GLOSSARY Last Change 11/6/94.
A20
(Address line 20) The 80286 and higher CPUs allow addresses in
real mode to extend slightly beyond the one megabyte mark, which causes an incompatibility with some older programs which expect such addresses to wrap back to the beginning of the address space. For complete compatibility with the 8088, newer machines thus contain circuitry which permits the twenty-first address line (A20) to be disabled. The CPU then effectively has only twenty address lines in real mode, just as the 8088 does, and addresses which would extend beyond the one megabyte mark wrap to the beginning of the address space. See also High Memory Area, Real Mode.
ABIOS
(Advanced BIOS) The IBM XT/286 and PS/2 models with 80286 or
higher processors contain two separate BIOSes. The ABIOS is a protected-mode BIOS which is used by OS/2. For machines without an ABIOS, such as the IBM AT, OS/2 loads the equivalent of the ABIOS from disk. see also CBIOS
API
(Application Program[ming] Interface) The defined set of calls
which a program may make to interact with or request services of the operating system or environment under which it is running. Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes.
APL
(A Programming Language) An interactive, mathematically-
oriented language which is well-suited to manipulating matrices. Originally using greek letters and numerous special symbols, thus requiring a special display, versions are now available which use keywords in place of the special symbols.
ASCIZ
A NUL-terminated ASCII string. The ASCIZ string "ABC" consists
of the four bytes 41h, 42h, 43h, and 00h. Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL.
AVATAR
(Advanced Video Attribute Terminal Assembler and Recreator) A
set of control codes which may be used to affect the output of characters to the screen on systems equipped with an appropriate driver. Similar in intent to ANSI sequences, AVATAR has shorter command sequences and provides additional PC-specific functionality. AVATAR is primarily used by the Opus and Maximus bulletin board systems (it was designed by one of the developers of the Opus system).
BASIC
(Beginner's All-purpose Symbolic Instruction Code) A
programming language originally designed as a means of teaching FORTRAN. There are many variations of BASIC with differing capabilities; the majority are interpreted but compiled BASIC is becoming more popular. All genuine IBM personal computers (including the latest PS/2 models) come equipped with a cassette-based BASIC interpreter in ROM.
BCD
(Binary Coded Decimal) A method of data storage where two
decimal digits are stored in each byte, one in the upper four bits and the other in the lower four bits. Since only the values 0 through 9 are used in each half of a byte, BCD values can be read as decimal numbers on a hexadecimal display of memory or a file.
Big-Endian
One of the two major ways of organizing multi-byte numeric
values in memory. A big-endian layout places the most significant byte of the value in the lowest (first) memory location, i.e. 12345678h is stored as 12h 34h 56h 78h. Motorola processors are big-endian. Compare Little-Endian.
BIOS
(Basic Input/Output System) A set of standardized calls giving
low-level access to the hardware. The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly.
BIOS Parameter Block
The BIOS Parameter Block stores the low-level layout of a
drive. See also INT 21h Function 53h.
Boot
To start up the computer or operating system. The term "boot"
is a contraction of "bootstrap", which in turn comes from the expression "to lift oneself by one's boot straps." The ROM BIOS on IBM PCs and compatibles reads in the first sector of the disk, which contains a short (less than 500 bytes) program that reads in a portion of the operating system, which in turn reads in the remainder of the operating system.
Boot Drive
The disk drive from which the operating system was booted. See
also Boot.
BPB
see BIOS Parameter Block
Breakpoint
When debugging, a memory location which when accessed causes a
break in the normal flow of execution and the invocation of the debugger. Used to let a program run at full speed until a certain instruction is reached or (less frequently) a particular data item is accessed or changed.
Cache
Caching is a method of increasing performance by keeping
frequently-used data in a location which is more quickly accessed. The most common caches are disk caches (store disk sectors in RAM) and RAM caches (store portions of main memory in special high-speed RAM which may be accessed as fast as the CPU is capable of accessing memory). See also Delayed Write, Write-Through.
Callback
A call to a specified function made by the operating system or
operating environment when a request (usually an asynchronous request) completes. This permits the calling program to continue operating while the request is processed yet still be aware of its completion immediately without the need to constantly poll the request's status. See also Callout.
