes that are performed with FCB function calls. This are is known as the disk transfer area. This disk transfer area (DTA) is sometimes called a buffer. It can be located anywhere in the data area of your application program and should be set by your program.

Only one DTA can be in effect at a time, so your program must tell DOS what memory location to use before using any disk read or write functions. Use function call 1Ah (Set Disk Transfer Address) to set the disk transfer address. Use function call 2Fh (Get Disk Transfer Address) to get the disk transfer address. Once set, DOS continues to use that area for all disk operations until another function call 1Ah is issued to define a new DTA. When a program is given control by COMMAND.COM, a default DTA large enough to hold 128 bytes is established at 80h into the program's Program Segment Prefix.

For file reads and writes that are performed with the extended function calls, there is no need to set a DTA address. Instead, specify a buffer address when you issue the read or write call.

DOS Program Segment

When you enter an external command or call a program through the EXEC function call, DOS determines the lowest availible address space to use as the start of available memory for the program being started. This area is called the Program Segment. At offset 0 within the program segment, DOS builds the Program Segment Prefix control block. EXEC loads the program after the Program Segment Prefix (at offset 100h) and gives it control. The program returns from EXEC by a jump to offset 0 in the Program Segment Prefix, by issuing an int 20h, or by issuing an int 21h with register AH=00h or 4Ch, or by calling location 50h in the PSP with AH=00h or 4Ch. It is the responsibility of all programs to ensure that the CS register contains the segment address of the Program Segment Prefix when terminating by any of these methods except call 4Ch.

All of these methods result in returning to the program that issued the EXEC. During this returning process, interrupt vectors 22h, 23h, and 24h (Terminate, Ctrl-Break, and Critical Error Exit addresses) are restored from the values saved in the PSP of the terminating program. Control is then given to the terminate address.

When a program receives control, the following conditions are in effect:

For all programs:

1) The segment address of the passed environment is contained at offset 2Ch in

 the Program Segment Prefix.

2) The environment is a series of ASCII strings totalling less than 32k bytes

 in the form:       NAME=parameter      The default environment is 160 bytes.
  Each string is terminated by a byte of zeroes, and the entire set of strings
 is terminated by abother byte of zeroes. Following the byte of zeroes that 
 terminates the set of environment string is a set of initial arguments passed
 to a program that contains a word count followed by an ASCIIZ string. The 
 ASCIIZ string contains the drive, path, and filename.ext of the executable 
 program. Programs may use this area to determine where the program was loaded
 from. The environment built by the command processor (and passed to all 
 programs it invokes) contains a COMSPEC=string at a minimum (the parameter on
 COMSPEC is the path used by DOS to locate COMMAND.COM on disk). The last PATH
 and PROMPT commands issued will also be in the environment, along with any 
 environment strings entered through the SET command. 
  The environment that you are passed is actually a copy of the invoking 
 process's environment. If your application terminates and stays resident 
 through int 27h, you should be aware that the copy of the environment passed 
 to you is static. That is, it will not change even if subsequent PATH,
 PROMPT, or SET commands are issued.

The environment can be used to transfer information between processes or to

 store strings for later use by application programs. The environment is
 always located on a paragraph boundary. This is its format:
      byte    ASCIIZ string 1
      byte    ASCIIZ string 2
          ....
      byte    ASCIIZ string n
      byte    of zeros (0)
 Typically the environment strings have the form:
      parameter = value
 Following the byte of zeros in the environment, a WORD indicates the number 
 of other strings following. 
 If the environment is part of an EXECed command interpreter, it is followed 
 by a copy of the DS:DX filename passed to the child process. A zero value 
 causes the newly created process to inherit the parent's environment.

3) Offset 80h in the PSP contains code to invoke the DOS function dispatcher.

 Thus, by placing the desired function number in AH, a program can issue a
 long call to PSP+50h to invoke a DOS function rather than issuing an int 21h.

4) The disk transfer address (DTA) is set to 80h (default DTA in PSP).

5) File Control Blocks 5Ch and 6Ch are formatted from the first two parameters

 entered when the command was invoked. Note that if either parameter contained
 a path name, then the corresponding FCB will contain only a valid drive
 number. The filename field will not be valid.

6) An unformatted parameter area at 81h contains all the characters entered

 after the command name (including leading and imbedded delimiters), with 80h
 set to the number of characters. If the <, >, or | parameters were entered
 on the command line, they (and the filenames associated with them) will not
 appear in this area, because redirection of standard input and output is
 transparent to applications.

(For EXE files only) 7) DS and ES registers are set to point to the PSP.

8) CS, IP, SS, and SP registers are set to the values passed by the linker.

(For COM files only) 9) For COM files, offset 6 (one word) contains the number of bytes availible in

 the segment.

10) Register AX reflects the validity of drive specifiers entered with the

  first two parameters as follows:
      AL=0FFh is the first parameter contained an invalid drive specifier,
              otherwise AL=00h.
      AL=0FFh if the second parameter contained an invalid drive specifier, 
              otherwise AL=00h.

11) All four segment registers contain the segment address of the inital

  allocation block, that starts within the PSP control block. All of user
  memory is allocated to the program. If the program needs to invoke another
  program through the EXEC function call (4Bh), it must first free some memory
  through the SETBLOCK function call to provide space for the program being
  invoked.

12) The Instruction Pointer (IP) is set to 100h.

13) The SP register is set to the end of the program's segment. The segment size

  at offset 6 is rounded down to the paragraph size.

14) A word of zeroes is placed on top of the stack.

The PSP (with offsets in hexadecimal) is formatted as follows:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ P R O G R A M S E G M E N T P R E F I X ³ ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ offset³ size ³ C O N T E N T S ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0000h ³ 2 bytes ³ int 20h ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0002h ³ 2 bytes ³ segment address, end of allocation block ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0004h ³ 1 byte ³ reserved, normally 0 ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0005h ³ 5 bytes ³ long call to MSDOS function dispatcher ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 000Ah ³ 4 bytes ³ previous termination handler interrupt vector (int 22h) ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 000Eh ³ 4 bytes ³ previous contents of ctrl-C interrupt vector (int 23h) ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0012h ³ 4 bytes ³ prev. critical error handler interrupt vector (int 24h) ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0016h ³ 22 bytes ³ reserved for DOS ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 002Ch ³ 2 bytes ³ segment address of environment block ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 002Eh ³ 34 bytes ³ reserved, DOS work area ³ ÀÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

      ³  4 bytes ³  stores the calling process's stack pointer when          ³
      ³          ³  switching to DOS's internal stack.                       ³

ÚÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0050h ³ 3 bytes ³ int 21h, RETF instructions ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0053h ³ 2 bytes ³ reserved ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0055h ³ 7 bytes ³ reserved, or FCB#1 extension ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 005Ch ³ ³ default File Control Block #1 ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 006Ch ³ ³ default File Control Block #2 (overlaid if FCB #1 opened) ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0080h ³ 1 byte ³ parameter length ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0081h ³ ³ parameters ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 00FFh ³ 128 bytes³ command tail and default Disk Transfer Area (DTA) ³ ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

1. The first segment of availible memory is in segment (paragraph) form. For

 example, 1000h would respresent 64k.

2. The word at offset 6 contains the number of bytes availible in the segment.

3. Offset 2Ch contains the segment address of the environment.

4. Programs must not alter any part of the PSP below offset 5Ch.

offset 0 contains hex bytes CD 20, the int 20h opcode. A program can end

     by making a jump to this location when the CS points to the PSP.
     For normal cases, int 21, function 4Ch should be used.

offset 2 contains the segment-paragraph address of the end of memory as

      reported by DOS. (which may not be the same as the real end of RAM).
      Multiply this number by 10h or 16 to get the amount of memory availible.

offset 4 reserved

offset 05 contains a long call to the DOS function dispatcher. Programs may

     jump to this address instead of calling int 21 if they wish. 

offsets 10, 14, 18 vectors

offset 2C is the segment:offset address of the environment for the program

     using this particular PSP.

offset 2E The DWORD at PSP+2EH is used by DOS to store the calling process's

     stack pointer when switching to DOS's own private stack - at the end of
     a DOS function call, SS:SP is restored from this address.

offset 50h contains a long call to the DOS function dispatcher.

offsets 5C, 65, 6C contain FCB information for use with FCB function calls.

      The first FCB may overlay the second if it is an extended call; your
      program should revector these areas to a safe place if you intend to
      use them.

offset 80h and 81h contain th elength and value of parameters passed on the

     command line. 

offset FF contains the DTA

STANDRD FILE CONTROL BLOCK

The standard file control block is defined as follows, with the offsets in decimal:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ F I L E C O N T R O L B L O C K ³ ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ Bytes ³ Function ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0 ³ Drive number. For example: ³ ³ ³ Before open: 00h = default drive ³ ³ ³ 01h = drive A: ³ ³ ³ 02h = drive B: etc. ³ ³ ³ After open: 00h = drive C: ³ ³ ³ 01h = drive A: ³ ³ ³ 02h = drive B: etc. ³ ³ ³ 0 is replaced by the actual drive number during open. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1-8 ³ Filename, left justified with trailing blanks. If a reserved device ³ ³ ³ name is placed here (such as PRN) do not include the optional colon. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 9-11 ³ Filename extension, left justified with trailing blanks. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 12-13 ³ Current block number relative to beginning of file, starting with 0 ³ ³ ³ (set to 0 by the open function call). A block consists of 128 ³ ³ ³ records, each of the size specified in the logical record size field.³ ³ ³ The current block number is used with the current record field ³ ³ ³ (below) for sequential reads and writes. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 14-15 ³ Logical record size in bytes. Set to 80h by the OPEN function call. ³ ³ ³ If this is not correct, you must set the value because DOS uses it ³ ³ ³ to determine the proper locations in the file for all disk reads and ³ ³ ³ writes. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 16-19 ³ File size in bytes. In this 2 word field, the first word is the ³ ³ ³ low-order part of the size. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 20-21 ³ Date the file was created or last updated. The mm/dd/yy are mapped ³ ³ ³ as follows: ³ ³ ³ 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 ³ ³ ³ y y y y y y y m m m m d d d d d ³ ³ ³ where: mm is 1-12 ³ ³ ³ dd is 1-31 ³ ³ ³ yy is 0-119 (1980-2099) ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 22-31 ³ Reserved for system use. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 32 ³ Current relative record number (0-127) within the current block. ³ ³ ³ (See above). You must set this field before doing sequential ³ ³ ³ read/write operations to the diskette. This field is not initialized ³ ³ ³ by the open function call. ³ ³ ³ If the record size is less than 64 bytes, both words are used. ³ ³ ³ Otherwise, only the first 3 bytes are used. Note that if you use the ³ ³ ³ File Control Block at 5Ch in the program segment, the last byte of ³ ³ ³ the FCB overlaps the first byte of the unformatted parameter area. ³ ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

note 1) An unopened FCB consists of the FCB prefix (if used), drive number, and

      filename.ext properly filled in. An open FCB is one in which the 
      remaining fields have been filled in by the CREAT or OPEN function 
      calls.
   2) Bytes 0-5 and 32-36 must be set by the user program. Bytes 16-31 are set
      by DOS and must not be changed by user programs.
   3) All word fields are stored with the least significant byte first. For 
      example, a record length of 128 is stored as 80h at offset 14, and 00h 
      at offset 15.

EXTENDED FILE CONTROL BLOCK

The extended file control block is used to create or search for files in the disk directory that have special attributes.

It adds a 7 byte prefix to the FCB, formatted as follows:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ E X T E N D E D F I L E C O N T R O L B L O C K ³ ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ Bytes ³ Function ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0 ³ Flag byte containing 0FFh to indicate an extended FCB. ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1-6 ³ Reserved ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 6-7 ³ Attribute byte. Refer to function call 11h (search first) for ³ ³ ³ details on using the attribute bits during directory searches. This ³ ³ ³ function is present to allow applications to define their own files ³ ³ ³ as hidden (and thereby excluded from normal directory searches) and ³ ³ ³ to allow selective directory searches. ³ ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

Any reference in the DOS function calls to an FCB, whether opened or unopened, may use either a normal or extended FCB. If you are using an extended FCB, the appropriate register should be set to the first byte of the prefix, rather than the drive-number field.

MEMORY CONTROL BLOCKS

DOS keeps track of allocated and availible memory blocks, and provides three function calls for application programs to communicate their memory needs to DOS. These calls are 48h to allocate a memory block, 49h to free a previously allocated memory block, and 4Ah (SETBLOCK) to change the size of an allocated memory block.

CONTROL BLOCK

DOS manages memory as follows:

DOS build a control block for each block of memory, whether free or allocated. For example, if a program issues an "allocate" (48h), DOS locates a block of free memory that satisfies the request, and then "carves" the requested memory out of that block. The requesting program is passed the location of the first byte of the block that was allocated for it - a memory management control block, describing the allocated block, has been built for the allocated block and a second memory management control block describes the amount of space left in the original free block of memory. When you do a setblock to shrink an allocated block, DOS builds a memory management control block for the area being freed and adds it to the chain of control blocks. Thus, any program that changed memory that is not allocated to it stands a chance of destroying a DOS memory management control block. This causes unpredictable results that don't show up until an activity is performed where DOS uses its chain of control blocks. The normal result is a memory allocation error, which means a system reset will be required.

When a program (command or application program) is to be loaded, DOS uses the EXEC function call 4Bh to perform the loading.

This is the same function call that is availible to applications programs for loading other programs. This function call has two options:

    Function 00h, to load and execute a program (this is what the command
                  processor uses to load and execute external commands)
    Function 03h, to load an overlay (program) without executing it.

Although both functions perform their loading in the same way (relocation is performed for EXE files) their handling of memory management is different.

FUNCTION 0: For function 0 to load and execute a program, EXEC first allocates the largest availible block of memory (the new program's PSP will be at offset 0 in that block). Then EXEC loads the program. Thus, in most cases, the new program owns all the memory from its PSP to the end of memory, including memory occupied by the transient parent of COMMAND.COM. If the program were to issue its own EXEC function call to load and execute another program, the request would fail because no availible memory exists to load the new program into.

NOTE: For EXE programs, the amount of memory allocated is the size of the

    program's memory image plus the value in the MAX_ALLOC field of the file's
    header (offset 0Ch, if that much memory is availible. If not, EXEC 
    allocates the size of the program's memory image plus the value in the 
    MIN_ALLOC field in the header (offset 0Ah). These fields are set by the 
    Linker).

A well-behaved program uses the SETBLOCK function call when it receives control, to shrink its allocated memory block down to the size it really needs. A COM program should remember to set up its own stack before doing the SETBLOCK, since it is likely that the default stack supplied by DOS lies in the area of memory being used. This frees unneeded memory, which can be used for loading other programs.

If the program requires additional memory during processing, it can obtain the memory using the allocate function call and later free it using the free memory function call.

When a program is loaded using EXEC function call 00h exits, its initial allocation block (the block beginning with its PSP) is automatically freed before the calling program regains control. It is the responsibility of all programs to free any memory they allocate before exiting to the calling program.

FUNCTION 3: For function 3, to load an overlay, no PSP is built and EXEC assumes the calling program has already allocated memory to load the new program into - it will NOT allocate memory for it. Thus the calling program should either allow for the loading of overlays when it determines the amount of memory to keep when issuing the SETBLOCK call, or should initially free as much memory as possible. The calling program should then allocate a block (based on the size of the program to be loaded) to hold the program that will be loaded using the "load overlay" call. Note that "load overlay" does not check to see if the calling program actually owns the memory block it has been instructed to load into - it assumes the calling program has followed the rules. If the calling program does not own the memory into which the overlay is being loaded, there is a chance the program being loaded will overlay one of the control blocks that DOS uses to keep track of memory blocks.

Programs loaded using function 3 should not issue any SETBLOCK calls since they don't own the memory they are operating in. (This memory is owned by the calling program)

Because programs loaded using function 3 are given control directly by (and return contrrol directly to) the calling program, no memory is automatically freed when the called program exits. It is up to the calling program to determine the disposition of the memory that had been occupied by the exiting program. Note that if the exiting program had itself allocated any memory, it is responsible for freeing that memory before exiting.

MEMORY CONTROL BLOCKS

Only the first 5 bytes of the memory control block are used. The first byte will always have the value of 4Dh or 5Ah. The value 5Ah indicates the block is the last in a chain, all memory above it is unused. 4Dh means that the block is intermediate in a chain, the memory above it belongs to the next program or to DOS.

The next two bytes hold the PSP segment address of the program that owns the corresponding block of memory. A value of 0 means the block is free to be claimed, any other value represents a segment address. Bytes 3 and 4 indicate the size in paragraphs of the memory block. If you know the address of the first block, you can find the next block by adding the length of the memory block plus 1 to the segment address of the control block.

Finding the first block can be difficult, as this varies according to the DOS version and the configuration.

The remaining 11 bytes are not currently used by DOS, and may contain "trash" characters left in memory from previous applications.

If DOS determines that the allocation chain of memory control blocks has been 

corrupted, it will halt the system and display the message "Memory Allocation Error", and the system will halt, requiring a reboot.

Each memory block consists of a signature byte (4Dh or 5Ah) then a word which is the PSP value of the owner of the block (which allocated it), followed by a word which is the size in paragraphs of the block. The last block has a signature of 5Ah. All others have 4Dh. If the owner is 0000 then the block is free.

User memory is allocated from the lowest end of available memory that will satisfy the request for memory.

CHAPTER 7

Programming Technical Reference - IBM Copyright 1988, Dave Williams

                           DOS File Structure

File Management Functions

Use DOS function calls to create, open, close, read, write, rename, find, and erase files. There are two sets of function calls that DOS provides for support of file management. They are:

* File Control Block function calls (0Fh-24h) * Handle function calls (39h-62h)

Handle function calls are easier to use and are more powerful than FCB calls. Microsoft recommends that the handle function calls be used when writing new programs. DOS 3.0 up have been curtailing use of FCB function calls; it is possible that future versions of DOS may not support FCB function calls. The following table compares the use of FCB calls to Handle function calls:

              FCB Calls                       Handle Calls
      Access files in current         Access files in ANY directory
      directory only.
      Requires the application        Does not require use of an FCB.
      program to maintain a file      Requires a string with the drive,
      control block to open,          path, and filename to open, create,
      create, rename or delete        rename, or delete a file. For file
      a file. For I/O requests,       I/O requests, the application program
      the application program         must maintain a 16 bit file handle
      also needs an FCB               that is supplied by DOS.

The only reason an application should use FCB function calls is to maintain the ability to run under DOS 1.x. To to this, the program may use only function calls 00h-2Eh.

FCB FUNCTION CALLS

FCB function calls require the use of one File Control Block per open file, which is maintained by the application program and DOS. The application program supplies a pointer to the FCB and fills in ther appropriate fields required by the specific function call. An FCB function call can perform file management on any valid drive, but only in the current logged directory. By using the current block, current record, and record length fields of the FCB, you can perform sequential I/O by using the sequential read or write function calls. Random I/O can be performed by filling in the random record and record length fields.

Several possible uses of FCB type calls are considered programming errors and should not be done under any circumstances to avoid problems with file sharing and compatibility with later versions of DOS. Some errors are: 1) If program uses the same FCB structure to access more than one open file. By

 opening a file using an FCB, doing I/O, and then replacing the filename field
 in the file control block with a new filename, a program can open a second
 file using the same FCB. This is invalid because DOS writes control info-
 rmation about the file into the reserved fields of the FCB. If the program
 then replaces the filename field with the original filename and then tries to
 perform I/O on this file, DOS may become confused because the control info-
 rmation has been changed. An FCB should never be used to open a second file
 without closing the one that is currently open. If more than one File Control
 Block is to be open concurrently, separate FCBs should be used.

2) A program should never try to use the reserved fields in the FCB, as the

 function of the fields changes with different versions of DOS.

3) A delete or a rename on a file that is currently open is considered an error

 and should not be attempted by an application program.

It is also good programming practice to close all files when I/O is done. This avoids potential file sharing problems that require a limit on the number of files concurrently open using FCB function calls.

HANDLE FUNCTION CALLS

The recommended method of file management is by using the extended "handle" set of function calls. These calls are not restricted to the current directory. Also, the handle calls allow the application program to define the type of access that other processes can have concurrently with the same file if the file is being shared.