Callout
A call made by the operating system, operating environment, or
an application program on various events, which may be intercepted by other software which is interested in the current state of the system. See also Callback, External Device Interface.
CAS
see Communicating Applications Specification
CBIOS
(Compatibility BIOS) The IBM XT/286 and PS/2 models with 80286
or higher processors contain two separate BIOSes. The CBIOS is a real-mode BIOS which is compatible with the earlier products in the IBM PC family and PS/2 models with 8086 processors. See also ABIOS.
CDS
see Current Directory Structure
CGA
(Color/Graphics Adapter) One of the two video display boards
introduced together with the original IBM PC. See also HGC, MDA.
Clock Tick
1/18.2 second, or approximately 55 milliseconds. This is the
rate at which the IBM PC's system clock is updated.
CMOS
(Complementary Metal-Oxide-Silicon) A type of integrated
circuit design known for its low power consumption.
CMOS RAM
A small amount (typically 64 or 128 bytes) of memory in the system's
real-time clock chip that is preserved by the clock's battery and is used for storing configuration information. See also Real-Time Clock.
Communicating Applications Specification
DCA and Intel's standard programmatic interface for sending and
receiving FAXes via any of a number of internal FAX boards.
CP/M
(Control Program for Microcomputers) An early operating system
for micros based on the 8-bit Intel 8080 CPU (and later the compatible 8085 and Zilog Z80 CPUs). MSDOS version 1.0 was essentially a clone of CP/M for the Intel 8086.
CP/M-86
One of the three operating systems offered by IBM for its
original PC (the other two were MSDOS and the UCSD p-System). It has since evolved into DR-DOS version 6.
CPU
(Central Processing Unit) The microprocessor which executes
programs on your computer.
Current Directory Structure
The data record used by DOS to keep track of the current
directory on a drive; whether the drive is valid, network, SUBSTituted, or JOINed; and other pertinent information. See also INT 21h Function 52h.
Cylinder
The set of concentric tracks of data located at the same
position on each data-bearing surface of the disk. A double-sided floppy will contain two tracks per cylinder.
DAC
(Digital-to-Analog Converter) A hardware device (in its simplest
form, nothing more than a set of interconnected resistors) which converts a digital number into an analog signal whose voltage is proportional to the value of the digital number. VGA and later color video boards use DACs to convert color values into the analog signals sent to the display; sound boards normally use DACs as well.
DCC
(Display Combination Code) A number which indicates both the
type of display adapter board and the type of monitor attached to the video board.
Delayed Write
A form of caching in which control is returned before the data is
actually written to the storage media. See also Cache, Write-Through.
Device Driver
An interface module between the device-independent portions of
the operating system and an actual hardware device which converts device-independent requests into the actual sequence of device operations to perform the requested action. IO.SYS contains the standard, built-in MSDOS device drivers such as CON, COM1, AUX, PRN, etc. See also INT 21h Function 52h and INT 2Fh Function 0802h.
Device Driver Request Header
The data structure passed to a device driver which contains the
command to be executed, its parameters, and space for a returned status and data values. See INT 2Fh Function 0802h.
DGIS
(Direct Graphics Interface Standard)
Direct Memory Access
A method whereby peripherals may transfer data into or out of
main memory without the involvement of the CPU.
Disk Transfer Address
The Disk Transfer Address indicates where functions which do
not take an explicit data address will read or store data. Although the name implies that only disk accesses use this address, other functions use it as well. See INT 21h Function 4Eh for an example of the DTA's use.
DLL
see Dynamic Link Library
DMA
see Direct Memory Access
DOS Extender
A program which allows a program to run in protected mode while still
retaining access to real-mode MSDOS services. See also Protected Mode.
DOS Parameter List
The DOS Parameter List is used to pass arguments to SHARE and
network functions. See also INT 21h Function 5D00h.
DOS Protected-Mode Interface
An API which provides basic services for protected-mode programs to
allocate memory, invoke real-mode software, etc. See also Virtual Control Program Interface.