To create or open a file, the application supplies a pointer to an ASCIIZ string giving the name and location of the file. The ASCIIZ string contains an optional drive letter, optional path, mandatory file specification, and a terminal byte of 00h. The following is an example of an ASCIIZ string:

                format [drive][path] filename.ext,0
                    DB "A:\path\filename.ext",0

If the file is being created, the application program also supplies the attribute of the file. This is a set of values that defines the file read only, hidden, system, directory, or volume label.

If the file is being opened, the program can define the sharing and access modes that the file is opened in. The access mode informs DOS what operations your program will perform on this file (read-only, write-only, or read/write) The sharing mode controls the type of operations other processes may perform concurrently on the file. A program can also control if a child process inherits the open files of the parent. The sharing mode has meaning only if file sharing is loaded when the file is opened.

To rename or delete a file, the appplication program simply needs to provide a pointer to the ASCIIZ string containing the name and location of the file and another string with the neew name if the file is being renamed.

The open or create function calls return a 16-bit value referred to as the file handle. To do any I/O to a file, the program uses the handle to reference the file. Once a file is opened, a program no longer needs to maintain the ASCIIZ string pointing to the file, nor is there any need to stay in the same directory. DOS keeps track of the location of the file regardless of what directory is current.

Sequential I/O can be performed using the handle read (3Fh) or write (40h) function calls. The offset in the file that IO is performed to is automatically moved to the end of what was just read or written. If random I/O is desired, the LSEEK (42h) function call can be used to set the offset into the file where I/O is to be performed.

SPECIAL FILE HANDLES

DOS reserves five special file handles for use by itself and applications programs. They are:

            0000h   STDIN   Standard Input Device
            0001h   STDOUT  Standard Output Device
            0002h   STDERR  Standard Error Output Device
            0003h   STDAUX  Standard Auxiliary Device
            0004h   STDPRN  Standard Printer Device

These handles are predefined by DOS and can be used by an application program. They do not need to be opened by a program, although a program can close these handles. STDIN should be treated as a read-only file, and STDOUT and STDERR should be treated as write-only files. STDIN and STDOUT can be redirected. All handles inherited by a process can be redirected, but not at the command line.

These handles are very useful for doing I/O to and from the console device. For example, you could read input from the keyboard using the read (3Fh) function call and file handle 0000h (STDIN), and write output to the console screen with the write function call (40h) and file handle 0001h (STDOUT). If you wanted an output that could not be redirected, you could output it using file handle 0002h (STDERR). This is very useful for error messages that must be seen by a user.

File handles 0003h (STDAUX) and 0004h (STDPRN) can be both read from and written to. STDAUX is typically a serial device and STDPRN is usually a parallel device.

ASCII and BINARY MODE

I/O to files is done in binary mode. This means that the data is read or written without modification. However, DOS can also read or write to devices in ASCII mode. In ASCII mode, DOS does some string processing and modification to the characters read and written. The predefined handles are in ASCII mode when initialized by DOS. All other file handles that don't refer to devices are in binary mode. A program, can use the IOCTL (44h) function call to set the mode that I/O is to a device. The predefined file handles are all devices, so the mode can be changed from ASCII to binary via IOCTL. Regular file handles that are not devices are always in binary mode and cannot be changed to ASCII mode.

The ASCII/BINARY bit was called "raw" in DOS 2.x, but it is called ASCII/BINARY in DOS 3.x.

The predefined file handles STDIN (0000h) and STDOUT (0001h) and STDERR (0002h) are all duplicate handles. If the IOCTL function call is used to change the mode of any of these three handles, the mode of all three handles is changed. For example, if IOCTL was used to change STDOUT to binary mode, then STDIN and STDERR would also be changed to binary mode.

FILE I/O IN BINARY (RAW) MODE

The following is true when a file is read in binary mode:

1) The characters ^S (scroll lock), ^P (print screen), ^C (control break) are

  not checked for during the read. Therefore, no printer echo occurs if ^S or
  ^P are read.

2) There is no echo to STDOUT (0001h). 3) Read the number of specified bytes and returns immediately when the last

  byte is received or the end of file reached.

4) Allows no editing of the ine input using the function keys if the input is

  from STDIN (0000h).

The following is true when a file is written to in binary mode:

1) The characters ^S (scroll lock), ^P (print screen), ^C (control break) are

  not checked for during the write. Therefore, no printer echo occurs.

2) There is no echo to STDOUT (0001h). 3) The exact number of bytes specified are written. 4) Does not caret (^) control characters. For example, ctrl-D is sent out as

  byte 04h instead of the two bytes ^ and D.

5) Does not expand tabs into spaces.

FILE I/O IN ASCII (COOKED) MODE

The following is true when a file is read in ASCII mode:

1) Checks for the characters ^C,^S, and ^P. 2) Returns as many characters as there are in the device input buffer, or the

  number of characters requested, whichever is less. If the number of 
  characters requested was less than the number of characters in the device 
  buffer, then the next read will address the remaining characters in the 
  buffer.

3) If there are no more bytes remaining in the device input buffer, read a

  line (terminated by ^M) into the buffer. This line may be edited with the 
  function keys. The characters returned terminated with a sequence of 0Dh,
  0Ah (^M,^J) if the number of characters requested is sufficient to include
  them. For example, if 5 characters were requested, and only 3 were entered
  before the carriage return (0Dh or ^M) was presented to DOS from the console
  device, then the 3 characters entered and 0Dh and 0Ah would be returned. 
  However, if 5 characters were requested and 7 were entered before the 
  carriage return, only the first 5 characters would be returned. No 0Dh,0Ah 
  sequence would be returned in this case. If less than the number of 
  characters requested are entered when the carriage return is received, the
  characters received and 0Dh,0Ah would be returned. The reason the 0Ah 
  (linefeed or ^J) is added to the returned characters is to make the devices
  look like text files.

4) If a 1Ah (^Z) is found, the input is terminated at that point. No 0Dh,0Ah

  (CR,LF) sequence is added to the string.

5) Echoing is performed. 6) Tabs are expanded.

The following is true when a file is written to in ASCII mode:

1) The characters ^S,^P,and ^C are checked for during the write operation. 2) Expands tabs to 8-character boundaries and fills with spaces (20h). 3) Carets control characters, for example, ^D is written as two bytes, ^ and D. 4) Bytes are output until the number specified is output or a ^Z is

  encountered. The number actually output is returned to the user.

NUMBER OF OPEN FILES ALLOWED

The number of files that can be open concurrently is restricted by DOS. This number is determined by how the file is opened or created (FCB or handle function call) and the number specified by the FCBS and FILES commands in the CONFIG.SYS file. The number of files allowed open by FCB function calls and the number of files that can be opened by handle type calls are independent of one another.

RESTRICTIONS ON FCB USAGE

If file sharing is not loaded using the SHARE command, there are no restrictions on the nuumber of files concurrently open using FCB function calls.

However, when file sharing is loaded, the maximum number of FCBs open is set by the the FCBS command in the CONFIG.SYS file.

The FCBS command has two values you can specify, 'm' and 'n'. The value for 'm' specifies the number of files that can be opened by FCBs, and the value 'n' specifies the number of FCBs that are protected from being closed.

When the maximum number of FCB opens is exceeded, DOS automatically closes the least recently used file. Any attempt to access this file results in an int 24h critical error message "FCB not availible". If this occurs while an application program is running, the value specified for 'm' in the FCBS command should be increased.

When DOS determines the least recently used file to close, it does not include the first 'n' files opened, therefore the first 'n' files are protected from being closed.

RESTRICTIONS ON HANDLE USAGE

The number of files that can be open simultaneously by all processes is determined by the FILES command in the CONFIG.SYS file. The number of files a single process can open depends on the value specified for the FILES command. If FILES is greater than or equal to 20, a single process can open 20 files. If FILES is less than 20, the process can open less than 20 files. This value includes three predefined handles: STDIN, STDOUT, and STDERR. This means only 17 additional handles can be added. DOS 3.3 includes a function to use more than 20 files per application.

ALLOCATING SPACE TO A FILE

Files are not nescessarily written sequentially on a disk. Space is allocated as needed and the next location availible on the disk is allocated as space for the next file being written. Therefore, if considerable file generation has taken place, newly created files will not be written in sequential sectors. However, due to the mapping (chaining) of file space via the File Allocation Table (FAT) and the function calls availible, any file may be used in either a sequential or random manner.

Space is allocated in increments called clusters. Cluster size varies according to the media type. An application program should not concern itself with the way that DOS allocates space to a file. The size of a cluster is only important in that it determines the smallest amount of space that can be allocated to a file. A disk is considered full when all clusters have been allocated to files.

MSDOS / PCDOS DIFFERENCES

There is a problem of compatibility between MS-DOS and IBM PC-DOS having to do with FCB Open and Create. The IBM 1.0, 1.1, and 2.0 documentation of OPEN (call 0Fh) contains the following statement:

"The current block field (FCB bytes C-D) is set to zero [when an FCB is opened]."

This statement is NOT true of MS-DOS 1.25 or MS-DOS 2.00. The difference is intentional, and the reason is CP/M 1.4 compatibility. Zeroing that field is not CP/M compatible. Some CP/M programs will not run when machine translated if that field is zeroed. The reason it is zeroed in the IBM versions is that IBM specifically requested that it be zeroed. This is the reason for the complaints from some vendors about the fact that IBM MultiPlan will not run under MS-DOS. It is probably the reason that some other IBM programs don't run under MS-DOS.

NOTE: Do what all MS/PC-DOS Systems programs do: Set every single FCB field you want to use regardless of what the documentation says is initialized.

.EXE FILE STRUCTURE

The EXE files produced by the Linker program consist of two parts, control and relocation information and the load module itself.

The control and relocation information, which is described below, is at the beginning of the file in an area known as the header. The load module immediately follows the header. The load module begins in the memory image of the module contructed by the Linker.

When you are loading a file with the name *.EXE, DOS does NOT assume that it is an EXE format file. It looks at the first two bytes for a signature telling it that it is an EXE file. If it has the proper signature, then the load proceeds. Otherwise, it presumes the file to be a .COM format file.

If the file has the EXE signature, then the internal consistency is checked. Pre-2.0 versions of MSDOS did not check the signature byte for EXE files.

The .EXE format can support programs larger than 64K. It does this by allowing separate segments to be defined for code, data, and the stack, each of which can be up to 64K long. Programs in EXE format may contain explicit references to segment addresses. A header in the EXE file has information for DOS to resolve these references.

The .EXE header is formatted as follows: ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ Offset ³ C O N T E N T S ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 00h ³ 4Dh ³ This is the Linker's signature to mark the file as a valid ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄ´ .EXE file (The ASCII letters M and Z, for Mark Zbikowski, ³ ³ 01h ³ 5Ah ³ one of the major designers of DOS at Microsoft) ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 02h-03h ³ Length of the image mod 512 (remainder after dividing the load ³ ³ ³ module image by 512) ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 04h-05h ³ Size of the file in 512 byte pages including the header. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 06h-07h ³ Number of relocation table items. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 08h-09h ³ Size of the header in 16 byte increments (paragraphs). This is ³ ³ ³ used to locate the beginning of the load module in the file. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0Ah-0Bh ³ Minimum number of 16 byte paragraphs required above the end of ³ ³ ³ the loaded program. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0Ch-0Dh ³ Maximum number of 16 byte paragraphs required above the end of ³ ³ ³ the loaded program. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 0Eh-0Fh ³ Displacement in paragraphs of stack segment within load module. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 10h-11h ³ Offset to be in SP register when the module is given control. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 12h-13h ³ Word Checksum - negative sum of all the words in the file, ³ ³ ³ ignoring overflow. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 14h-15h ³ Offset to be in the IP register when the module is given control. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 16h-17h ³ Displacement in paragraphs of code segment within load module. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 18h-19h ³ Displacement in bytes of the first relocation item in the file. ³ ÃÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1Ah-1Bh ³ Overlay number (0 for the resident part of the program) ³ ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

THE RELOCATION TABLE

The word at 18h locates the first entry in the relocation table. The relocation table is made up of a variable number of relocation items. The number of items is contained at offset 06-07. The relocation item contains two fields - a 2 byte offset value, followed by a 2 byte segment value. These two fields represent the displacement into the load module before the module is given control. The process is called relocation and is accomplished as follows:

1. A Program Segment Prefix is built following the resident portion of the

 program that is performing the load operation.

2. The formatted part of the header is read into memory (its size is at

 offset 08-09)

3. The load module size is determined by subtracting the header size from the

 file size. Offsets 04-05 and 08-09 can be used for this calculation. The 
 actual size is downward adjusted based on the contents of offsets 02-03. 
 Note that all files created by the Linker programs prior to version 1.10 
 always placed a value of 4 at this location, regardless of the actual 
 program size. Therefore, Microsoft recommends that this field be ignored if 
 it contains a value of 4. Based on the setting of the high/low loader switch,
 an appropriate segment is determined for loading the load module. This
 segment is called the start segment.

4. The load module is read into memory beginning at the start segment. The

 relocation table is an ordered list of relocation items. The first relocation
 item is the one that has the lowest offset in the file. 

5. The relocation table items are read into a work area one or more at a time.

6. Each relocation table item segment value is added to the start segment value.

 The calculated segment, in conjunction with the relocation item offset value,
 points to a word in the load module to which is added the start segment 
 value. The result is placed back into the word in the load module.

7. Once all the relocation items have been processed, the SS and SP registers

 are set from the values in the header and the start segment value is added 
 to SS. The ES and DS registers are set to the segment address of the program 
 segment prefix. The start segment value is added to the header CS register 
 value. The result, along with the header IP value, is used to give the 
 module control.

"NEW" .EXE FORMAT (Microsoft Windows and OS/2)

The "old" EXE format is documented here. The "new" EXE format puts more information into the header section and is currently used in applications that run under Microsoft Windows. The linker that creates these files comes with the Microsoft Windows Software Development Kit and is called LINK4. If you try to run a Windows-linked program under DOS, you will get the error message "This program requires Microsoft Windows".

CHAPTER 8

Programming Technical Reference - IBM Copyright 1988, Dave Williams

                        DOS DISK INFORMATION

THE DOS AREA

All disks and diskettes formatted by DOS are created with a sector size of 512 bytes. The DOS area (entire area for a diskette, DOS partition for hard disks) is formatted as follows:

      Boot record - 1 sector
      First copy of the FAT - variable size
      Second copy of the FAT - same size as first copy
      Root directory - variable size
      Data area

The following sections describe each of the allocated areas:

THE BOOT RECORD

The boot record resides on track 0, sector 1, side 0 of every diskette formatted by the DOS FORMAT program. It is put on all disks to provide an error message is you try to start up with a nonsystem disk in drive A:. For hard disks the boot record resides on the first sector of the DOS partition.

THE DOS FILE ALLOCATION TABLE (FAT)

This section explains how DOS uses the FAT to convert the clusters of a file into logical sector numbers. We recommend that system utilities use the DOS handle calls rather than interpreting the FAT.

The FAT is used by DOS to allocate disk space for files, one cluster at a time.

The FAT consists of a 12 bit entry (1.5 bytes) for each cluster on the disk or a 16 bit (2 bytes) entry when a hard disk has more than 20740 sectors as is the case with fixed disks larger than 10Mb. The first two FAT entries map a portion of the directory; these FAT entries contain indicators of the size and format of the disk. The FAT can be in a 12 or 16 bit format. DOS determines whether a disk has a 12 or 16 bit FAT by looking at the total number of allocation units on a disk. For all diskettes and hard disks with DOS partitions less than 20,740 sectors, the FAT uses a 12 bit value to map a cluster. For larger partitions, DOS uses a 16 bit value. The second, third, and fourth bit applicable for 16 bit FAT bytes always contains 0FFFFh. The first byte is used as follows:

   hex value              meaning                     normally used
     0F8h      hard disk                         bootable hard disk at C:800
               double sided  18 sector diskette   PS/2 1.44 meg DSQD
     0F9h      double sided  15 sector diskette   AT 1.2 meg DSQD
               double sided  9  sector diskette   Convertible 720k DSHD
     0FCh      single sided  9  sector diskette   DOS 2.0, 180k SSDD
     0FDh      double sided  9  sector diskette   DOS 2.0, 360k DSDD
     0FEh      single sided  8  sector diskette   DOS 1.0, 160k SSDD
     0FFh      double sided  8  sector diskette   DOS 1.1, 320k SSDD

The third FAT entry begins mapping the data area (cluster 002).

NOTE: These values are provided as a reference. Therefore, programs should not

    make use of these values.

Each entry contains a hexadecimal character (or 4 for 16 bit FATs). () indicates the high order four bit value in the case of 16 bit FAT entries. They can be:

          (0)000h  if the cluster is unused and availible

(0F)FF8h - (0F)FFFh to indicate the last cluster of a file

          (X)XXXh  any other hexadecimal numbers that are the cluster number
                   of the next cluster in the file. The cluster number is the
                   first cluster in the file that is kept in the file's 
                   directory entry.

The values (0F)FF0h - (0F)FF7h are used to indicate reserved clusters. (0F)FF7h indicates a bad cluster if it is not part of the allocation chain. (0F)FF8h - (0F)FFFh are used as end of file markers.

The file allocation table always occupies the sector or sectors immediately following the boot record. If the FAT is larger than 1 sector, the sectors occupy consecutive sector numbers. Two copies of the FAT are written, one following the other, for integrity. The FAT is read into one of the DOS buffers whenever needed (open, allocate more space, etc).

USE OF THE 12 BIT FILE ALLOCATION TABLE

Obtain the starting cluster of the file from the directory entry.

Now, to locate each subsequent sector of the file:

1. Multiply the cluster number just used by 1.5 (each FAT entry is 1.5

 bytes long).

2. The whole part of the product is offset into the FAT, pointing to the entry

 that maps the cluster just used. That entry contains the cluster number of 
 the next cluster in the file.

3. Use a MOV instruction to move the word at the calculated FAT into a register. 4. If the last cluster used was an even number, keep the low order 12 bits of

 the register, otherwise, keep the high order 12 bits.

5. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters are in the file.

 Otherwise, the next 12 bits contain the cluster number of the next cluster in
 the file. 
To convert the cluster to a logical sector number (relative sector, such as 

that used by int 25h and 26h and DEBUG):

1. Subtract 2 from the cluster number 2. Multiply the result by the number of sectors per cluster. 3. Add the logical sector number of the beginning of the data area.

USE OF THE 16 BIT FILE ALLOCATION TABLE

Obtain the starting cluster of the file from the directory entry. Now to locate each subsequent cluster of the file:

1. Multiply the cluster number used by 2 (each FAT entry is 2 bytes long). 2. Use the MOV word instruction to move the word at the calculated FAT offset

  into a register.

3. If the resultant 16 bits are (0FF8h-0FFFFh) no more clusters are in the

  file. Otherwise, the 16 bits contain the cluster number of the next cluster 
  in the file. 

Compaq DOS makes availible a new disk type (6) with 32 bit partition values, allowing 512 megabytes per hard disk (Compaq DOS 3.3.1)

DOS DISK DIRECTORY

The FORMAT command initially builds the root directory for all disks. Its location (logical sector number) and the maximum number of entries are availible through the device driver interfaces.

DIRECTORY ENTRIES

Since directories other than the root directory are actually files, there is no limit to the number of entries that they may contain.

All directory entries are 32 bytes long, and are in the following format (byte and offset are decimal). The following paragraphs describe the directory entry bytes:

*BYTES 0-7 Bytes 0-7 represent the filename. The first byte of the filename indicates the status of the filename. The status of a filename can contain the following values:

 00h Filename never used. This is used to limit the length of directory 
     searches, for performance reasons.
 05h Indicates that the first character of the filename actually has an 0Edh 
     character.
0E5h Filename has been used but the file has been erased.
 2Eh This entry is for a directory. If the second byte is also 2Eh, the 
     cluster field contains the cluster number of this directory's parent 
     directory.  (0000h if the parent directory is the root directory).
                                               

Any other character is the first character of a filename.

*BYTES 8-10 These bytes indicate the filename extension.

*BYTE 11 This byte indicates the file's attribute. The attribute byte is mapped as follows (values are in hexadecimal):

NOTE: Attributes 08h and 10h cannot be changed using function call 43h (CHMOD).

The system files IBMBIO.COM and IBMDOS.COM (or customized equivalent) are marked as read-only, hidden, and system files. Files can be marked hidden when they are created. Also, the read-only, hidden, and system and archive attributes may be changed through the CHMOD function call.

01h Indicates that the file is marked read-only. An attempt to open the file

  for output using function call 3Dh results in an error code being returned. 
  This value can be used with other values below.

02h Indicates a hidden file. The file is excluded from normal directory

  searches.

04h Indicates a system file. This file is excluded from normal directory

  searches.

08h Indicates that the entry contains the volume label in the first 11 bytes.

  The entry contains no other usable information and may exist only in the 
  root directory.

20h Indicates an archive bit. This bit is set on whenever the file is written

  to and closed. It is used by BACKUP and RESTORE.

All other bits are reserved, and must be 0.

*BYTES 12-21 reserved by DOS

*BYTES 22-23 These bytes contain the time when the file was created or last updated. The time is mapped in the bits as follows:

    ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
    ³         B Y T E    23         ³         B Y T E    22         ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³ F   E   D   C   B   A   9   8 ³ 7   6   5   4   3   2   1   0 ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³ H   H   H   H   H ³ M   M   M   M   M   M ³ D   D   D   D   D ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³ binary # hrs 0-23 ³ binary # minutes 0-59 ³ bin. # 2-sec incr ³
    ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

NOTE: The time is stored with the least significant byte first.

*BYTES 24-25 This area contains the date when the file was created or last updated. The mm/dd/yy are mapped in the bits as follows:

    ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
    ³         B Y T E    25         ³         B Y T E    24         ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³ F   E   D   C   B   A   9   8 ³ 7   6   5   4   3   2   1   0 ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³ Y   Y   Y   Y   Y   Y   Y ³ M   M   M   M ³ D   D   D   D   D ³
    ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
    ³      0-119 (1980-2099)    ³     1-12      ³       1-31        ³
    ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

NOTE: The date is stored with the least significant byte first.

*BYTES 26-27 This area contains the starting cluster number of the first cluster in the file. The first cluster for data space on all fixed disks and floppy disks is always cluster 002. The cluster number is stored with the least significant byte first.

*BYTES 28-31 This area contains the file size in bytes. The first word contains the low order part of the size. Both words are stored with the least significant byte first.

                      File Allocation Table

offset size description

 3       8 bytes system id (such as IBM 3.3)
11      2 bytes number of bytes per sector, ie 512 bytes is 200h
13      1 byte  sectors per cluster (1 or 2)
14      2 bytes number of reserved sectors at beginning, 1 for floppies
16      1 byte  number of copies of FAT, 2 for floppies
17      2 bytes number of root directory entries (64, 112, 256,etc)
19      2 bytes total sectors per disk
21      1 byte  format ID (F8, F9, FC, FF, etc)
22      2 bytes number of sectors per FAT (1 or 2)
24      2 bytes number of sectors per track (8 or 9, 17)
26      2 bytes number of sides, heads, or cylinders (1 or 2 for floppy)
28      2 bytes number of special reserved sectors

THE DATA AREA

Allocation of space for a file (in the data area) is done only when needed (it is not preallocated). The space is allocated one cluser (unit allocation) at a time. A cluster is always one or more consecutive sector numbers, and all of the clusters in a file are "chained" together in the FAT.

The clusters are arranged on disk to minimize head movement for multisided media. All of the space on a track (or cylinder) is allocated before moving on to the next track. This is accomplished by using the sequential sector numbers on the lowest-numbered head, then all the sector numbers on the next head, and so on until all sectors of all heads of the track are used. Then the next sector used will be sector 1 of head 0 on the next track.

An interesting innovation that was introduced in MS-DOS 3.0: disk space that is freed by erasing a file is not re-used immediately, unlike earlier versions of DOS. Instead, free space is obtained from the area not yet used during the current session, until all of it is used up. Only then will space that is freed during the current session be re-used.

This feature minimizes fragmentation of files, since never-before-used space is always contiguous. However, once any space has been freed by deleting a file, that advantage vanishes at the next system boot. The feature also greatly simplifies un-erasing files, provided that the need to do an un-erase is found during the same session and also provided that the file occupies contiguous clusters.

However, when one is using programs which make extensive use of temporary files, each of which may be created and erased many times during a session, the feature becomes a nuisance; it forces the permanent files to move farther and farther into the inner tracks of the disk, thus increasing rather than decreasing the amount of fragmentation which occurs.

The feature is implemented in DOS by means of a single 16-bit "last cluster used" (LCU) pointer for each physical disk drive; this pointer is a part of the physical drive table maintained by DOS. At boot time, the LCU pointer is zeroed. Each time another cluster is obtained from the free-space pool (the FAT), its number is written into the LCU pointer. Each time a fresh cluster is required, the FAT is searched to locate a free one; in older versions of DOS this search always began at Cluster 0000, but in 3.x it begins at the cluster pointed to by the LCU pointer.

For hard disks, the size of the file allocation table and directory are determined when FORMAT initializes it and are based on the size of the DOS partition.

The following table gives the specifications for floppy disk formats:

             # of   sectors  FAT size    DIR        DIR     sectors  total

disk DOS ver sides /track (sectors) (sectors) (entries) /cluster sectors

(5-1/4 inch) 160k (DOS 1.0) 1 8 (40) 1 4 64 1 320 320k (DOS 1.1) 2 8 (40) 1 7 112 2 360 180k (DOS 2.0) 1 9 (40) 2 4 64 1 640 360k (DOS 2.0) 2 9 (40) 2 7 112 2 720 1.2M (DOS 3.0) 2 15 (80) 7 14 224 1 2400

(3-1/2 inch) 720k (DOS 3.2) 2 9 (80) 3 7 112 2 1440 1.44M(DOS 3.3) 2 18 (80) 9 14 224 1 2880

Files in the data area are not nescessarily written sequentially on the first. The data area space is allocated one cluster at a time, skipping over clusters already allocated. The first free cluster found is the next cluster allocated, regardless of its physical location on the disk. This permits the most efficient utilization of disk space because clusters freed by erasing files can be allocated for new files. Refer back to the description of the DOS FAT in this chapter for more information.

Hard Disk Layout

The DOS hard disk routines perform the following services:

1) Allow multiple operating systems to utilize the hard disk without the need

 to backup and restore files when changing operating systems.

2) Allow a user-selected operating system to be started from the hard disk.

 
 I) In order to share the hard disk among operating systems, the disk may be 
    logically divided into 1 to 4 partitions. The space within a given 
    partition is contiguous, and can be dedicated to a specific operating 
    system. Each operating system may "own" only one partition in DOS versions
    2.0 through 3.2. PCDOS 3.3 introduced the "Extended DOS Partition" which 
    allows multiple DOS partitions on the same hard disk. The FDISK.COM (or 
    similar program from other DOS vendors) utility allows the user to select
    the number, type, and size of each partition. The partition information is
    kept in a partition table that is embedded in the master fixed disk boot 
    record on the first sector of the disk. The format of this table varies 
    from version to version of DOS.
II) An operating system must consider its partition to be the entire disk, 
    and must ensure that its functions and utilities do not access other 
    partitions on the disk. 

III) Each partition may contain a boot record on its first sector, and any

    other programs or data that you choose - including a copy of an operating 
    system. For example, the DOS FORMAT command may be used to format and 
    place a copy of DOS in the DOS partition in the same manner that a 
    diskette is formatted. With the FDISK utility, you may designate a 
    partition as "active" (bootable). The master hard disk boot record causes
    that partition's boot record to receive control when the system is started
    or reset. Additional disk partitions could be FORTH, UNIX, Pick, CP/M-86,
    or the UCSD p-System.

SYSTEM INITIALIZATION

The boot sequence is as follows:

1. System initialization first attempts to load an operating system from

 diskette drive A. If the drive is not ready or a read error occurs, it then 
 attempts to read a master hard disk boot record on the first sector of the 
 first hard disk in the system. If unsuccessful, or if no hard disk is 
 present, it invokes ROM BASIC in an IBM PC or displays a disk error 
 message on most compatibles.

2. If initialization is successful, the master hard disk boot record is given

 control and it examines the partition table embedded within it. If one of 
 the entries indicates an active (bootable) partition, its boot record is 
 read from the partition's first sector and given control.

3. If none of the partitions is bootable, ROM BASIC is invoked on an IBM PC or

  a disk error on most compatibles.

4. If any of the boot indicators are invalid, or if more than one indicator is

 marked as bootable, the message INVALID PARTITION TABLE is displayed and the 
 system stops.

5. If the partition's boot record cannot be successfully read within five

 retries due to read errors, the message ERROR LOADING OPERATING SYSTEM 
 appears and the system stops.

6. If the partition's boot record does not contain a valid "signature", the

 message MISSING OPERATING SYSTEM appears, and the system stops.

NOTE: When changing the size or location of any partition, you must ensure that

    all existing data on the disk has been backed up. The partitioning program
    will destroy the data on the disk.

BOOT RECORD/PARTITION TABLE

A boot record must be written on the first sector of all hard disks, and must contain the following:

1. Code to load and give control to the boot record for one of four possible

 operating systems.

2. A partition table at the end of the boot record. Each table entry is 16

 bytes long, and contains the starting and ending cylinder, sector, and head 
 for each of four possible partitions, as well as the number of sectors 
 preceding the partition and the number of sectors occupied by the partition. 
 The "boot indicator" byte is used by the boot record to determine if one of 
 the partitions contains a loadable operating system. FDISK initialization 
 utilities mark a user-selected partition as "bootable" by placing a value 
 of 80h in the corresponding partition's boot indicator (setting all other 
 partitions' indicators to 0 at the same time). The presence of the 80h tells 
 the standard boot routine to load the sector whose location is contained in 
 the following three bytes. That sector is the actual boot record for the 
 selected operating system, and it is responsible for the remainder of the 
 system's loading process (as it is from the diskette). All boot records are 
 loaded at absolute address 0:7C00.

The partition table with its offsets into the boot record is: (except for Wyse DOS 3.2 with 32 bit allocation table, and DOS 3.3-up) ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿ ³ Offset / Purpose ³ ³ Head ³ Sector ³ Cylinder ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ´ ³ 1BEh partition 1 begin ³ boot ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ´ ³ 1C2h partition 1 end ³ syst ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ´ ³ 1C6h partition 1 relative sector ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1CAh partition 1 # sectors ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ´ ³ 1CEh partition 2 begin ³ boot ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ´ ³ 1D2h partition 2 end ³ syst ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ´ ³ 1D6h partition 2 relative sector ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1DAh partition 2 # sectors ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ´ ³ 1DEh partition 3 begin ³ boot ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ´ ³ 1E2h partition 3 end ³ syst ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ´ ³ 1E6h partition 3 relative sector ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1EAh partition 3 # sectors ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ´ ³ 1EEh partition 4 begin ³ boot ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ´ ³ 1F2h partition 4 end ³ syst ind ³ H ³ S ³ cyl ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ´ ³ 1F6h partition 4 relative sector ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 1FAh partition 4 # sectors ³ low word ³ high word ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ 1FEh signature ³ hex 55 ³ hex AA ³ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÙ

HARD DISK TECHNICAL INFORMATION

Boot indicator (boot ind): The boot indicator byte must contain 0 for a non- bootable partition or 80h for a bootable partition. Only one partition can be marked as bootable at a time.

System Indicator (sys ind): The sys ind field contains an indicator of the operating system that "owns" the partition.

        The system indicators are:
                     00h unknown (unspecified or non-DOS)
                     01h DOS 12 bit FAT
                     02h DOS 16 bit FAT

Cylinder (CYL) and Sector (S): The 1 byte fields labelled CYL contain the low order 8 bits of the cylinder number - the high order 2 bits are in the high order 2 bits of the sector (S) field. This corresponds with the ROM BIOS interrupt 13h (disk I/O) requirements, to allow for a 10 bit cylinder number.

The fields are ordered in such a manner that only two MOV instructions are required to properly set up the DX and CX registers for a ROM BIOS call to load the appropriate boot record (hard disk booting is only possible from the first hard disk in the system, where a BIOS drive number of 80h corresponds to the boot indicator byte).

All partitions are allocated in cylinder multiples and begin on sector 1, head 0.

EXCEPTION: The partition that is allocated at the beginning of the disk starts at sector 2, to account for the hard disk's master boot record.

Relative Sector (rel sect): The number of sectors preceding each partition on the disk is kept in the 4 byte field labelled "rel sect". This value is obtained by counting the sectors beginning with cylinder 0, sector 1, head 0 of the disk, and incrementing the sector, head, and then track values up to the beginning of the partition. This, if the disk has 17 sectors per track and 4 heads, and the second partition begins at cylinder 1, sector 1, head 0,and the partition's starting relative sector is 68 (decimal) - there were 17 sectors on each of 4 heads on 1 track allocated ahead of it. The field is stored with the least significant word first.

Number of sectors (#sects): The number of sectors allocated to the partition is kept in the "# of sects" field. This is a 4 byte field stored least significant word first.

Signature: The last 2 bytes of the boot record (55AAh) are used as a signature to identify a valid boot record. Both this record and the partition boot record are required to contain the signature at offset 1FEh.

The master disk boot record invokes ROM BASIC if no indicator byte reflects a bootable system.

When a partition's boot record is given control. It is passed its partition table entry address in the DS:SI registers.

DETERMINING FIXED DISK ALLOCATION

DOS determines disk allocation using the following formula:

            
                                       D * BPD
                          TS - RS -  ÄÄÄÄÄÄÄÄÄÄÄ
                                         BPS
                    SPF = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
                                      BPS * SPC
                               CF + ÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
                                         BPC

where:

   TS      total sectors on disk
   RS      the number of sectrs at the beginning of the disk that are reserved
           for the boot record. DOS reserves 1 sector.
   D       The number of directory entries in the root directory.
   BPD     the number of bytes per directory entry. This is always 32.
   BPS     the number of bytes per logical sector. Typically 512, but you can
           specify a different number with VDISK.
   CF      The number of FATS per disk. Usually 2. VDISK is 1.
   SPF     the number of sectors per FAT. Maximum 64.
   SPC     The number of sectors per allocation unit.
   BPC     the number of bytes per FAT entry. BPC is 1.5 for 12 bit FATs.
           2 for 16 bit FATS.

To calculate the minimum partition size that will force a 16-bit FAT:

      CYL = (max clusters * 8)/(HEADS * SPT)

where:

   CYL     number of cylinders on the disk
   max clusters  4092 (maximum number of clusters for a 12 bit FAT)
   HEADS   number of heads on the hard disk
   SPT     sectors per track  (normally 17 on MFM)

note: DOS 2.0 uses a "first fit" algorithm when allocating file space on the hard disk. Each time an application requests disk space, it will scan from the beginning of the FAT until it finds a contiguous peice of storage large enough for the file. DOS 3.0 keeps a pointer into the disk space, and begins its search from the point it last left off. This pointer is lost when the system is rebooted. This is called the "next fit" algorithm. It is faster than the first fit and helps minimize fragmentation. In either case, if the FCB function calls are used instead of the handle function calls, the file will be broken into peices starting with the first availible space on the disk.

Comment to 826. Comment(s).


Better late than never… A partition table entry for the IBM AT is set up as follows:

      DB      drive   ; 0 or 80H, 80H marks a bootable, active partition
      DB      head1   ; starting heads
      DW      trksec1 ; starting track/sector (CX value for INT 13)
      DB      system  ; see below
      DB      head2   ; ending head
      DW      trksec2 ; ending track/sector
      DD      sector1 ; absolute # of starting sector
      DD      sector2 ; absolute # of last sector

The system byte is different for different O/S entries:

      1      DOS, 12-bit FAT entries
      4      DOS, 16-bit FAT entries
      DB     Concurrent DOS
      F2     2nd partition for Sperry machines with large disks
      And so on.  There are bytes for XENIX, Prologue and lots of other O/S. 

Many manufacturers diddle with these system bytes to implement more than 1 DOS partition per disk. The only one I know about who violates the rule that only one DOS partition (1 or 4) per disk may exist is Tandon.

CHAPTER 9

Programming Technical Reference - IBM Copyright 1988, Dave Williams

                     INSTALLABLE DEVICE DRIVERS

DEVICE DRIVER FORMAT

A device driver is a COM or EXE file that contains all of the code needed to control an add-in device. It has a special header to identify it as a device, define the strategy and interrupt entry points, and define its various attributes.

NOTE: For device drivers the COM file must not use the ORG 100h. Since the

    driver does not use the program segment prefix, it is simply loaded
    without offset. Therefore the memory image file must have an origin of 0 
    (ORG 0 or no ORG statement).

TYPES OF DEVICES

There are two types of devices: Character devices and Block devices. Their attributes are as follows:

Character devices are designed to do character I/O in a serial manner like CON, AUX, and PRN. These devices have names like CON, AUX, CLOCK$, and you can open channels (handles or FCBs) to do input and output with them. Because character devices have only one name, they can only support one device.

Block devices are the fixed disk or diskette drives on a system. They can do random I/O in peices called blocks, which are usually the physical sector size of the disk. These devices are not named as character devices are, and cannot be opened directly. Instead they are mapped by using the drive letters A,B,C etc. Block devices can have units within them. In this way, a single block driver can be responsible for one or more disk drives. For example, the first block device driver can be responsible for drives A,B,C,and D. This means it has four units defined and therefore takes up four drive letters. The position of the driver in the chain of all drives determines the way in which the drive letters correspond. For example, if the device driver is the first block driver in the device chain, and it defines four units, then these devices are called A,B,C, and D. If the second device driver defines three units, then those units are E,F,and G. DOS 1.x allows 16 devices. DOS 2.x allows 63, and DOS 3.x allows 26. It is recommended that drivers limit themselves to 26 devices for compatibility with DOS 3.x.

DOS doesn't care about the position of installed character devices versus block devices. The installed character devices get put into the chain ahead of resident character devices so that you can override the system's default driver for CON etc.

Although it is sometimes beleived that installed block devices get linked into the chain BEHIND the resident block devices, if you look at the actual device chain, this is not true (though it is true in the sense that installed block devices get assigned drive letters in sequence, starting with the next letter after the last one assigned to a resident block device).

DEVICE HEADER

A device driver requires a device header at the beginning of the file. This is the format of the device header:

              Field                           Length
      Pointer to next device header field     dword
      Attribute                               word
      Pointer to device strategy routine      word
      Pointer to device interrupt routine     word
      Name/Unit field                         8 bytes

POINTER TO NEXT DEVICE HEADER FIELD

The device header field is a pointer to the device header of the next device driver. It is a doubleword field that is set by DOS at the time the device driver is loaded. The first word is an offset and the second word is the segment. If you are loading only one device driver, set the device header field to -1 before loading the device. If you are loading more than one device driver, set the first word of the device driver header to the offset of the next device driver's header. Set the device driver header field of the last device driver to -1.

ATTRIBUTE FIELD

The attribute field is a word field that describes the attributes of the device driver to the system. The attributes are:

      word    bits (decimal)
              15      1       character device
                      0       block device
              14      1       supports IOCTL
                      0       doesn't support IOCTL
              13      1       non-IBM format (block only)
                      0       IBM format
              12      not documented - unknown
              11      1       supports removeable media
                      0       doesn't support removeable media
              10      reserved for DOS
           through
              4       reserved for DOS
              3       1       current block device
                      0       not current block device
              2       1       current NUL device
                      0       not current NUL device
              1       1       current standard output device
                      0       not current standard output device

BIT 15 is the device type bit. Use it to tell the system the that driver is a

      block or character device.

BIT 14 is the IOCTL bit. It is used for both character and block devices. Use

      it to tell DOS whether the device driver can handle control strings
      through the IOCTL function call 44h.
       If a device driver cannot process control strings, it should set bit
      14 to 0. This way DOS can return an error is an attempt is made through
      the IOCTL function call to send or receive control strings to the
      device. If a device can process control strings, it should set bit 14 
      to 1. This way, DOS makes calls to the IOCTL input and output device 
      function to send and receive IOCTL strings.
       The IOCTL functions allow data to be sent to and from the device 
      without actually doing a normal read or write. In this way, the device
      driver can use the data for its own use, (for example, setting a baud
      rate or stop bits, changing form lengths, etc.) It is up to the device
      to interpret the information that is passed to it, but the information
      must not be treated as a normal I/O request.

BIT 13 is the non-IBM format bit. It is used for block devices only. It affects

      the operation of the Get BPB (BIOS parameter block) device call.

BIT 11 is the open/close removeable media bit. Use it to tell DOS if the

      device driver can handle removeable media. (DOS 3.x only)

BIT 3 is the clock device bit. It is used for character devices only. Use it

      to tell DOS if your character device driver is the new CLOCK$ device.

BIT 2 is the NUL attribute bit. It is used for character devices only. Use it

      to tell DOS if your character device driver is a NUL device. Although
      there is a NUL device attribute bit, you cannot reassign the NUL device.
      This is an attribute that exists for DOS so that DOS can tell if the NUL
      device is being used.