DPB
see Drive Paramter Block
DPL
see DOS Parameter List
DPMI
see DOS Protected-Mode Interface
DRAM
(Dynamic Random Access Memory) RAM memory which essentially consists
of a tiny capacitor for each bit of memory. Since capacitors do not hold a charge indefinitely, DRAM must be constantly refreshed to avoid losing its contents. Also, the process of reading the contents of the memory are destructive, meaning extra time must be spent restoring the contents of memory addresses which are accessed, so DRAM is slower than SRAM. See also Refresh, SRAM.
Drive Parameter Block
The DOS Drive Parameter Block stores the description of the
media layout for a logical drive, as well as some housekeeping information. See also INT 21h Function 1Fh and INT 21h Function 32h.
DTA
see Disk Transfer Address
DWORD
Doubleword; four bytes. Commonly used to hold a 32-bit
segment:offset or selector:offset address.
Dynamic Link Library
A collection of subroutines which are linked with a program at
the time it is loaded into memory rather than permanently placed in the executable. This has the advantage of allowing a single copy of the subroutine library to reside on disk or in memory even when it is used by many programs. It also permits all programs using the DLL to be updated without recompiling simply by installing a new version of the library.
EGA
(Enhanced Graphics Adapter) IBM's second color video board for the
IBM PC family, capable of a maximum resolution of 640x350 pixels in 16 simultaneous colors of a total of 64 possible colors.
EISA
(Enhanced Industry-Standard Architecture) A 32-bit superset of the
IBM AT's expansion bus (which is now known as the ISA or Industry-Standard Architecture bus).
EMS
see Expanded Memory Specification
EOI
(End of Interrupt) A particular command sent to the interrupt
controller to indicate that the interrupt has been handled by software and that new interrupts of the same or lower priority may now be signalled by the interrupt controller.
ESDI
(Enhanced Small Device Interface) A disk drive interface type
which was briefly popular before IDE took over. An ESDI drive can transfer data between the drive and controller at 10, 15, or 20 megabits per second, which is faster than an MFM or RLL controller but slower than what is possible with an IDE or SCSI drive. See also IDE.
Exception
A signal by the CPU that some error condition has been encountered
that it can not deal with without a program's intervention. The most commonly encountered exceptions on Intel processors are Exceptions 12 and 13, which are stack and general problems, respectively. Exception 13 is typically caused by a memory access which wraps from the end of a segment back to the beginning.
Expanded Memory Specification
A specification devised by Lotus, Intel, and Microsoft for
accessing more than one megabyte of memory by bank-switching additional memory into the one megabyte real mode address space.
Extended BIOS Data Area
A block of memory, typically the 1K at the top of conventional
memory, which is used to store additional data for use by the BIOS which does not fit into the 256-byte data area at segment 0040h.
Extended File Control Block
A DOS File Control Block which has had an additional seven
bytes prepended to permit control of file attributes (which are stored in the appendage). See also FCB.
Extended Memory
Memory beyond the one megabyte address which is available only
on 80286 and higher machines. Except for a small portion (the High Memory Area), extended memory is only accessible from protected mode.
Extended Memory Specification
A specification devised by Microsoft which allows multiple
programs to share extended (above 1 megabyte) memory and noncontiguous memory above 640K. See also Upper Memory Block.
External Device Interface
A series of calls made by the DESQview multitasker on various
"interesting" events, which may be intercepted by programs which wish to keep track of the current system state. See also Callout.
FAT
see File Allocation Table
FCB
see File Control Block
File Allocation Table
A data structure on disk that records which clusters are free,
which are unusable, and which have been allocated. The clusters occupied by a file are linked into a list in the file allocation table, allowing DOS to find the contents of the file.
File Control Block
A data record in the calling program's address space which is
used by DOS 1.x functions to record the state of an open file. See also INT 21h Function 13h.
File Handle
A small positive integer used to identify the previously-opened
file on which a program wishes to perform an operation.
Flush
To force the copying of any data still stored in temporary
buffers to its final destination.
FM
(Frequency Modulation) A method of encoding data as a series of
magnetic flux reversals on disk or tape, commonly known as single-density recording. In frequency modulation, a series of clock pulses are written at regular intervals, with one data bit for each clock pulse. See also MFM, RLL.