BIT 0 are the standard input and output bits. They are used for character

&     devices only. Use these bits to tell DOS if your character device 

BIT 1 driver is the new standard input device or standard output device.

POINTER TO STRATEGY AND INTERRUPT ROUTINES

These two fields are pointers to the entry points of the strategy and input routines. They are word values, so they must be in the same segment as the device header.

NAME/UNIT FIELD

This is an 8-byte field that contains the name of a character device or the unit of a block device. For the character names, the name is left-justified and the space is filled to 8 bytes. For block devices, the number of units can be placed in the first byte. This is optional because DOS fills in this location with the value returned by the driver's INIT code.

CREATING A DEVICE DRIVER

To create a device driver that DOS can install, perform the following:

1) Create a memory image file or an EXE file with a device header at the start

 of the file.

2) Originate the code (including the device header) at 0, not 100h. 3) Set the next device header field. Refer to "Pointer to Next Device Header

 Attribute Field" for more information.

4) Set the attribute field of the device header. Refer to "Attribute Field" for

 more information.

5) Set the entry points for the interrupt and strategy routines. 6) Fill in the name/unit field with the name of the character device or the unit

 number of the block device.

DOS always processes installable character device drivers before handling the default devices. So to install a new CON device, simply name the device CON. Be sure to set the standard input device and standard output device bits in the attribute field of a new CON device. The scan of the device list stops on the first match so the installable device driver takes precedence.

NOTE: Because DOS can install the device driver anywhere in memory, care

    must be taken in any FAR memory references. You should not expect that 
    your driver will be loaded in the same place every time.

INSTALLING DEVICE DRIVERS

DOS installs new device drivers dynamically at boot time by reading and processing the DEVICE command in the config.sys file. For example, if you have written a device driver called DRIVER1, to install it put this command in the CONFIG.SYS file:

                           DEVICE=DRIVER1

DOS calls a device driver at its strategy entry point first, passing in a request header the information describing what DOS wants the device driver to do. This strategy routine does not perform the request but rather queues the request or saves a pointer to the request header. The second entry point is the interrupt routine and is called by DOS immediately after the strategy routine returns. The interrupt routine is called with no parameters. Its function is to perform the operation based on the queued request and set up any return infromation. DOS passes the pointer to the request header in ES:BX. This structure consists of a fixed length header (Request Header) followed by data pertinent to the operation to be performed.

NOTE: It is the responsibility of the device driver to preserve the machine

    state. For example, save all registers on entry and restore them on exit.

The stack used by DOS has enough room on it to save all the registers. If more stack space is needed, it is the device driver's responsibility to allocate and maintain another stack. All calls to execute device drivers are FAR calls. FAR returns should be executed to return to DOS.

INSTALLING CHARACTER DEVICES

One of the functions defined for each device is INIT. This routine is called only once when the device is installed and never again. The INIT routine returns the following:

A) A location to the first free byte of memory after the device driver, like a

 TSR that is stored in the terminating address field. This way, the 
 initialization code can be used once and then thrown away to save space.

B) After setting the address field, a character device driver can set the status

 word and return.

INSTALLING BLOCK DEVICES

Block devices are installed in the same way as character devices. The difference is that block devices return additional information. Block devices must also return:

A) The number of units in the block device. This number determines the logical

 names the devices will have. For example, if the current logical device 
 letter is F at the time of the install call, and the block device driver INIT
 routine returns three logical units, the letters G, H, and I are assigned to 
 the units. The mapping is determined by the position of the driver in the 
 device list and the number of units in the device. The number of units 
 returned by INIT overrides the value in the name/unit field of the device 
 header.

B) A pointer to a BPB (BIOS parameter block) pointer array. This is a pointer

 to an array of *n* word pointers there *n* is the number of units defined. 
 These word pointers point to BPBs. This way, if all of the units are the 
 same, the entire array can point to the same BPB to save space.
  The BPB contains information pertinent to the devices such as the sector 
 size, number of sectors per allocation unit, and so forth. The sector size of
 the BPB cannot be greater than the maximum allotted size set at DOS 
 initialization time.
 NOTE: This array must be protected below the free pointer set by the return.

C) The media descriptor byte. This byte is passed to devices so that they know

 what parameters DOS is currently using for a particular drive unit.

Block devices can take several approaches. They can be *dumb* or *smart*. A dumb device would define a unit (and therefore a BPB) for each possible media drive combination. Unit 0=drive 0;single side, unit 1=drive 0;double side, etc. For this approach, the media descriptor bytes would mean nothing. A smart device would allow multiple media per unit. In this case, the BPB table returned at INIT must define space large enough to acommodate the largest possible medias supported (sector size in BPB must be as large as maximum sector size DOS is currently using). Smart drivers will use the media byte to pass information about what media is currently in a unit.

REQUEST HEADER

The request header passes the information describing what DOS wants the device driver to do.

ÚÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ Length ³ F i e l d ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ BYTE ³ Length in bytes of the request header plus any data at end ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ BYTE ³ Unit code. The subunit the operation is for (minor device) ³ ³ ³ Has no meaning for character devices. ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ Command code ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 8 BYTES ³ Deserved for DOS ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³(variable)³ Data appropriate for the operation ³ ÀÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

UNIT CODE FIELD

The unit code field identifies which unit in a block device driver the request is for. For example, if a block device driver has three units defined, then the possible values of the unit code field would be 0,1,and 2.

COMMAND CODE FIELD

The command code field in the request header can have the following values:

    CODE         FUNCTION
      0       INIT
      1       MEDIA CHECK      (block only,NOP for character)
      2       BUILD BPB        (block only, NOP for character)
      3       IOCTL input      (called only if IOCTL bit is 1)
      4       INPUT            (read)
      5       NONDESTRUCTIVE INPUT NO WAIT (character devices only)
      6       INPUT STATUS     (character devices only)
      7       INPUT FLUSH      (character devices only)
      8       OUTPUT           (write)
      9       OUTPUT           (write with verify)
      10      OUTPUT STATUS    (character devices only)
      11      OUTPUT FLUSH     (character devices only)
      12      IOCTL OUTPUT     (called only if IOCTL bit is 1)
      13      DEVICE OPEN      (called only if OPEN/CLOSE/RM bit is set)
      14      DEVICE CLOSE     (called only if OPEN/CLOSE/RM bit is set)
      15      REMOVEABLE MEDIA (called only if OPEN/CLOSE/RM bit is set and
                                device is block)

NOTE: Command codes 13,14,and 15 are for use with DOS versions 3.x.

STATUS FIELD The status field in the request header contains:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ D E V I C E D R I V E R S T A T U S F I E L D ³ ÃÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ B ³ 0 ³ ³ ³ ³ 1 ³ ³ ³ Y ³ 2 ³ ³ ³ ³ 3 ³ Error message return code ³ ³ T ³ 4 ³ (with bit 15=1) ³ ³ ³ 5 ³ ³ ³ E ³ 6 ³ ³ ³ ³ 7 ³ ³ ÃÄÄÄÄÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ bit ³ 8 ³ DONE ³ ÃÄÄÄÄÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ bit ³ 9 ³ BUSY ³ ÃÄÄÄÄÄÄÄÅÄÄÄÁÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ bits ³ 10 - 14 ³ Reserved ³ ÃÄÄÄÄÄÄÄÅÄÄÄÄÂÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ bit ³ 15 ³ Error ³ ÀÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

The status word field is zero on entry and is set by the driver interrupt routine on return.

BIT 15 is the error bit. If this bit is set, the low 8 bits of the status word

      (7-0) indicate the error code.

BITS 14-10 are reserved.

BIT 9 is the busy bit. It is only set by status calls and the removable media

      call. See "STATUS" and "REMOVABLE MEDIA" in this chapter for more 
      information about the calls.

BIT 8 is the done bit. If it is set, it means the operation is complete. The

      driver sets the bit to 1 when it exits.

The low 8 bits of the status word define an error message if bit 15 is set. These errors are:

      00h  Write protect violation   01h  Unknown unit
      02h  Device not ready          03h  Unknown command
      04h  CRC error                 05h  Bad drive request structure length
      06h  seek error                07h  unknown media
      08h  sector not found          09h  printer out of paper
      0Ah  write fault               0Bh  read fault
      0Ch  general failure           0Dh  reserved
      0Eh  reserved                  0Fh  invalid disk change

DEVICE DRIVER FUNCTIONS

All strategy routines are called with ES:BX pointing to the request header. The interrupt routines get the pointers to the request header from the queue the strategy routines store them in. The command code in the request header tells the driver which function to perform. NOTE: all DWORD pointers are stored offset first, then segment.

The following function call parameters are described:

      INIT
      MEDIA CHECK
      BUILD BPB (BIOS PARAMETER BLOCK)
      MEDIA DESCRIPTOR BYTE
      INPUT OR OUTPUT
      NONDESTRUCTIVE INPUT NO WAIT
      STATUS
      FLUSH
      OPEN OR CLOSE
      REMOVABLE MEDIA

INIT Command code=0

      ES:BX   pointer to request header. Format of header:
              length           field
              13 bytes  request header
               dword    number of units (not set by character devices)
               dword    Ending address of resident program code
               dword    Pointer to BPB array (not set by character devices)
                        /pointer to remainder of arguments
                byte    Drive number (3x only)

The driver must do the following:

      A) set the number of units (block devices only)
      B) set up the pointer to the BPB array (block devices only)
      C) perform any initialization code (to modems, printers, etc)
      D) Set the ending address of the resident program code
      E) set the status word in the request header.

To obtain information obtained from CONFIG.SYS to a device driver at INIT time, the BPB pointer field points to a buffer containing the information passed from CONFIG.SYS following the =. The buffer that DOS passes to the driver at INIT after the file specification contains an ASCII string for the file OPEN. The ASCII string (ending in 0h) is terminated by a carriage return (0Dh) and linefeed (0Ah). If there is no parameter information after the file specification, the file specification is immediately followed by a linefeed (0Ah). This information is read-only and only system calls 01h-0Ch and 30h can be issued by the INIT code of the driver. The last byte parameter contains the drive letter for the first unit of a block driver. For example, 0=A, 1=B etc. If an INIT routine determines that it cannot set up the device and wants to abort without using any memory, follow this procedure:

      A) set the number of units to 0
      B) set the ending offset address at 0
      C) set the ending offsret segment address to the code segment (CS)

NOTE: If there are multiple device drivers in a single memory image file, the

    ending address returned by the last INIT called is the one DOS uses. It is
    recommended that all device drivers in a single memory image file return
    the same ending address.

MEDIA CHECK command code=1

      ES:BX   pointer to request header. Format of header:
              length          field
              13 bytes  request header
              byte      media descriptor from DOS
              byte      return
              dword     returns a pointer to the previous volume ID (if bit
                        11=1 and disk change is returned) (DOS 3.x)

When the command code field is 1, DOS calls MEDIA CHECK for a drive unit and passes its current media descriptor byte. See "Media Descriptor Byte" later in this chapter for more information about the byte. MEDIA CHECK returns one of the following:

      A) media not changed             C) not sure
      B) media changed                 D) error code

The driver must perform the following:

      A) set the status word in the request header
      B) set the return byte
             -1       media has been changed
              0       don't know if media has been changed
              1       media has not been changed

DOS 3.x: If the driver has set the removable media bit 11 of the device header attribute word to 1 and the driver returns -1 (media changed), the driver must set the DWORD pointer to the previous volume identification field. If DOS determines that the media changed is an error, DOS generates an error 0Fh (invalid disk change) on behalf of the device. If the driver does not implement volume identification support, but has bit 11 set to 1, the driver should set a pointer to the string "NO NAME",0.

MEDIA DESCRIPTOR Currently the media descriptor byte has been defined for a few media types. This byte should be idetnical to the media byte if the device has the non-IBM format bit off. These predetermined values are:

media descriptor byte ⇒ 1 1 1 1 1 0 0 0 (numerical order) 7 6 5 4 3 2 1 0

     BIT                MEANING        
      0       1=2 sided       0=not 2 sided
      1       1=8 sector      0=not 8 sector
      2       1=removeable    0=nonremoveable
     3-7      must be set to 1

Examples of current DOS media descriptor bytes:

         media      sides   sectors  ID byte
      hard disk       *       *       0F8h
      5-1/4 floppy    2       15      0F9h
      5-1/4 floppy    1       9       0FCh
      5-1/4 floppy    2       9       0FDh
      5-1/4 floppy    2       8       0FFh
      5-1/4 floppy    1       8       0FEh
      8" floppy       1       26      0FEh *
      8" floppy       2       26      0FDh
      8" floppy       2       8       0FEh *

*NOTE: The two Media Descriptor Bytes that are the same for 8" diskettes (0FEh) are not a misprint. To determine whether you are using a single sided or double sided diskette, attempt to read the second side, and if an error occurs you can assume the diskette is single sided.

BUILD BPB (BIOS Parameter Block) command code =2

      ES:BX   pointer to request header. Format:
              length          field
              13 bytes  request header
              byte      media descriptor from DOS
              dword     transfer address (buffer address)
              dword     pointer to BPB table

DOS calls BUILD BPB under the following two conditions:

A) If "media changed" is returned B) If "not sure" is returned, there are no used buffers. Used buffers are

 buffers with changed data that has not yet been written to the disk.

The driver must do the following:

A) set the pointer to the BPB B) set the status word in the request header.

The driver must determine the correct media type currently in the unit to return the pointer to the BPB table. The way the buffer is used (pointer passed by DOS) is determined by the non-IBM format bit in the attribute field of the device header. If bit 13=0 (device is IBM compatible), the buffer contains the first sector of the FAT (most importantly the FAT ID byte). The driver must not alter this buffer in this case. If bit 13=1 the buffer is a one sector scratch area which can be used for anything. For drivers that support volume identification and disk change, the call should cause a new volume identification to be read off the disk. This call indicates that the disk has been legally changed. If the device is IBM compatible, it must be true that the first sector of the first FAT is located at the same sector for all possible media. This is because the FAT sector is read before the media is actually determined. The information relating to the BPB for a particular media is kept in the boot sector for the media. In particular, the format of the boot sector is:

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ For DOS 2.x, 3 byte near jump (0E9h) For DOS 3.x, 2 byte near jump (0EBh) ³ ³ followed by a NOP (90h) ³ ÃÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ 8 bytes ³ OEM name and version ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ BYTE ³ ³ sectors per allocation unit (must be a power of 2) ³ ÃÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ B ³ reserved sectors (strarting at logical sector 0) ³ ÃÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ BYTE ³ ³ number of FATs ³ ÃÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ P ³ max number of root directory entries ³ ÃÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ ³ number of sectors in logical image (total number of ³ ³ ³ ³ sectors in media, including boot sector directories, etc.) ³ ÃÄÄÄÄÄÄÄÄÄÄ´ B ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ BYTE ³ ³ media descriptor ³ ÃÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ ³ number of sectors occupied by a single FAT ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ sectors per track ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ number of heads ³ ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ WORD ³ number of hidden sectors ³ ÀÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

The three words at the end return information about the media. The number of heads is useful for supporting different multihead drives that have the same storage capacity but a different number of surfaces. The number of hidden sectors is useful for drive partitioning schemes.

INPUT / OUTPUT command codes=3,4,8,9,and 12

      ES:BX   pointer to request header. Format:
              length          field
              13 bytes  request header
              byte      media descriptor byte
              dword     transfer address (buffer address)
              word      byte/sector count
              dword     (DOS 3.x) pointer to the volume ID if error code 0Fh
                        is returned

The driver must perform the following:

      A) set the status word in the request header
      B) perform the requested function
      C) set the actual number of sectors or bytes tranferred

NOTE: No error checking is performed on an IOCTL I/O call. However the driver

    must set the return sector or byte count to the actual number of bytes 
    transferred.

The following applies to block device drivers:

Under certain circumstances the device driver may be asked to do a write operation of 64k bytes that seems to be a *wrap around* of the transfer address in the device driver request packet. This arises due to an optimization added to write code in DOS. It will only happen in writes that are within a sector size of 64k on files that are being exetended past the current end of file. It is allowable for the device driver to ignore the balance of the write that wraps around, if it so chooses. For example, a write of 10000h bytes worth of sectors with a transfer address of XXXX:1 ignores the last two bytes.

Remember: A program that uses DOS function calls can never request an input or

        output function of more than 0FFFFh bytes, therefore, a wrap around 
        in the transfer (buffer) segment can never occur. It is for this 
        reason you can ignore bytes that would have wrapped around in the 
        tranfer segment.

If the driver returns an error code of 0Fh (invalid disk change) it must put a DWORD pointer to an ASCIIZ string which is the correct volume ID to ask the user to reinsert the disk.

DOS 3.x: The reference count of open files on the field (maintained by the OPEN and CLOSE calls) allows the driver to determine when to return error 0Fh. If there are no open files (reference count=0) and the disk has been changed, the I/O is all right, and error 0Fh is not returned. If there are open files (reference count > 0) and the disk has been changed, an error 0Fh condition may exist.

NONDESTRUCTIVE INPUT NO WAIT command code=5

      ES:BX   pointer to request header. Format:
              length          field
              13 bytes  request header
              byte      read from device

The driver must do the following:

      A) return a byte from the device
      B) set the status word in the request header.

If the character device returns busy bit=0 (characters in the buffer), then the next character that would be read is returned. This character is not removed form the buffer (hence the term nondestructive input). This call allows DOS to look ahead one character.

STATUS command codes=8 and 10

      ES:BX   pointer to a request header. Format:
              length          field
              13 bytes  request header

This driver must perform the following:

      A) perform the requested function
      B) set the busy bit
      C) set the status word in the request header.

The busy bit is set as follows:

For input on character devices: if the busy bit is 1 on return, a write request would wait for completion of a current request. If the busy bit is 0, there is no current request. Therefore, a write request would start immediately.

For input on character devices with a buffer: if the busy bit is 1 on return, a read request does to the physical device. If the busy bit is 0, there are characters in the device buffer and a read returns quickly. It also indicates that a user has typed something. DOS assumes all character devices have a type- ahead input buffer. Devices that do not have this buffer should always return busy=0 so that DOS does not hang waiting for information to be put in a buffer that does not exist.

FLUSH command codes=7 and 11

      ES:BX   pointer
              length          field
              13 bytes  request header

This call tells the driver to flush (terminate) all pending requests that it has knowledge of. Its primary use is to flush the input queue on character devices. The driver must set the status word in the request header upon return.

OPEN or CLOSE (3.x) command codes=13 and 14

      ES:BX   pointer
              length          field
              13 bytes  static request header

These calls are designed to give the device information about the current file activity on the device if bit 11 of the attribute word is set. On block devices, these calls can be used to manage local buffering. The device can keep a reference count. Every OPEN causes the device to increment the reference count. Every CLOSE causes the device to decrement the reference count. When the reference count is 0, if means there are no open files in the device. Therefore, the device should flush buffers inside the device it has written to because now the user can change the media on a removeable media drive. If the media had been changed, it is advisable to reset the reference count to 0 without flushing the buffers. This can be thought of as "last close causes flush". These calls are more useful on character devices. The OPEN call can be used to send a device initialization string. On a printer, this could cause a string to be sent to set the font, page size, etc. so that the printer would always be in a known state in the I/O stream. Similarly, a CLOSE call can be used to send a post string (like a form feed) at the end of an I/O stream. Using IOCTL to set these pre and post strings provides a flexible mechanism of serial I/O device stream control.

NOTE: Since all processes have access to STDIN,STDOUT,STDERR,STDAUX, and STDPRN

    (handles 0,1,2,3,and 4) the CON, AUX, and PRN devices are always open.

REMOVABLE MEDIA (DOS 3.x) command code=15

      ES:BX   pointer
              length          field
              13 bytes  status request header

To use this call, set bit 11 of the attribute field to 1. Block devices can only use this call through a subfunction of the IOCTL function call (44h). This call is useful because it allows a utility to know whether it is dealing with a nonremovable media drive or with a removable media drive. For example, the FORMAT utility needs to know whether a drive is removable or nonremovable because it prints different versions of some prompts.

The information is returned in the BUSY bit of the status word. If the busy bit is 1, the media is nonremovable.

NOTE: No error checking is performed. It is assumed that this call always

    succeeds.

THE CLOCK$ DEVICE

To allow a clock board to be integrated into the system for TIME and DATE, the CLOCK$ device is used. This device defines and performs functions like any other character device (most functions will be reset done bit, reset error bit, and return). When a read or write to this device occurs, 6 bytes are transferred. The first 2 bytes are a word, which is the count of days since 01-01-80. The third byte is minutes, the fourth is hours, the fifth is hundredths of a second, and the sixth is seconds. Reading the CLOCK$ device gets the date and time, writing to it sets the date and time.

CHAPTER 10

Programming Technical Reference - IBM Copyright 1988, Dave Williams

         LOTUS-INTEL-MICROSOFT  EXPANDED MEMORY SPECIFICATION

The Expanded Memory Manager …………………………………….. 10-

      History ........................................................ 10-
      Page Frames .................................................... 10-

Expanded Memory Services ……………………………………….. 10- AST/Quadram/Ashton-Tate Enhanced EMM …………………………….. 10-

      Calling the Manager ............................................ 10-
      Common EMS Functions (hex calls)
               1 (40h) Get Manager Status ............................ 10-
               2 (41h) Get Page Frame Segment ........................ 10-
               3 (42h) Get Number of Pages ........................... 10-
               4 (43h) Get Handle and Allocate Memory ................ 10-
               5 (44h) Map Memory .................................... 10-
               6 (45h) Release Handle and Memory ..................... 10-
               7 (46h) Get EMM Version ............................... 10-
               8 (47h) Save Mapping Context .......................... 10-
               9 (48h) Restore Mapping Context ....................... 10-
              10 (49h) Reserved ...................................... 10-
              11 (4Ah) Reserved ...................................... 10-
              12 (4Bh) Get Number of EMM Handles ..................... 10-
              12 (4Ch) Get Pages Owned By Handle ..................... 10-
              14 (4Dh) Get Pages for All Handles ..................... 10-
              15 (4Eh) Get Or Set Page Map ........................... 10-
     new LIM 4.0 specification:
              16 (4Fh) Get/Set Partial Page Map ...................... 10-
              17 (50h) Map/Unmap Multiple Pages ...................... 10-
              18 (51h) Reallocate Pages .............................. 10-
              19 (52h) Handle Attribute Functions .................... 10-
              20 (53h) Get Handle Name ............................... 10-
              21 (54h) Get Handle Directory .......................... 10-
              22 (55h) Alter Page Map & Jump ......................... 10-
              23 (56h) Alter Page Map & Call ......................... 10-
              24 (57h) Move Memory Region ............................ 10-
              25 (58h) Get Mappable Physical Address Array ........... 10-
              26 (59h) Get Expanded Memory Hardware .................. 10-
              27 (5Ah) Allocate Raw Pages ............................ 10-
              28 (5Bh) Get Alternate Map Register Set ................ 10-
              29 (5Ch) Prepare Expanded Memory Hardware .............. 10-
              30 (5Dh) Enable OS/E Function Set ...................... 10-
              31 (5Eh) Unknown ....................................... 10-
              32 (5Fh) Unknown ....................................... 10-
              33 (60h) Unknown ....................................... 10-
              34 (61h) AST Generic Accelerator Card Support .......... 10-

Expanded Memory Manager Error Codes ……………………………… 10-

THE EXPANDED MEMORY MANAGER

History

The Lotus/Intel/Microsoft Expanded Memory Manager was originally a Lotus and Intel project and was announced as version 3.0 in the second quarter of 1985 primarily as a means of running larger Lotus worksheets by transparently paging unused sections to bank-switched memory. Shortly afterward Microsoft announced support of the standard and version 3.2 was subsequently released with support for Microsoft Windows. LIM 3.2 supported up to 8 megabytes of paged memory. The LIM 4.0 supports up to 32 megabytes of paged memory.

AST/QUADRAM/ASHTON-TATE ENHANCED EXPANDED MEMORY SPECIFICATION

The AQA EEMS maintains upward compatibility with the LIM, but is a superset of functions.

The AQA EEMS permits its pages to be scattered throughout the unused portion of the machine's address space.

On August 19, 1987, the new version of the Expanded Memory Specification (EMS) was announced by Lotus, Intel and Microsoft. This new version of the specification includes many features of the Enhanced Expanded Memory Specification (EEMS) originally developed by AST Reserach, Quadram and Ashton- Tate, although the three original sponsoring companies elected not to make the new specification upward compatible with EEMS. AST Research says that they will endorse EMS 4.0 without reservation.

The definitive document for the LIM-EMS is Intel part number 300275-004, August, 1987.

                                                     32M ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
                                                        /³              ³
                                                         ³              ³
                                                   /     ³              ³
                                                         ³              ³
                                              /          ³              ³
                                                         ³              ³
                                         /               ³              ³
                                                         ³   Expanded   ³
                                    /                    ³    Memory    ³
        1024K ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿                           ³              ³
              ³ / / / / / /  ³  /                        ³              ³
         960K ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´                           ³              ³
              ³  Page Frame  ³                           ³              ³
              ³              ³                           ³              ³
              ³ 12 16K-Byte  ³                           ³              ³
              ³   Physical   ³                           ³              ³
              ³    Pages     ³                           ³              ³
         768K ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´                           ³ Divided into ³
              ³ / / / / / /  ³ \                         ³   logical    ³
         640K ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´                           ³    pages     ³
              ³              ³   \                       ³              ³
              ³              ³                           ³              ³
              ³              ³     \                     ³              ³
              ³              ³                           ³              ³
              ³ 24 16K-Byte  ³       \                   ³              ³
              ³   Physical   ³                           ³              ³
              ³    Pages*    ³         \                 ³              ³
              ³              ³                           ³              ³
              ³              ³           \               ³              ³
              ³              ³                           ³              ³
              ³              ³             \             ³              ³
         256K ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´                           ³              ³
              ³              ³               \           ³              ³
              ³ / / / / / /  ³                           ³              ³
              ³              ³                 \         ³              ³
              ³ / / / / / /  ³                           ³              ³
              ³              ³                   \       ³              ³
              ³ / / / / / /  ³                           ³              ³
              ³              ³                     \     ³              ³
            0 ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ                           ³              ³
                                                     \   ³              ³
                                                         ³              ³
                                                       \ ³              ³
                                                       0 ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

The page frame is located above the 640k system RAM area, anywhere from 0A000h to 0FFFFh. This area is used by the video adapters, network cards, and add-on ROMs (as in hard disk controllers). The page frames are mapped around areas that are in use.

               WRITING PROGRAMS THAT USE EXPANDED MEMORY

In order to use expanded memory, applications must perform these steps in the following order:

1. Determine if EMM is installed. 2. Determine if enough expanded memory pages exist for your application.

 (Function 3)

3. Allocate expanded memory pages. (Function 4 or 18) 4. Get the page frame base address. (Function 2) 5. Map in expanded memory pages. (Function 5 or 17) 6. Read/write/execute data in expanded memory, just as if it were conventional

 memory.

7. Return expanded memory pages to expanded memory pool before exiting. Function

 6 or 18)

Programming Guidelines

The following section contains guidelines for programmers writing applications that use EMM.

A) Do not put a program's stack in expanded memory.

B) Do not replace interrupt 67h. This is the interrupt vector the EMM uses.

 Replacing interrupt 67h could result in disabling the Expanded Memory
 Manager.

C) Do not map into conventional memory address space your application doesn't

 own. Applications that use the EMM to swap into conventional memory space,
 must first allocate this space from the operating system. If the operating
 system is not aware that a region of memory it manages is in use, it will
 think it is available. This could have disastrous results. EMM should not be
 used to "allocate" conventional memory. DOS is the proper manager of
 conventional memory space. EMM should only be used to swap data in
 conventional memory space previously allocated from DOS.

D) Applications that plan on using data aliasing in expanded memory must check

 for the presence of expanded memory hardware. Data aliasing occurs when
 mapping one logical page into two or more mappable segments. This makes one
 16K-byte expanded memory page appear to be in more than one 16K-byte memory
 address space. Data aliasing is legal and sometimes useful for applications.
  Software-only expanded memory emulators cannot perform data aliasing. A
 simple way to distinguish software emulators from actual expanded memory
 hardware is to attempt data aliasing and check the results. For example, map
 one logical page into four physical pages. Write to physical page 0. Read
 physical pages 1-3 to see if the data is there as well. If the data appears
 in all four physical pages, then expanded memory hardware is installed in the
 system, and data aliasing is supported.

E) Applications should always return expanded memory pages to the expanded

 memory manager upon termination. These pages will be made available for other
 applications. If unneeded pages are not returned to the expanded memory
 manager, the system could run out of expanded memory pages or expanded
 memory handles.

F) Terminate and stay resident programs (TSRs) should always save the state of

 the map registers before changing them. Since TSRs may interrupt other
 programs which may be using expanded memory, they must not change the state
 of the page mapping registers without first saving them. Before exiting, TSRs
 must restore the state of the map registers.
  The following sections describe the three ways to save and restore the state
 of the map registers.
 1) Save Page Map and Restore Page Map (Functions 8 and 9). This is the
    simplest of the three methods. The EMM saves the map register contents in
    its own data structures -- the application does not need to provide extra
    storage locations for the mapping context. The last mapping context to be
    saved, under a particular handle, will be restored when a call to Restore
    Page Map is issued with the same handle. This method is limited to one
    mapping context for each handle and saves the context for only LIM
    standard 64K-byte page frames.
 2) Get/Set Page Map (Function 15). This method requires the application to
    allocate space for the storage array. The EMM saves the mapping context in
    an array whose address is passed to the EMM. When restoring the mapping
    context with this method, an application passes the address of an array
    which contains a previously stored mapping context. This method is
    preferable if an application needs to do more than one save before a
    restore. It provides a mechanism for switching between more than one
    mapping context.
 3) Get/Set Partial Page Map (Function 16). This method provides a way for
    saving a partial mapping context. It should be used when the application
    does not need to save the context of all mappable memory. This function
    also requires that the storage array be part of the application's data.

G) All functions using pointers to data structures must have those data

 structures in memory which will not be mapped out. Functions 22 and 23
 (Alter Map & Call and Alter Map & Jump) are the only exceptions.

EMS 4.0 SPECIFICATIONS

Page Frames

The bank switched memory chunks are referred to as "page frames". These frame consist of four 16K memory blocks mapped into some of the normally unused system ROM address area, 0C0000-0EFFFF. Each 16K page is independent of the other and they can map to discrete or overlapping areas of the 8 megabyte expanded memory address area. Most cards allow selection of addresses to prevent conflict with other cards, such as hard disk controllers and other expanded memory boards.

Calling the Manager

Applications programs communicate with the EMM device driver directly via user interrupt 67h. All communication between the application program and the driver bypasses DOS completely. To call the driver, register AH is loaded with the number of the EMM service requested; DX is loaded with the file handle; and interrupt 67h is called. ES:DI is used to pass the address of a buffer or array if needed. On return AH contains 0 if the call was successful or an error code from 80h to 8Fh if unsuccessful.

        TESTING FOR THE PRESENCE OF THE EXPANDED MEMORY MANAGER

Before an application program can use the Expanded Memory Manager, it must determine whether the manager is present. The two recommended methods are the "open handle" technique and the "get interrupt vector" technique.

The majority of application programs can use either the "open handle" or the "get interrupt vector" method. However, if your program is a device driver or if it interrupts DOS during file system operations, you must use only the "get interrupt vector" method.

Device drivers execute from within DOS and can't access the DOS file functions; programs that interrupt DOS during file operations have a similar restriction. During their interrupt processing procedures, they can't access the DOS file functions because another program may be using the system. Since the "get interrupt vector" method doesn't require the DOS file functions, you must use it for programs of this type.

The "Open Handle" Method

Most application programs can use the DOS "Open Handle" method to test for the presence of the EMM. To use this method, follow these steps in order:

1) Issue an "open handle" command (DOS function 3Dh) in "read only" access mode

 (register AL = 0). This function requires your program to point to an ASCII
 string which contains the path name of the file or device in which you're
 interested (register set DS:DX contains the pointer). In this case the file
 is actually the reserved name of the expanded memory manager.
 you should format the ASCII string as follows:
 ASCII_device_name  DB  'EMMXXXX0', 0
 The ASCII codes for the capital letters EMMXXXX0 are terminated by a byte
 containing a value of zero.

2) If DOS returns no error code, skip Steps 3 and 4 and go to Step 5. If DOS

 returns a "Too many open files" error code, go to Step 3. If DOS returns a
 "File/Path not found" error code, skip Step 3 and go to Step 4.

3) If DOS returns a "Too many open files" (not enough handles) status code, your

 program should invoke the "open file" command before it opens any other
 files. This will guarantee that at least one file handle will be available to
 perform the function without causing this error.
  After the program performs the "open file" command, it should perform the
 test described in Step 6 and close the "file handle" (DOS function 3Eh).
 Don't keep the manager "open" after this status test is performed since
 "manager" functions are not available through DOS. Go to Step 6.

4) If DOS returns a "File/Path not found," the memory manager is not installed.

 If your application requires the memory manager, the user will have to reboot
 the system with a disk containing the memory manager and the appropriate
 CONFIG.SYS file before proceeding.

5) If DOS doesn't return an error status code you can assume that either a

 device with the name EMMXXXX0 is resident in the system, or a file with this
 name is on disk in the current disk drive. Go to Step 6.

6) Issue an "I/O Control for Devices" command (DOS function 44h) with a "get

 device information" command (register AL = 0). DOS function 44h determines
 whether EMMXXXX0 is a device or a file.
  You must use the file handle (register BX) which you obtained in Step 1 to
 access the "EMM" device.
 This function returns the "device information" in a word (register DX).
 Go to Step 7.

7. If DOS returns any error code, you should assume that the memory manager

 device driver is not installed. If your application requires the memory
 manager, the user will have to reboot the system with a disk containing the
 memory manager and the appropriate CONFIG.SYS file before proceeding.

8) If DOS didn't return an error status, test the contents of bit 7 (counting

 from 0) of the "device information" word (register DX) the function
 returned. Go to Step 9.

9) If bit 7 of the "device information" word contains a zero, then EMMXXXX0 is

 a file, and the memory manager device driver is not present. If your
 application requires the memory manager, the user will have to reboot the
 system with a disk containing the memory manager and the appropriate
 CONFIG.SYS file before proceeding.
  If bit 7 contains a one, then EMMXXXX0 is a device. Go to Step 10.

10) Issue an "I/O Control for Devices" command (DOS function 44h) with a "get

  output status" command (register AL = 7). You must use the file handle you
  obtained in Step 1 to access the "EMM" device (register BX). Go to Step 11.

11) If the expanded memory device driver is ready, the memory manager passes

  a status value of 0FFh in register AL. The status value is 00h if the device
  driver is not ready.
   If the memory manager device driver is "not ready" and your application
  requires its presence, the user will have to reboot the system with a disk
  containing the memory manager and the appropriate CONFIG.SYS file before
  proceeding.
   If the memory manager device driver is "ready," go to Step 12.

12) Issue a "Close File Handle" command (DOS function 3Eh) to close the expanded

  memory device driver. You must use the file handle you obtained in Step 1 to
  close the "EMM" device (register BX).

The "Get Interrupt Vector" technique

Any type of program can use this method to test for the presence of the EMM.

Use this method (not the "Open Handle" method) if your program is a device driver or if it interrupts DOS during file system operations.

Follow these steps in order:

1) Issue a "get vector" command (DOS function 35h) to obtain the contents of

 interrupt vector array entry number 67h (addresses 0000:019Ch thru
 0000:019Fh).
  The memory manager uses this interrupt vector to perform all manager
 functions. The offset portion of this interrupt service routine address is
 stored in the word located at address 0000:019Ch; the segment portion is
 stored in the word located at address 0000:019Eh.

2) Compare the "device name field" with the contents of the ASCII string which

 starts at the address specified by the segment portion of the contents of
 interrupt vector address 67h and a fixed offset of 000Ah. If DOS loaded the
 memory manager at boot time this name field will have the name of the device
 in it.
  Since the memory manager is implemented as a character device driver, its
 program origin is 0000h. Device drivers are required to have a "device
 header" located at the program origin. Within the "device header" is an 8
 byte "device name field." For a character mode device driver this name field
 is always located at offset 000Ah within the device header. The device name
 field contains the name of the device which DOS uses when it references the
 device.
  If the result of the "string compare" in this technique is positive, the
 memory manager is present.

Terminate and Stay Resident (TSR) Program Cooperation: In order for TSR's to cooperate with each other and with other applications, TSRs must follow this rule: a program may only remap the DOS partition it lives in. This rule applies at all times, even when no expanded memory is present.

EXPANDED MEMORY SERVICES

FUNCTIONS DEFINED IN EMS 3.2 SPECIFICATION

Interrupt 67h

Function 40h Get Manager Status LIM Function Call 1

           Returns a status code indicating whether the memory manager is
           present and the hardware is working correctly.

entry AH 40h return AH error status: 00h, 80h, 81h, 84h note 1) upward and downward compatible with both EMS and EEMS 3.2.

      this call can be used only after establishing that the EMS driver is in
      fact present
   2) uses register AX

Function 41h Get Page Frame Segment LIM Function Call 2

           Obtain segment address of the page frame used by the EMM.

entry AH 41h return AH error status: 00h, 80h, 81h, 84h

      BX      page frame segment address (error code 0)

note 1) upward and downward compatible with both EMS and EEMS 3.2.

   2) uses registers AX & BX

Function 42h Get Unallocated Page Count LIM Function Call 3

           Obtain total number of logical expanded memory pages present in
           the system and the number of those pages not already allocated.

entry AH 42h return AH error status: 00h, 80h, 81h, 84h

      BX      number of unallocated pages currently availible
      DX      total number of pages

note 1) upward and downward compatible with both EMS and EEMS 3.2. Note that EMS

      and EEMS 3.2 had no mechanism to return the maximum number of handles
      that can be allocated by programs. This is handled by the EMS 4.0 new
      function 54h/02h.
   2) uses registers AX, BX, DX

Function 43h Get Handle and Allocate Memory LIM Function Call 4

           Notifies the EMM that a program will be using extended memory,
           obtains a handle, and allocates a certain number of logical pages
           of extended memory to be controlled by that handle

entry AH 43h

      BX      number of 16k logical pages requested (zero OK)

return AH error status: 00h, 80h, 81h, 84h, 85h, 87h, 88h, 89h

      DX      unique EMM handle (see note 2)

note 1) upward compatible with both EMS and EEMS 3.2; EMS and EEMS 3.2 do not

      allow the allocation of zero pages (returns error status 89h). EMS 4.0
      does allow zero pages to be requested for a handle, allocating pages
      later using function 51h
   2) your program must use this EMM handle as a parameter in any function
      that requires it. You can use up to 255 handles. The uppermost byte of
      the handle will be zero and cannot be used by the application.
   3) regs AX & DX are used

Function 44h Map Memory LIM Function Call 5

           Maps one of the logical pages of expanded memory assigned to a
           handle onto one of the four physical pages within the EMM's page
           frame.

entry AH 44h

      AL      physical page to be mapped (0-3)
      BX      the logical page to be mapped (zero through [number of pages
              allocated to the EMM handle - 1]). If the logical page number
              is 0FFFFh, the physical page specified in AL will be unmapped
              (made inaccessible for reading or writing).
      DX      the EMM handle your program received from Function 4 (Allocate
              Pages).

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh note 1) downward compatible with both EMS and EEMS 3.2; EMS and EEMS 3.2 do not

      support unmap (logical page 0FFFFh) capability. Also, EEMS 3.2
      specified there were precisely four physical pages; EMS 4.0 uses the
      subfunctions of function 58h to return the permitted number of physical
      pages. This incorporates the functionality of function 69h ("function
      42") of EEMS.
   2) uses register AX

Function 45h Release Handle and Memory LIM Function Call 6

           Deallocates the logical pages of expanded memory currently
           assigned to a handle and then releases the handle itself.

entry AH 45h

      DX      handle

return AH error status: 00h, 80h, 81h, 83h, 84h, 86h note 1) upward and downward compatible with both EMS and EEMS 3.2.

   2) uses register AX
   3) when a handle is deallocated, its name is set to all ASCII nulls
      (binary zeros).
   4) a program must perform this function before it exits to DOS or no other
      programs can use these pages or the EMM handle.