Formatting
Preparing a storage medium (usually magnetic media such as a
disk or tape) for storing data. Low-level or physical formatting writes all necessary housekeeping data to enable the storage device to read the media and may also initialize the storage units on the media to a known state. High-level or logical formatting writes data used by the operating system, such as allocation information and directories onto media which has already been physically formatted. Formatting programs often perform both a low-level and a high-level format.
FOSSIL
(Fido/Opus/Seadog Standard Interface Layer) A standardized API
for performing serial I/O, originally used by the Fido and Opus bulletin- board software and Seadog bulletin-board mailer, but now in wider use.
FTP
(File Transfer Protocol) The standard protocol for copying files
from one machine to another on a TCP/IP (Internet) network. Also the program of the same name with which a user may transfer files.
Gather-Write
see Scatter/Gather
Handle
A short identifier, usually a small integer or a pointer, for
some other object which is maintained or controlled by the operating system or environment; a particular handle may be valid system-wide or may have meaning only for a particular process. See also File Handle.
HGC
(Hercules Graphics Controller) A monochrome video adapter capable
of 720x352 monochrome graphics. The HGC was the first non-IBM video adapter for the IBM PC. See also CGA, MDA.
High Memory Area
The first 65520 bytes (64K less 16 bytes) of extended memory.
This area is accessible from real mode on the 80286 and higher processors because these processors do not wrap addresses at one megabyte as the 8088 and 8086 do. See also A20, INT 2Fh Functions 4A01h.
HMA
see High Memory Area
Horizontal Retrace
When a monitor has finished displaying a single scan line, it must
move it electron beam(s) back to the left edge of the CRT, during which time it turns off the beam. On the original CGA (and some early clones), the only time one could access the display memory without causing "snow" was during the horizontal or vertical retrace periods, as the display adapter was not itself accessing the display memory during those times. See also Vertical Retrace.
IDE
(Integrated Drive Electronics) A type of disk drive interface
which essentially extends the PC's expansion bus all the way to the drive and places the drive controller on the disk drive itself. See also ESDI.
IFS
see Installable File System
Installable File System
An Installable File System which allows non-DOS format media to
be used by DOS. In most ways, an IFS is very similar to a networked drive, although an IFS would typically be local rather than remote. See also INT 21h Function 52h.
IP
(Internet Protocol) The lower level (transport layer) of the
TCP/IP protocol suite. See also TCP, TCP/IP.
IPC
(Inter-Process Communication) Any one of numerous methods for
allowing two or more separate processes to exchange data.
IPX
(Internetwork Packet Exchange) A low-level layer of Novell's NetWare networking software.
IRQ
(Interrupt ReQuest) A hardware line connected to the interrupt
controller chip which signals that a CPU interrupt should be generated.
ISA
(Industry-Standard Architecture) The expansion bus used by the
IBM PC/AT. See also EISA.
JFT
see Job File Table
Job File Table
The Job File Table (also called Open File Table) stored in a
program's PSP which translates handles into SFT numbers. See also INT 21h Function 26h.
LCD
(Liquid Crystal Display)
List of Lists
An internal DOS table of lists and other tables through which
most DOS-internal data structures may be reached. See INT 21h Function 52h.
Little-Endian
One of the two major ways of organizing multi-byte numeric
values in memory. A little-endian layout places the least significant byte of the value in the lowest (first) memory location, i.e. 12345678h is stored as 78h 56h 34h 12h. Intel processors are little-endian. Compare Big-Endian.
LPT
Abbreviation for Line PrinTer.
MCB
see Memory Control Block
MCGA
(Multi-Color Graphics Array) The low-end color adapter offered
in IBM's early PS/2 series machines.
MDA
(Monochrome Display Adapter) A text-only video adapter introduced
together with the original IBM PC. See also CGA, HGC.
Memory Control Block
The data structure containing the length and owner (among other
things) of a portion of the memory managed by DOS. See INT 21h Function 52h.
MFM
(Modified Frequency Modulation) A method of encoding data as a
series of magnetic flux reversals on disk