Function 46h Get EMM Version LIM Function Call 7

           Returns the version number of the Expanded Memory Manager software.

entry AH 46h return AH error status: 00h, 80h, 81h, 84h

      AL      version number byte (if AL=00h)
              binary coded decimal (BCD) format if version byte:
              high nibble: integer digit of the version number
              low nibble : fractional digit of version number
              i.e., version 4.0 is represented like this:
                        0100 0000
                          /   \
                         4  .  0

note 1) upward and downward compatible with both EMS and EEMS 3.2. It appears

      that the intended use for this function is to return the version of the
      vendor implementation of the expanded memory manager instead of the
      specification version.
   2) uses register AX

Function 47h Save Mapping Context LIM Function Call 8

           Save the contents of the expanded memory page-mapping registers on
           the expanded memory boards, associating those contents with a
           specific EMM handle.

entry AH 47h

      DX      caller's EMM handle (NOT current EMM handle)

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ch, 8Dh note 1) upward and downward compatible with both EMS and EEMS 3.2.

   2) This only saves the context saved in EMS 3.2 specification; if a driver,
      interrupt routine or TSR needs to do more, functions 4Eh (Page Map
      functions) or 4Fh (Partial Page Map functions) should be used.
   3) no mention is made about the number of save contexts to provide. AST
      recommends in their Rampage AT manual one save context for each handle
      plus one per possible interrupt (5 + <handles>).
   4) uses register AX
   5) this function saves the state of the map registers for only the 64K page
      frame defined in versions 3.x of the LIM. Since all applications written
      to LIM versions 3.x require saving the map register state of only this
      64K page frame, saving the entire mapping state for a large number of
      mappable pages would be inefficient use of memory. Applications that use
      a mappable memory region outside the LIM 3.x page frame should use
      functions 15 or 16 to save and restore the state of the map registers.

Function 48h Restore Page Map LIM Function Call 9

           Restores the contents of all expanded memory hardwere page-mapping
           registers to the values associated with the given handle by a
           previous function 08h (Save Mapping Context).

entry AH 48h

      DX      caller's EMM handle (NOT current EMM handle)

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Eh note 1) upward and downward compatible with both EMS and EEMS 3.2.

   2) This only restores the context saved in EMS 3.2 specification; if a
      driver, interrupt routine or TSR needs to do more, functions 4Eh (Page
      Map functions) or 4Fh (Partial Page Map functions) should be used.
   3) uses register AX
   4) this function saves the state of the map registers for only the 64K page
      frame defined in versions 3.x of the LIM. Since all applications written
      to LIM versions 3.x require saving the map register state of only this
      64K page frame, saving the entire mapping state for a large number of
      mappable pages would be inefficient use of memory. Applications that use
      a mappable memory region outside the LIM 3.x page frame should use
      functions 15 or 16 to save and restore the state of the map registers.

Function 49h Reserved LIM Function Call 10

           This function was used in EMS 3.0, but was no longer documented in
           EMS 3.2. It formerly returned the page mapping register I/O port
           array. Use of this function is discouraged, and in EMS 4.0 may
           conflict with the use of the new functions 16 through 30 (4Fh
           through 5Dh) and functions 10 and 11. Functions 10 and 11 are
           specific to the hardware on Intel expanded memory boards and may
           not work correctly on all vendors' expanded memory boards.

Function 4Ah Reserved LIM Function Call 11

           This function was used in EMS 3.0, but was no longer documented in
           EMS 3.2. It was formerly Get Page Translation Array. Use of this
           function is discouraged, and in EMS 4.0 may conflict with the use
           of the new functions (4Fh through 5Dh).

Function 4Bh Get Number of EMM Handles LIM Function Call 12

           The Get Handle Count function returns the number of open EMM
           handles (including the operating system handle 0) in the system.

entry AH 4Bh return AH error status: 00h, 80h, 81h, 84h

      BX      handle count (AH=00h) (including the operating system handle
              [0]). max 255.

note 1) upward and downward compatible with EMS and EEMS 3.2.

   2) uses registers AX and BX

Function 4Ch Get Pages Owned by Handle LIM Function Call 13

           Returns number of logical expanded memory pages allocated to a
           specific EMM handle.

entry AH 4Ch

      DX      handle

return AH error status: 00h, 80h, 81h, 83h, 84h

      BX      pages allocated to handle, max 2048 because the EMM allows a
              maximum of 2048 pages (32M bytes) of expanded memory.

note 1) This function is upward compatible with EMS and EEMS 3.2.

   2) programmers should compare the number returned in BX with the maximum
      number of pages returned by function 42h register DX, total number of
      EMM pages. This should be an UNSIGNED comparison, just in case the spec
      writers decide to use 16 bit unsigned numbers (for a maximum space of
      one gigabyte) instead of signed numbers (for a maximum space of 512
      megabytes). Unsigned comparisons will work properly in either case
   3) uses registers AX and BX

Function 4Dh Get Pages for All Handles LIM Function Call 14

           Returns an array containing all active handles and the number of
           logical expanded memory pages associated with each handle.

entry AH 4Dh

      ES:DI   pointer to 1020 byte array to receive information on an array of
              structures where a copy of all open EMM handles and the number
              of pages allocated to each will be stored.

return AH error status: 00h, 80h, 81h, 84h

      BX      number of active handles (1-255); array filled with 2-word
              entries, consisting of a handle and the number of pages
              allocated to that handle. (including the operating system handle
              [0]). BX cannot be zero because the operating system handle is
              always active and cannot be deallocated.

note 1) NOT COMPATIBLE with EMS or EEMS 3.2, since the new special OS handle

      0000h is returned as part of the array. Unless benign use of this
      information is used (such as displaying the handle and count of pages
      associated with the handle) code should be changed to only work with
      handles between 01h and FFh and to specifically ignore handle 00h.
   2) The array consists of an array of 255 elements. The first word of each
      element is the handle number, the second word contains the number of
      pages allocated.
   3) There are two types of handles, "standard" and "raw". The specification
      does not talk about how this function works when both raw and standard
      handles exist in a given system. There is no currently known way to
      differentiate between a standard handle and a raw handle in EMS 4.0.
   4) uses registers AX and BX

Function 4Eh Get or Set Page Map LIM Function Call 15

           Gets or sets the contents of the EMS page-mapping registers on the
           expanded memory boards.
            This group of four subfunctions is provided for context switching
           required by operating environments and systems. These functions are
           upward and downward compatible with both EMS and EEMS 3.2; in
           addition, these functions now include the functionality of EEMS
           function 6Ah ("function 43") involving all pages.
            The size and contents of the map register array will vary from
           system to system based on hardware vendor, software vendor, number
           of boards and the capacity of each board in the system. Note the
           array size can be determined by function 4Eh/03h.
            Use these functions (except for 03h) instead of Functions 8 and 9
           if you need to save or restore the mapping context but don't want
           (or have) to use a handle.
      00h  Get Page Map
           This call saves the mapping context for all mappable memory regions
           (conventional and expanded) by copying the contents of the mapping
           registers from each expanded memory board to a destination array.
           The application must pass a pointer to the destination array.

entry AH 4Eh

      AL      00h
      ES:DI   pointer to target array

return AH error status: 00h, 80h, 81h, 84h, 8Fh note 1) uses register AX

   2) does not use an EMM handle
       01h  Set Page Map
           This call the mapping context for all mappable memory regions
           (conventional and expanded) by copying the contents of a source
           array into the mapping registers on each expanded memory board in
           the system. The application must pass a pointer to the source array.

entry AH 4Eh

      AL      01h
      DS:SI   pointer to source array

return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h note 1) uses register AX

   2) does not use an EMM handle
      02h  Get & Set Page Map
           This call simultaneously saves the current mapping context and
           restores a previous mapping context for all mappable memory regions
           (both conventional and expanded). It first copies the contents of
           the mapping registers from each expanded memory board in the system
           into a destination array. Then the subfunction copies the contents
           of a source array into the mapping registers on each of the
           expanded memory boards.

entry AH 4Eh

      AL      02h
      DS:SI   pointer to source array
      ES:DI   pointer to target array

return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h note 1) uses register AX

        03h  Get Size of Page Map Save Array

entry AH 4Eh

      AL      03h

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      AL      size in bytes of array

note 1) this subfunction does not require an EMM handle

   2) uses register AX

FUNCTIONS NEW TO EMS 4.0

Function 4Eh Get or Set Page Map LIM Function Call 16 entry AH 4Eh

      AL      00h     if getting mapping registers
              01h     if setting mapping registers
              02h     if getting and setting mapping registers at once
              03h     if getting size of page-mapping array
      DS:SI   pointer to array holding information (AL=01/02)
      ES:DI   pointer to array to receive information (AL=00/02)

return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h note 1) this function was designed to be used by multitasking operating systems

      and should not ordinarily be used by appplication software.

Function 4Fh Get/Set Partial Page Map LIM Function Call 16

           These four subfunctions are provided for context switching required
           by interrupt routines, operating environments and systems. This set
           of functions provides extended functionality over the EEMS function
           6Ah (function 43) involving subsets of pages. In EEMS, a subset of
           pages could be specified by starting position and number of pages;
           in this function a list of pages is specified, which need not be
           contiguous.
            Interrupt routines can use this function in place of functions 47h
           and 48h, especially if the interrupt routine wants to use more than
           the standard four physical pages.
      AH      4Fh
      AL      subfunction
              00h     get partial page map
                      DS:SI   pointer to structure containing list of
                              segments whose mapping contexts are to be saved
                      ES:DI   pointer to array to receive page map
              01h     set partial page map
                      DS:SI   pointer to structure containing saved partial
                              page map
              02h     get size of partial page map
                      BX      number of mappable segments in the partial map
                              to be saved

return AH error status (00h): 00h, 80h, 81h, 84h, 8Bh, 8Fh, 0A3h

              error status (01h): 00h, 80h, 81h, 84h, 8Fh, 0A3h
              error status (02h): 00h, 80h, 81h, 84h, 8Bh, 8Fh
      AL      size of partial page map for subfunction 02h
      DS:SI   (call 00h) pointer to array containing the partial mapping
              context and any additional information necessary to restore this
              context to its original state when the program invokes a Set
              subfunction.

note uses register AX

Function 50h Map/Unmap Multiple Pages LIM Function Call 17 entry AH 50h

      AL      00h     (by physical page)
              01h     (by segment number)
      CX      contains the number of entries in the array. For example, if the
              array contained four pages to map or unmap, then CX would
              contain 4.
      DX      handle
      DS:SI   pointer to an array of structures that contains the information
              necessary to map the desired pages.

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh note 1) New function permits multiple logical-to-physical assignments to be made

      in a single call.(faster than mapping individual pages)
   2) The source map array is an array of word pairs. The first word of a
      pair contains the logical page to map (0FFFFh if the physical page is
      to be totally unmapped) and the second word of a pair contains the
      physical page number (subfunction 00h) or the segment selector
      (subfunction 01h) of the physical page in which the logical page shall
      be mapped.
   3) A map of available physical pages (by physical page number and segment
      selectors) can be obtained using function 58h/00h, Get Mappable
      Physical Address Array.
   4) uses register AX
   5) Both mapping and unmapping pages can be done simultaneously.
   6) If a request to map or unmap zero pages is made, nothing is done and no
      error is returned.
   7) Pages can be mapped or unmapped using one of two methods. Both methods
      produce identical results.
       A) A logical page and a physical page at which the logical page is to
          be mapped. This method is an extension of Function 5 (Map Handle
          Page).
       B) Specifys both a logical page and a corresponding segment address at
          which the logical page is to be mapped. While functionally the same
          as the first method, it may be easier to use the actual segment
          address of a physical page than to use a number which only
          represents its location. The memory manager verifies whether the
          specified segment address falls on the boundary of a mappable
          physical page. The manager then translates the segment address
          passed to it into the necessary internal representation to map the
          pages.

Function 51h Reallocate pages LIM Function Call 18

           This function allows an application to change the number of logical
           pages allocated to an EMM handle.

entry AH 51h

      BX      number of pages desired at return
      DX      handle

return AH error status: 00h, 80h, 81h, 83h, 84h, 87h, 88h

      BX      number of pages now associated with handle

note 1) uses registers AX, BX

   2) Logical pages which were originally allocated with Function 4 are called
      pages and are 16K bytes long. Logical pages which were allocated with
      Function 27 are called raw pages and might not be the same size as pages
      allocated with Function 4.
   3) If the status returned in BX is not zero, the value in BX is equal to
      the number of pages allocated to the handle prior to calling this
      function. This information can be used to verify that the request
      generated the expected results.

Function 52h Get/Set Handle Attributes LIM Function Call 19 entry AH 52h

      AL      subfunction
              00h     get handle attributes
              01h     set handle attributes
                      BL      new attribute
                              00h     make handle volatile
                              01h     make handle non-volatile
              02h     get attribute capability
      DX      handle

return AH error status: (function 00h) 00h, 80h, 81h, 83h, 84h, 8Fh, 91h

              error status: (function 01h) 00h, 80h, 81h, 83h, 84h, 8Fh, 90h,
                                           91h
              error status: (function 02h) 00h, 80h, 81h, 84h, 8Fh
      AL      attribute (for subfunction 00h)
              00h     handle is volatile
              01h     handle is nonvolatile
      AL      attribute capability (for subfunction 02h)
              00h     only volatile handles supported
              01h     both volatile and non-volatile supported

note 1) uses register AX

   2) A volatile handle attribute instructs the memory manager to deallocate
      both the handle and the pages allocated to it after a warm boot. If all
      handles have the volatile attribute (default) at warm boot the handle
      directory will be empty and all expanded memory will be initialized to
      zero immediately after a warm boot.
   3) If the handle's attribute has been set to non-volatile, the handle, its
      name (if it is assigned one), and the contents of the pages allocated to
      the handle are all maintained after a warm boot.
   4) Most PCs disable RAM refresh signals for a considerable period during a
      warm boot. This can corrupt some of the data in memory boards. Non-
      volatile handles should not be used unless it is definitely known that
      the EMS board will retain proper function through a warm boot.
   5) subfunction 02h can be used to determine whether the memory manager can
      support the non-volatile attribute.
   6) Currently the only attribute supported is non-volatile handles and
      pages, indicated by the least significant bit.

Function 53h Handle Name Functions LIM Function Call 20

           EMS handles may be named. Each name may be any eight characters.
           At installation, all handles have their name initialized to ASCII
           nulls (binary zeros). There is no restriction on the characters
           which may be used in the handle name (ASCII chars 00h through
           0FFh). A name of eight nulls (zeroes) is special, and indicates a
           handle has no name. Nulls have no special significance, and they
           can appear in the middle of a name. The handle name is 64 bits of
           binary information to the EMM.
            Functions 53h and 54h provide a way of setting and reading the
           names associated with a particular handle. Function 53h manipulates
           names by number.
            When a handle is assigned a name, at least one character in the
           name must be a non-null character in order to distinguish it from
           a handle without a name.
      00h  Get Handle Name
           This subfunction gets the eight character name currently
           assigned to a handle.
            The handle name is initialized to ASCII nulls (binary zeros)
           three times:  when the memory manager is installed, when a handle
           is allocated, and when a handle is deallocated.

entry AH 53h

      AL      00h
      DX      handle
      ES:DI   pointer to 8-byte handle name array into which the name
              currently assigned to the handle will be copied.

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Fh note uses register AX

      01h  Set Handle Name
           This subfunction assigns an eight character name to a handle.
           A handle can be renamed at any time by setting the handle's
           name to a new value. When a handle is deallocated, its name is
           removed (set to ASCII nulls).

entry AH 53h

      AL      01h
      DX      handle
      DS:SI   pointer to 8-byte handle name array that is to be assigned to
              the handle. The handle name must be padded with nulls if the
              name is less than eight characters long.

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Fh, 0A1h note uses register AX

Function 54h Handle Directory Functions LIM Function Call 21

           Function 54h manipulates handles by name.
      00h  Get Handle Directory
           Returns an array which contains all active handles and the names
           associated with each.

entry AH 54h

      AL      00h
      ES:DI   pointer to 2550 byte target array

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      AL      number of active handles

note 1) The name array consists of 10 byte entries; each entry has a word

      containing the handle number, followed by the eight byte (64 bit) name.
   2) uses register AX
   3) The number of bytes required by the target array is:
              10 bytes * total number of handles
   4) The maximum size of this array is:
              (10 bytes/entry) * 255 entries = 2550 bytes.
      01h  Search for Named Handle
           Searches the handle name directory for a handle with a particular
           name. If the named handle is found, this subfunction returns the
           handle number associated with the name.

entry AH 54h

      AL      01h
      DS:SI   pointer to an 8-byte string that contains the name of the
              handle being searched for

return AH error status: 00h, 80h, 81h, 84h, 8Fh, A0h, 0A1h

      DX      handle number

note 1) uses registers AX and DX

      02h  Get Total Handles
           Returns the total number of handles the EMM supports, including
           the operating system handle (handle value 0).

entry AH 54h

      AL      02h

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      BX      total number of handles availible

note 1) This is NOT the current number of handles defined, but the maximum

      number of handles that can be supported in the current environment.
   2) uses registers AX and BX

Function 55h Alter Page Map and Jump (cross page branch) LIM Function Call 22

           Alters the memory mapping context and transfers control to the
           specified address. Analogous to the FAR JUMP in the 8086 family
           architecture. The memory mapping context which existed before
           calling function is lost.

entry AH 55h

      AL      00h     physical page numbers provided by caller
              01h     segment addresses provided by caller
      DX      handle
      DS:SI   pointer to structure containing map and jump address

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh note 1) Flags and all registers except AX are preserved across the jump.

   2) uses register AX
   3) Values in registers which don't contain required parameters maintain the
      values across the jump. The values in registers (with the exception of
      AX) and the flag state at the beginning of the function are still in the
      registers and flags when the target address is reached.
   4) Mapping no pages and jumping is not considered an error. If a request to
      map zero pages and jump is made, control is transferred to the target
      address, and this function performs a far jump.

Function 56h Alter Page Map and Call (cross page call) LIM Function Call 23

      00h and 01h
             This subfunction saves the current memory mapping context,
             alters the specified memory mapping context, and transfers
             control to the specified address.

entry AH 56h

      AL      00h physical page numbers provided by caller
              01h segment addresses provided by caller
      DS:SI   pointer to structure containing page map and call address
      DX      handle

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh note 1) Flags and all registers except AX are preserved to the called routine.

      On return, flags and all registers except AX are preserved; AL is set to
      zero and AX is undefined.
   2) uses register AX
   3) Values in registers which don't contain required parameters maintain
      the values across the call. The values in registers (with the exception
      of AX) and the flag state at the beginning of the function are still in
      the registers and flags when the target address is reached.
   4) Developers using this subfunction must make allowances for the
      additional stack space this subfunction will use.
      02h  Get Page Map Stack Space Size
           Since the Alter Page Map & Call function pushes additional
           information onto the stack, this subfunction returns the number of
           bytes of stack space the function requires.

entry AH 56h

      AL      02h

return: BX number of bytes of stack used per call

      AH      error status: 00h, 80h, 81h, 84h, 8Fh

note 1) if successful, the target address is called. Use a RETF to return and

      restore mapping context
   2) uses registers AX, BX

Function 57h Move/Exchange Memory Region LIM Function Call 24

      00h  Move Memory Region
           Moves data between two memory areas. Includes moves between paged
           and non-paged areas, or between two different paged areas.

entry AH 57h

      AL      00h
      SI      offset to request block
      DS      segment selector to request block

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Fh, 92h, 93h, 94h,

              95h, 96h, 98h, 0A2h

note 1) uses register AX

      01h  Exchange Memory Region
           Exchanges data between two memory areas. Includes exchanges between
           paged and non-paged areas, or between two different paged areas.

entry AH 57h

      AL      01h
      DS:SI   pointer to the data structure which contains the source and
              destination information for the exchange.

return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Fh, 93h, 94h, 95h,

              96h, 97h, 98h, 0A2h

note 1) The request block is a structure with the following format:

      dword   region length in bytes
      byte    0=source in conventional memory
              1=source in expanded memory
      word    source handle
      word    source offset in page or selector
      word    source logical page (expanded) or selector (conventional)
      byte    0=target in conventional memory
              1=target in expanded memory
      word    target handle
      word    target offset in page or selector
      word    target logical page (expanded) or selector (conventional)
   2) Expanded memory allocated to a handle is considered to be a linear
      array, starting from logical page 0 and progressing through logical page
      1, 2, ... n, n+1, ... up to the last logical page in the handle.
   3) uses register AX

Function 58h Mappable Physical Address Array LIM Function Call 25

           These functions let you obtain a complete map of the way physical
           memory is laid out in a vendor independent manner. This is a
           functional equivalent of EEMS function 68h ("function 41"). EEMS
           function 60h ("function 33") is a subset call of 68h.
      00h  Get Array
           Returns an array containing the segment address and physical page
           number for each mappable physical page in a system. This array
           provides a cross reference between physical page numbers and the
           actual segment addresses for each mappable page in the system.

entry AH 58h

      AL      00h
      ES:DI   pointer to target array

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      CX      entries in target array

note 1) The information returned is in an array composed of word pairs. The

      first word is the physical page's segment selector, the second word the
      physical page number. Note that values are not necessarily returned in a
      particular order, either ascending/decending segment selector values or
      as ascending/decending physical page number.
   2) For compatibility with earlier EMS specifications, physical page zero
      contains the segment selector value returned by function 41h, and
      physical pages 1, 2 and 3 return segment selector values that corrospond
      to the physical 16 KB blocks immediately following physical page zero.
   3) uses registers AX and CX
   4) The array is sorted in ascending segment order. This does not mean that
      the physical page numbers associated with the segment addresses are
      also in ascending order.
      01h   Get Physical Page Address Array Entries.
            Returns a word which represents the number of entries in the
            array returned by the previous subfunction. This number also
            indicates the number of mappable physical pages in a system.

entry AH 58h

      AL      01h

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      CX      number of entries returned by 58h/00h

note 1) multiply CX by 4 for the byte count.

   2) uses registers AX and CX

Function 59h Get Expanded Memory Hardware Information LIM Function Call 26

           These functions return information specific to a given hardware
           implementation and to use of raw pages as opposed to standard
           pages. The intent is that only operating system code ever need use
           these functions.
      00h  Get EMS Hardware Info
           Returns an array containing expanded memory hardware configuration
           information for use by an operating system.

entry AH 59h

      AL      00h
      ES:DI   pointer to 10 byte target array
              The target array has the following format:
              word: raw page size in paragraphs (multiples of 16 bytes)
              word: number of alternate register sets
              word: size of page maps (function 4Eh [15])
              word: number of alternate registers sets for DMA
              word: DMA operation -- see full specification

return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A4h note 1) uses register AX

   2) This function is for use by operating systems only.
   3) This function can be disabled at any time by the operating system.
      01h  Get Unallocated Raw Page Count
           Returns the number of unallocated non-standard length mappable
           pages as well as the total number of non-standard length mappable
           pages of expanded memory

entry AH 59h

      AL      01h

return AH error status: 00h, 80h, 81h, 84h, 8Fh

      BX      unallocated raw pages availible for use
      DX      total raw 16k pages of expanded memory

note 1) uses registers AX, BX, CX

   2) An expanded memory page which is a sub-multiple of 16K is termed a raw
      page. An operating system may deal with mappable physical page sizes
      which are sub-multiples of 16K bytes.
   3) If the expanded memory board supplies pages in exact multiples of 16K
      bytes, the number of pages this function returns is identical to the
      number Function 3 (Get Unallocated Page Count) returns. In this case,
      there is no difference between a page and a raw page.

Function 5Ah Allocate Raw Pages LIM Function Call 27

           Allocates the number of nonstandard size pages that the operating
           system requests and assigns a unique EMM handle to these pages.

entry AH 5Ah

      AL      00h     allocate standard pages
              01h     allocate raw pages
      BX      number of pages to allocate

return AH error status: 00h, 80h, 81h, 84h, 85h, 87h, 88h

      DX      unique raw EMM handle (1-255)

note 1) it is intended this call be used only by operating systems

   2) uses registers AX and DX
   3) For all functions using the raw handle returned in DX, the length of
      the physical and logical pages allocated to it are some nonstandard
      length (that is, not 16K bytes).
   4) this call is primarily for use by operating systems or EMM drivers
      supporting hardware with a nonstandard EMS page size.

Function 5Bh Alternate Map Register Set - DMA Registers LIM Function Call 28 entry AH 00h Get Alternate Map Register Set

              01h     Set Alternate Map Register Set
                      BL      new alternate map register set number
                      ES:DI   pointer to map register context save area if
                              BL=0
              02h     Get Alternate Map Save Array Size
              03h     Allocate Alternate Map Register Set
              04h     Deallocate Alternate Map Register Set
                      BL      number of alternate map register set
              05h     Allocate DMA Register Set
              06h     Enable DMA on Alternate Map Register Set
                      BL      DMA register set number
                      DL      DMA channel number
              07h     Disable DMA on Alternate Map Register Set
                      BL      DMA register set number
              08h     Deallocate DMA Register Set
                      BL      DMA register set number

return AH status: 00h, 02h 00h, 80h, 84h, 81h, 8Fh, 0A4h

                      01h        00h, 80h, 81h, 84h, 8Fh, 9Ah, 9Ch, 9Dh,
                                 0A3h, 0A4h
                      03h, 05h   00h  80h  81h  84h, 8Fh, 9Bh, 0A4h
                      04h        00h, 80h, 81h, 84h, 8Fh, 9Ch, 9Dh, 0A4h
                      06h, 07h   00h, 80h, 81h, 84h, 8Fh, 9Ah, 9Ch, 9Dh, 9Eh,
                                 9Fh, 0A4h
      BL      current active alternate map register set number if nonzero
              (AL=0)
      BL      number of alternate map register set; zero if not supported
              (AL=3)
      DX      array size in bytes (subfunction 02h)
      ES:DI   pointer to a map register context save area if BL=0 (AL=0)

note 1) this call is for use by operating systems only, and can be enabled

      or disabled at any time by the operating system
   2) This set of functions performs the same functions at EEMS function 6Ah
      subfunctions 04h and 05h ("function 43").
   3) 00h uses registers AX, BX, ES:DI
      01h uses register AX
      02h uses registers AX and DX
      03h uses registers AX and BX
      04h uses register AX
      05h uses registers AX, BX
      06h uses register AX
      07h uses register AX

Function 5Ch Prepare EMS Hardware for Warm Boot LIM Function Call 29

           Prepares the EMM hardware for a warm boot.

entry AH 5Ch return AH error status: 00h, 80h, 81h, 84h note 1) uses register AX

   2) this function assumes that the next operation that the operating system
      performs is a warm boot of the system.
   3) in general, this function will affect the current mapping context, the
      alternate register set in use, and any other expanded memory hardware
      dependencies which need to be initialized at boot time.
   4) if an application decides to map memory below 640K, the application must
      trap all possible conditions leading to a warm boot and invoke this
      function before performing the warm boot itself.

Function 5Dh Enable/Disable OS Function Set Functions LIM Function Call 30

           Lets the OS allow other programs or device drivers to use the OS
           specific functions. This capability is provided only for an OS
           which manages regions of mappable conventional memory and cannot
           permit programs to use any of the functions which affect that
           memory, but must be able to use these functions itself.

entry AH 5Dh

      AL      00h     enable OS function set
              01h     disable OS function set
              02h     return access key (resets memory manager, returns access
                      key at next invocation)
      BX,CX   access key returned by first invocation

return BX,CX access key, returned only on first invocation of function

      AH      status  00h, 80h, 81h, 84h, 8Fh, 0A4h

note 1) this function is for use by operating systems only. The operating system

      can disable this function at any time.
   2) 00h uses registers AX, BX, CX
      01h uses registers AX, BX, CX
      02h uses register AX
   3) 00h, 01h: The OS/E (Operating System/Environment) functions these
      subfunctions affect are:
      Function 26. Get Expanded Memory Hardware Information.
      Function 28. Alternate Map Register Sets.
      Function 30. Enable/Disable Operating System Functions.

Function 5Eh Unknown LIM Function call (not defined under LIM)

Function 5Fh Unknown LIM Function call (not defined under LIM)

Function 60h EEMS - Get Physical Window Array LIM Function call (not defined under LIM) entry AH 60h

      ES:DI   pointer to buffer

return AH status

      AL      number of entries
      buffer at ES:DI filled

Function 61h Generic Accelerator Card Support LIM Function Call 34

           Contact AST Research for a copy of the Generic Accelerator Card
           Driver (GACD) Specification

note Can be used by accelerator card manufacturer to flush RAM cache,

      ensuring that the cache accurately reflects what the processor would
      see without the cache.

Function 68h EEMS - Get Addresses of All Page Frames in System LIM Function Call (not defined under LIM) entry AH 68h

      ES:DI   pointer to buffer

return AH status

      AL      number of entries
      buffer at ES:DI filled

note Equivalent to LIM 4.0 function 58h

Function 69h EEMS - Map Page Into Frame LIM Function Call (not defined under LIM) entry AH 69h

      AL      frame number
      BX      page number
      DX      handle

return AH status note Similar to EMS function 44h

Function 6Ah EEMS - Page Mapping LIM Function Call (not defined under LIM) entry AH 6Ah

      AL      00h save partial page map
                      CH      first page frame
                      CL      number of frames
                      ES:DI   pointer to buffer which is to be filled
              01h restore partial page map
                      CH      first page frame
                      CL      number of frames
                      DI:SI   pointer to previously saved page map
              02h save and restore partial page map
                      CH      first page frame
                      CL      number of frames
                      ES:DI   buffer for current page map
                      DI:SI   new page map
              03h get size of save array
                      CH      first page frame
                      CL      number of frames
              return  AL      size of array in bytes
              04h switch to standard map register setting
              05h switch to alternate map register setting
              06h deallocate pages mapped to frames in conventional memory
                      CH      first page frame
                      CL      number of frames

return AH status note Similar to LIM function 4Eh, except that a subrange of pages can

      be specified

EXPANDED MEMORY MANAGER ERROR CODES

EMM error codes are returned in AH after a call to the EMM (int 67h).

code meaning

00h function successful 80h internal error in EMM software (possibly corrupted driver) 81h hardware malfunction 82h EMM busy (dropped in EEMS 3.2) 83h invalid EMM handle 84h function requested not defined - unknown function code in AH. 85h no more EMM handles availible 86h error in save or restore of mapping context 87h more pages requested than exist 88h allocation request specified more logical pages than currently

      availible in system (request does not exceed actual physical number of
      pages, but some are already allocated to other handles); no pages
      allocated

89h zero pages; cannot be allocated (dropped in EMS 4.0) 8Ah logical page requested to be mapped outside range of logical pages

      assigned to handle

8Bh illegal page number in mapping request (valid numbers are 0 to 3) 8Ch page-mapping hardware state save area full 8Dh save of mapping context failed; save area already contains context

      associated with page handle

8Eh retore of mapping context failed; save area does not contain context

      for requested handle

8Fh subfunction parameter not defined (unknown function)

LIM 4.0 extended error codes:

90h attribute type undefined 91h warm boot data save not implemented 92h move overlaps memory 93h move/exchange larger than allocated region 94h conventional/expanded regions overlap 95h logical page offset outside of logical page 96h region larger than 1 MB 97h exchange source/destination overlap 98h source/destination undefined or not supported 99h (no status assigned) 9Ah alternate map register sets supported, specified set is not 9Bh all alternate map & DMA register sets allocated 9Ch alternate map & DMA register sets not supported 9Dh alternate map register or DMA set not defined, allocated or is currently

      defined set

9Eh dedicated DMA channels not supported 9Fh dedicated DMA channels supported; specifed channel is not 0A0h named handle could not be found 0A1h handle name already exists 0A2h move/exchange wraps around 1 MB boundry 0A3h data structure contains corrupted data 0A4h access denied

This is a user-supported technical reference. If you find this information 

to be of use, please mail your check or money order for $15 to:

      Dave Williams
      PO Box 181
      Jacksonville, AR 72087-0181
      USA

In return for your support you will receive the very latest edition of this manual on a disk, plus one disk of appendixes and references and a third disk with source code. That's about two megabytes of raw data when uncompressed, or the equivalent of ten manuals the size of the technical reference manuals from IBM or Microsoft.

In addition, supporting users may obtain updates by merely mailing a disk and return postage whenever they feel like it. INDEX

DOS TECHNICAL INFORMATION Programming Technical Reference - IBM Copyright 1988, Dave Williams

CHAPTER 1. SOME HISTORY THE OPERATING SYSTEM HIERARCHY DOS STRUCTURE DOS Initialization

CHAPTER 2. SYSTEM MEMORY MAP - OVERALL PC Port Assignment, Intel 8088, 80C88, 8086, 80286, 80386 CPUs Reserved Memory Locations in the IBM PC At Absolute Addresses The IBM PC System Interrupts (Overview) The IBM-PC System Interrupts (in detail) Interrupt 00h Divide by Zero (processor error). Interrupt 01h Single step Interrupt 02h Non-maskable interrupt Interrupt 03h Breakpoint Interrupt 04h Divide overflow Interrupt 05h Print Screen Interrupt 06h Reserved by IBM Interrupt 07h Reserved by IBM Interrupt 08h Timer Interrupt 09h Keyboard Interrupt 0Ah EGA Vertical Retrace Interrupt 0Bh Communications Controller (serial port) hdw. entry Interrupt 0Ch Communications Controller (serial port) hdw. entry Interrupt 0Dh Alternate Printer, PC/AT 80287 Interrupt 0Eh Diskette Interrupt 0Fh Reserved by IBM

CHAPTER 3. THE PC ROM BIOS Interrupt 10h Video I/O

Function 00h Determine or Set Video State
         01h Set Cursor Type
         02h Set Cursor Position
         03h Read Cursor Position
         04h Read Light Pen
         05h Select Active Page
         06h Scroll Page Up
         07h Scroll Page Down
         08h Read Character Attribute
         09h Write Character and Attribute
         0Ah Write Character
         0Bh Set Color Palette
         0Ch Write Dot
         0Dh Read Dot
         0Eh Write TTY
         0Fh Return Current Video State
         10h Set Palette Registers
         11h Character Generator Routine (EGA and after)
         12h Alternate Select (EGA and after)
         13h Write String
         14h Load LCD Character Font
         15h Return Physical Display Parameters
         1Ah Display Combination Code
         1Bh Functionality/State Information
         1Ch Save/Restore Video State
         70h Get Video RAM Address
         71h Get INCRAM Addresses
         72h Scroll Screen Right
         73h Scroll Screen Left
         81h DESQview video - Get something?
         82h DESQview - Get Current Window Info
         F0h Microsoft Mouse driver EGA support - Read One Register
         F1h Microsoft Mouse driver EGA support - Write One Register
         F2h Microsoft Mouse driver EGA support - Read Register Range
         F3h Microsoft Mouse driver EGA support - Write Register Range
         F4h Microsoft Mouse driver EGA support - Read Register Set
         F5h Microsoft Mouse driver EGA support - Read Register Set
         F6h Microsoft Mouse driver EGA support
         F7h Microsoft Mouse driver EGA support
         FAh Microsoft Mouse driver EGA support - Interrogate Driver
         FEh Get Alternate Screen Buffer Address (text mode only)
         FFh Update Real Display (text mode only)

Interrupt 11h Equipment Check Interrupt 12h Memory Size Interrupt 13h Disk I/O

Function 00h Reset
         01h Get Status of disk system
         02h Read Sectors
         03h Write Sectors
         04h Verify
         05h Format Track
         06h Hard Disk
         07h Hard Disk
         08h Read Drive Parameters
         09h Initialize Two Fixed Disk Base Tables
         0Ah Read Long   (Hard disk)
         0Bh Write Long
         0Ch Seek To Cylinder
         0Dh Alternate Disk Reset
         0Eh Read Sector Buffer
         0Fh Write sector buffer
         10h Test For Drive Ready
         11h Recalibrate Drive
         12h Controller RAM Diagnostic
         13h Drive Diagnostic
         14h Controller Internal Diagnostic
         15h Get Disk Type
         16h Change of Disk Status (diskette)
         17h Set Disk Type for Format (diskette)
         18h Set Media Type For Format  (diskette)
         19h Park Hard Disk Heads
         1Ah ESDI Hard Disk - Format

Interrupt 14h Initialize and Access Serial Port For Int 14

Function 01h Send Character in AL to Comm Port DX (0 or 1)
         02h Wait For A Character From Comm Port DX
         03h Fetch the Status of Comm Port DX (0 or 1)
         04h Extended Initialize
         05h Extended Communication Port Control

Interrupt 15h Cassette I/O

Function 00h Turn Cassette Motor On
         01h Turn Cassette Motor Off
         02h Read Blocks From Cassette
         03h Write Data Blocks to Cassette
         0Fh ESDI Format Unit Periodic Interrupt
         10h TopView API Function Calls
         20h PRINT.COM  (DOS internal)
         21h Power-On Self Test (POST) Error Log
         40h Read/Modify Profiles
         41h Wait On External Event
         42h Request System Power Off
         43h Read System Status
         44h (De)activate Internal Modem Power
         4Fh Keyboard Intercept
         80h Device Open
         81h Device Close
         82h Program Termination
         83h Event Wait
         84h Read Joystick Input Settings
         85h System Request (SysReq) Key Pressed
         86h Elapsed Time Wait
         88h Extended Memory Size Determine
         89h Switch Processor to Protected Mode
         91h Set Flag and Complete Interrupt
         C0h Get System Configuration
         C1h System
         C2h Pointing Device BIOS Interface (DesQview 2.x)
         C3h Enable/Disable Watchdog Timeout
         C4h Programmable Option Select
         DEh DesQview Services

Interrupt 16h Keyboard I/O

Function 00h Get Keyboard Input
         01h Check Keystroke Buffer
         02h Shift Status
         03h Keyboard
         04h Keyboard Click Toggle
         05h Keyboard Buffer Write
         10h Get Enhanced Keystroke And Read
         11h Check Enhanced Keystroke
         12h Extended Get Shift Status
         F0h Set CPU speed (Compaq 386)

Interrupt 17h Printer

Function 00h Print Character/send AL to printer DX (0, 1, or 2)
         01h Initialize Printer
         02h Printer Status

Interrupt 18h ROM BASIC Interrupt 19h Bootstrap Loader Interrupt 1Ah Time of Day

Function 00h Read System Time Counter
         01h Set Clock
         02h Read Real Time Clock Time
         03h Set Real Time Clock Time
         04h Read Real Time Clock Date
         05h Set Real Time Clock Date
         06h Set Real Time Clock Alarm
         07h Reset Real Time Clock Alarm
         08h Set Real Time Clock Activated Power On Mode
         09h Read Real Time Clock Alarm Time and Status
         0Ah Read System-Timer Day Counter
         0Bh Set System-Timer Day Counter
         80h Set Up Sound Multiplexor

Interrupt 1Bh Control-Break Interrupt 1Ch Timer Tick Interrupt 1Dh Vector of Video Initialization Parameters. Interrupt 1Eh Vector of Diskette Controller Parameters Interrupt 1Fh Pointer to Graphics Character Extensions (Graphics Set 2) Interrupt 20h PROGRAM TERMINATE Interrupt 20h DOS - Terminate Program Interrupt 20h Minix - Send/Receive Message

CHAPTER 4. DOS INTERRUPTS AND FUNCTION CALLS DOS Registers Interrupts Interrupt 21h Function Request (Overview) Calling the DOS Services Interrupt 21h Function Request (in detail)

Function 00h Program Terminate
         01h Keyboard Input
         02h Display Output
         03h Auxiliary Input
         04h Auxiliary Output
         05h Printer Output
         06h Direct Console I/O
         07h Direct Console Input Without Echo
         08h Console Input Without Echo
         09h Print String
         0Ah Buffered Keyboard Input
         0Bh Check Standard Input Status
         0Ch Clear Keyboard Buffer and Invoke a Kbd Function
         0Dh Disk Reset
         0Eh Select Disk
         0Fh Open File
         10h Close File
         11h Search for First Entry
         12h Search for Next Entry
         13h Delete File
         14h Sequential Read
         15h Sequential Write
         16h Create File
         17h Rename File
         18h Unknown
         19h Current Disk
         1Ah Set Disk Transfer Address
         1Bh Allocation Table Information
         1Ch Allocation Table Information for Specific Device
         1Dh Unknown
         1Eh Unknown
         1Fh Read DOS Disk Block (default drive)
         20h Unknown
         21h Random Read
         22h Random Write
         23h File Size
         24h Set Relative Record Field
         25h Set Interrupt Vector
         26h Create New Program Segment
         27h Random Block Read
         28h Random Block Write
         29h Parse Filename
         2Ah Get Date
         2Bh Get Date
         2Ch Get Time
         2Dh Set Time
         2Eh Set/Reset Verify Switch
         2Fh Get Disk Transfer Address (DTA)
         30h Get DOS Version Number
         31h Terminate Process and Stay Resident
         32h Read DOS Disk Block
         33h Ctrl-Break Check
         34h Return INDOS Flag
         35h Get Vector
         36h Get Disk Free Space
         37h Get/Set Switch Character (SWITCHAR)
         38h Return Country Dependent Information
         39h Create Subdirectory (MKDIR)
         3Ah Remove Subdirectory (RMDIR)
         3Bh Change Durrent Directory (CHDIR)
         3Ch Create a File (CREAT)
         3Dh Open a File
         3Eh Close a File Handle
         3Fh Read From a File or Device
         40h Write to a File or Device
         41h Delete a File from a Specified Directory (UNLINK)
         42h Move File Read/Write Pointer (LSEEK)
         43h Change File Mode (CHMOD)
         44h I/O Control for Devices (IOCTL)
         45h Duplicate a File Handle (DUP)
         46h Force a Duplicate of a Handle (FORCDUP)
         47h Get Current Directory
         48h Allocate Memory
         49h Free Allocated Memory
         4Ah Modify Allocated Memory Blocks (SETBLOCK)
         4Bh Load or Execute a Program (EXEC)
         4Ch Terminate a Process (EXIT)
         4Dh Get Return Code of a Subprocess (WAIT)
         4Eh Find First Matching File (FIND FIRST)
         4Fh Find Next Matching File (FIND NEXT)
         50h Set PSP
         51h Get PSP
         52h IN-VARS
         53h Translate BPB
         54h Get Verify Setting
         55h Create Child PSP
         56h Rename a File
         57h Get or Set Timestamp of a File
         58h Get/Set Allocation Strategy (DOS 3.x)
         59h Get Extended Error Code
         5Ah Create Unique Filename
         5Bh Create a New File
         5Ch Lock/Unlock File Access
         5Dh Network - Partial
         5Eh Network Printer
         5Fh Network Redirection
         60h Parse Pathname
         61h Unknown
         62h Get Program Segment Prefix (PSP) Address
         63h Get Lead Byte Table (DOS 2.25)
         64h Unknown
         65h Get Extended Country Information (DOS 3.3)
         66h Get/Set Global Code Page Table (DOS 3.3)
         67h Set Handle Count (DOS 3.3)
         68h Commit File (DOS 3.3)
         69h     Disk Serial Number  DOS 4.0 (US)
         6Ah     unknown  (DOS 4.0?)
         6Bh     unknown  (DOS 4.0?)
         6Ch     Extended Open/Create  DOS 4.0 (US)
         89h  DOS_Sleep

Aftermarket Application Installed Function Calls, Used by NetWare

Function B6h-FFh   Novell NetWare

CHAPTER 5. Interrupts 22h Through 86h Interrupt 22h Terminate Address Interrupt 23h Ctrl-Break Exit Address Interrupt 24h Critical Error Handler Interrupt 25h Absolute Disk Read Interrupt 26h Absolute Disk Write Interrupt 27h Terminate And Stay Resident Interrupt 28h (not documented by Microsoft) Interrupt 29h (not documented by Microsoft) Interrupt 2Ah Microsoft Networks - Session Layer Interrupt Interrupt 2Bh (not documented by Microsoft) Interrupt 2Ch (not documented by Microsoft) Interrupt 2Dh (not documented by Microsoft) Interrupt 2Eh (undocumented by Microsoft) Interrupt 2Fh Multiplex Interrupt Interrupt 30h (not a vector!) far jump instruction for CP/M-style calls Interrupt 31h Unknown Interrupt 32h Unknown Interrupt 33h Used by Microsoft Mouse Driver Interrupt 34h Turbo C/Microsoft languages - Floating Point emulation Interrupt 35h Turbo C/Microsoft languages - Floating Point emulation Interrupt 36h Turbo C/Microsoft languages - Floating Point emulation Interrupt 37h Turbo C/Microsoft languages - Floating Point emulation Interrupt 38h Turbo C/Microsoft languages - Floating Point emulation Interrupt 39h Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Ah Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Bh Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Ch Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Dh Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Eh Turbo C/Microsoft languages - Floating Point emulation Interrupt 3Fh Overlay manager interrupt (Microsoft LINK.EXE) Interrupt 40h Hard Disk BIOS Interrupt 41h Hard Disk Parameters Interrupt 42h Pointer to screen BIOS entry Interrupt 43h Pointer to EGA initialization parameter table Interrupt 44h Pointer to EGA graphics character table Interrupt 45h Reserved by IBM (not initialized) Interrupt 46h Pointer to second hard disk, parameter block Interrupt 47h Reserved by IBM (not initialized) Interrupt 48h Cordless Keyboard Translation Interrupt 49h Non-keyboard Scan Code Translation Table Address Interrupt 4Ah Real-Time Clock Alarm Interrupt 4Bh Reserved by IBM (not initialized) Interrupt 4Ch Reserved by IBM (not initialized) Interrupt 4Dh Reserved by IBM (not initialized) Interrupt 4Eh Reserved by IBM (not initialized) Interrupt 4Fh Reserved by IBM (not initialized) Interrupt 50-57 IRQ0-IRQ7 relocated by DesQview Interrupt 58h Reserved by IBM (not initialized) Interrupt 59h Reserved by IBM (not initialized) Interrupt 5Ah Reserved by IBM (not initialized) Interrupt 5Bh Reserved by IBM (not initialized) Interrupt 5Ah Cluster Adapter BIOS entry address Interrupt 5Bh Reserved by IBM (not initialized) (cluster adapter?) Interrupt 5Ch NETBIOS interface entry port Interrupt 5Dh Reserved by IBM (not initialized) Interrupt 5Eh Reserved by IBM (not initialized) Interrupt 5Fh Reserved by IBM (not initialized) Interrupt 60h-67h User Program Interrupts (availible for general use) Interrupt 67h Used by Lotus-Intel-Microsoft Expanded Memory Specification Interrupt 68h Not Used (not initialized) Interrupt 69h Not Used (not initialized) Interrupt 6Ah Not Used (not initialized) Interrupt 6Bh Not Used (not initialized) Interrupt 6Ch System Resume Vector (Convertible) (not initialized on PC) Interrupt 6Dh Not Used (not initialized) Interrupt 6Fh Not Used (not initialized) Interrupt 70h IRQ 8, Real Time Clock Interrupt Interrupt 71h IRQ 9, Redirected to IRQ 8 Interrupt 72h IRQ 10 (AT, XT/286, PS/2) Reserved Interrupt 73h IRQ 11 (AT, XT/286, PS/2) Reserved Interrupt 74h IRQ 12 Mouse Interrupt (AT, XT/286, PS/2) Interrupt 75h IRQ 13, Coprocessor Error, BIOS Redirect to int 2 (NMI) (AT) Interrupt 76h IRQ 14, Hard Disk Controller (AT, XT/286, PS/2) Interrupt 77h IRQ 15 (AT, XT/286, PS/2) Reserved Interrupt 78h Not Used Interrupt 79h Not Used Interrupt 7Ah Novell NetWare - LOW-LEVEL API Interrupt 7Bh-7Fh Not Used Interrupt 80h-85h Reserved by BASIC Interrupt 86h Relocated by NETBIOS int 18 Interrupt 86h-F0h Used by BASIC when BASIC interpreter is running Interrupt E4h Logitech Modula-2 v2.0 MONITOR Interrupt F1h-FFh (absolute addresses 3C4-3FF) Interrupt F8h Set Shell Interrupt (OEM) Interrupt F9h First of 8 SHELL service codes, reserved for OEM shell (WINDOW); Interrupt FAh USART ready (RS-232C) Interrupt FBh USART RS ready (keyboard) Interrupt FCh Unknown Interrupt FDh reserved for user interrupt Interrupt FEh AT/XT286/PS50+ - destroyed by return from protected mode Interrupt FFh AT/XT286/PS50+ - destroyed by return from protected mode

CHAPTER 6. DOS CONTROL BLOCKS AND WORK AREAS The Disk Transfer Area (DTA) DOS Program Segment STANDRD FILE CONTROL BLOCK EXTENDED FILE CONTROL BLOCK MEMORY CONTROL BLOCKS CONTROL BLOCK MEMORY CONTROL BLOCKS

CHAPTER 7. DOS File Structure File Management Functions FCB FUNCTION CALLS HANDLE FUNCTION CALLS SPECIAL FILE HANDLES ASCII and BINARY MODE FILE I/O IN BINARY (RAW) MODE FILE I/O IN ASCII (COOKED) MODE NUMBER OF OPEN FILES ALLOWED RESTRICTIONS ON FCB USAGE RESTRICTIONS ON HANDLE USAGE ALLOCATING SPACE TO A FILE MSDOS / PCDOS DIFFERENCES .EXE FILE STRUCTURE THE RELOCATION TABLE "NEW" .EXE FORMAT (Microsoft Windows and OS/2)

CHAPTER 8. DOS DISK INFORMATION THE DOS AREA THE BOOT RECORD THE DOS FILE ALLOCATION TABLE (FAT) USE OF THE 12 BIT FILE ALLOCATION TABLE USE OF THE 16 BIT FILE ALLOCATION TABLE DOS DISK DIRECTORY DIRECTORY ENTRIES THE DATA AREA HArd DISK LAYOUT SYSTEM INITIALIZATION THE BOOT SEQUENCE BOOT RECORD/PARTITION TABLE HARD DISK TECHNICAL INFORMATION DETERMINING FIXED DISK ALLOCATION

CHAPTER 9. INSTALLABLE DEVICE DRIVERS DEVICE DRIVER FORMAT TYPES OF DEVICES DEVICE HEADER POINTER TO NEXT DEVICE HEADER FIELD ATTRIBUTE FIELD POINTER TO STRATEGY AND INTERRUPT ROUTINES NAME/UNIT FIELD CREATING A DEVICE DRIVER INSTALLING DEVICE DRIVERS INSTALLING CHARACTER DEVICES INSTALLING BLOCK DEVICES REQUEST HEADER UNIT CODE FIELD COMMAND CODE FIELD STATUS FIELD DEVICE DRIVER FUNCTIONS INIT MEDIA CHECK MEDIA DESCRIPTOR BUILD BPB (BIOS Parameter Block) INPUT / OUTPUT NONDESTRUCTIVE INPUT NO WAIT STATUS FLUSH OPEN or CLOSE (3.x) REMOVABLE MEDIA (DOS 3.x) THE CLOCK$ DEVICE

CHAPTER 10. LOTUS-INTEL-MICROSOFT EXPANDED MEMORY SPECIFICATION The Expanded Memory Manager

History
Page Frames

Expanded Memory Services AST/Quadram/Ashton-Tate Enhanced EMM

Calling the Manager
Common EMS         s (hex calls)
  1 (40h) Get Manager Status
  2 (41h) Get Page Frame Segment
  3 (42h) Get Number of Pages
  4 (43h) Get Handle and Allocate Memory
  5 (44h) Map Memory
  6 (45h) Release Handle and Memory
  7 (46h) Get EMM Version
  8 (47h) Save Mapping Context
  9 (48h) Restore Mapping Context
 10 (49h) Reserved
 11 (4Ah) Reserved
 12 (4Bh) Get Number of EMM Handles
 13 (4Ch) Get Pages Owned By Handle
 14 (4Dh) Get Pages for All Handles
 15 (4Eh) Get Or Set Page Map
new LIM 4.0 specification:
 16 (4Fh) Get/Set Partial Page Map
 17 (50h) Map/Unmap Multiple Pages
 18 (51h) Reallocate Pages
 19 (52h) Handle Attribute Functions
 20 (53h) Get Handle Name
 21 (54h) Get Handle Directory
 22 (55h) Alter Page Map & Jump
 23 (56h) Alter Page Map & Call
 24 (57h) Move Memory Region
 25 (58h) Get Mappable Physical Address Array
 26 (59h) Get Expanded Memory Hardware
 27 (5Ah) Allocate Raw Pages
 28 (5Bh) Get Alternate Map Register Set
 29 (5Ch) Prepare Expanded Memory Hardware
 30 (5Dh) Enable OS/E Function Set
 31 (5Eh) Unknown
 32 (5Fh) Unknown
 33 (60h) Unknown
 34 (61h) AST Generic Accelerator Card Support
    (68h) EEMS - Get Addresses of All Page Frames in System
    (69h) EEMS - Map Page Into Frame
    (6Ah) EEMS - Page Mapping

Expanded Memory Manager Error Codes LIM 4.0 extended error codes

Programming Technical Reference - IBM Copyright 1988, Dave Williams

These scan codes are generated by pressing a key on the PC's keyboard. This is the 'make' code. A 'break' code is generated when the key is released. The break scancode is 128 higher than the make code, and is generated by setting bit 7 of the scan code byte to 1.

               IBM PC KEYBOARD EXTENDED CODES
      Normal          Shift          Control         Alt

ESC 1 1 2 0;120 2 3 0;121 3 4 0;122 4 5 0;123 5 6 0;124 6 7 0;125 7 8 0;126 8 9 0;127 9 10 0;128 0 11 0;129 - 12 0;130 = 13 0;131 TAB 15 0;15(backtab) backtab none 0;15 RETURN 28

       Normal      Shift     Control      Alt     NumLock

Home 0;71 0;119 none 7 UpArrow 0;72 none none 8 PgUp 0;73 0;132 none 9 gray - 0;74 0;74 LArrow 0;75 0;115 none 4 keypad 5 none none none 5 RArrow 0;77 0;116 6 gray + 0;78 0;78 End 0;79 0;117 none 1 DnArrow 0;80 2 PgDn 0;81 0;118 none 3 Ins 0;82 none 11 Del 0;83 0;128 none 52 PrtSc 55 0;114 L shift 42 R shift 54 alt key 56 capslock 58 spacebar 57 control key 29 numlock 69 scrollock 70 ; 39 [ 26 ] 27 " 40 \ 43 / 53 , 51 . 52

               IBM PC KEYBOARD EXTENDED CODES
       Normal      Shift     Control      Alt

a = 30 0;30 b = 48 0;48 c = 46 0;46 d = 32 0;32 e = 18 0;18 f = 33 0;33 g = 34 0;34 h = 35 0;35 i = 23 0;23 j = 36 0;36 k = 37 0;37 l = 38 0;38 m = 50 0;50 n = 49 0;49 o = 24 0;24 p = 25 0;25 q = 16 0;16 r = 19 0;19 s = 31 0;31 t = 20 0;20 u = 22 0;22 v = 47 0;47 w = 17 0;17 x = 45 0;45 y = 21 0;21 z = 44 0;44

       Normal      Shift     Control      Alt

F1 = 0;59 0;84 0;94 0;104 F2 = 0;60 0;85 0;95 0;105 F3 = 0;61 0;86 0;96 0;106 F4 = 0;62 0;87 0;97 0;107 F5 = 0;63 0;88 0;98 0;108 F6 = 0;64 0;89 0;99 0;109 F7 = 0;65 0;90 0;100 0;110 F8 = 0;66 0;91 0;101 0;111 F9 = 0;67 0;92 0;102 0;112 F10 = 0;68 0;93 0;103 0;113

      "Enhanced" 101/102 key keyboard scancodes
       Normal      Shift     Control      Alt

F11 = 0;152 0;162 0;172 0;182 | F12 = 0;153 0;163 0;173 0;183 | Tandy?

F11 = 0;133 0;135 0;137 0;139 F12 = 0;134 0;136 0;138 0;140

alt-home 0;151 UpArr 0;141 0;152 Ctrl - 0;142 Ctrl 5 0;143 Ctrl + 0;144 DnArr 0;145 0;160 Ins 0;146 0;162 Del 0;147 0;163 Tab 0;148 0;165 / 0;149 0;164 Ctrl-* 0;150 alt-Enter 0;166 alt-PgUp 0;153 alt-LArr 0;154 alt-RArr 0;155 alt-End 0;156 alt-PgDn 0;161

  BIOS keystroke codes, hexadecimal
  Key      Normal         Shift          Control        Alt
  Esc      011B           011B           011B            --
  1!       0231 '1'       0221 '!'        --            7800
  2@       0332 '2'       0340 '@'       0300           7900
  3#       0433 '3'       0423 '#'        --            7A00
  4$       0534 '4'       0524 '$'        --            7B00
  5%       0635 '5'       0625 '%'        --            7C00
  6^       0736 '6'       075E '^'       071E           7D00
  7&       0837 '7'       0826 '&'        --            7E00
  8*       0938 '8'       092A '*'        --            7F00
  9(       0A39 '9'       0A28 '('        --            8000
  0)       0B30 '0'       0B29 ')'        --            8100
  -_       0C2D '-'       0C5F '_'       0C1F           8200
  =+       0D3D '='       0D2B '+'        --            8300
  BkSpc    0E08           0E08           0E7F            --
  tab      0F09           0F00            --             --
  q        1071 'q'       1051 'Q'       1011           1000
  w        1177 'w'       1157 'W'       1117           1100
  e        1265 'e'       1245 'E'       1205           1200
  r        1372 'r'       1352 'R'       1312           1300
  t        1474 't'       1454 'T'       1414           1400
  y        1579 'y'       1559 'Y'       1519           1500
  u        1675 'u'       1655 'U'       1615           1600
  i        1769 'i'       1749 'I'       1709           1700
  o        186F 'o'       184F 'O'       180F           1800
  p        1970 'p'       1950 'P'       1910           1900
  [{       1A5B '['       1A7B '{'       1A1B            --
  ]}       1B5D ']'       1B7D '}'       1B1D            --
  enter    1C0D           1C0D           1C0A            --
  Ctrl      --             --             --             --
  a        1E61 'a'       1E41 'A'       1E01           1E00
  s        1F73 's'       1F53 'S'       1F13           1F00
  d        2064 'd'       2044 'D'       2004           2000
  f        2166 'f'       2146 'F'       2106           2100
  g        2267 'g'       2247 'G'       2207           2200
  h        2368 'h'       2348 'H'       2308           2300
  j        246A 'j'       244A 'J'       240A           2400
  k        256B 'k'       254B 'K'       250B           2500
  l        266C 'l'       264C 'L'       260C           2600
  ;:       273B ';'       273A ':'        --             --
  '"       2827 '''       2822 '"'        --             --
  `~       2960 '`'       297E '~'        --             --
  l shift   --             --             --             --
  \|       2B5C '\'       2B7C '|'       2B1C            --
  z        2C7A 'z'       2C5A 'Z'       2C1A           2C00
  x        2D78 'x'       2D58 'X'       2D18           2D00
  c        2E63 'c'       2E43 'C'       2E03           2E00
  v        2F76 'v'       2F56 'V'       2F16           2F00
  b        3062 'b'       3042 'B'       3002           3000
  n        316E 'n'       314E 'N'       310E           3100
  m        326D 'm'       324D 'M'       320D           3200
  ,<       332C ','       333C '<'        --             --
  .>       342E '.'       343E '>'        --             --
  /?       352F '/'       353F '?'        --             --
  r shift   --             --             --             --
  PrtSc    372A '*'        --            7200            --
  Alt       --             --             --             --
  spacebar 3920 ' '       3920 ' '       3920 ' '       3920 ' '
  CapsLock  --             --             --             --
  BIOS keystroke codes, hexadecimal, continued
  Key      Normal         Shift          Control        Alt
  F1       3B00           5400           5E00           6800
  F2       3C00           5500           5F00           6900
  F3       3D00           5600           6000           6A00
  F4       3E00           5700           6100           6B00
  F5       3F00           5800           6200           6C00
  F6       4000           5900           6300           6D00
  F7       4100           5A00           6400           6E00
  F8       4200           5B00           6500           6F00
  F9       4300           5C00           6600           7000
  F10      4400           5D00           6700           7100
  NumLock   --             --             --             --
  Scroll    --             --             --             --
  7 Home   4700           4737 '7'       7700            --
  8 up     4800           4838 '8'        --             --
  9 PgUp   4900           4939 '9'       8400            --
  grey -   4A2D '-'       4A2D '-'        --             --
  4 left   4B00           4B34 '4'       7300            --
  5         --            4C35 '5'        --             --
  6 right  4D00           4D36 '6'       7400            --
  grey +   4E2B '+'       4E2B '+'        --             --
  1 End    4F00           4F31 '1'       7500            --
  2 down   5000           5032 '2'        --             --
  3 PgDn   5100           5133 '3'       7600            --
  Ins      5200           5230 '0'        --             --
  Del      5300           532E '.'        --             --
  A table entry of "--" means you can't get that combination out of BIOS.
1)
rows+1)*(bytes/char
/data/webs/external/dokuwiki/data/pages/archive/programming/dostech.pro.txt · Last modified: 1999/08/01 17:20 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki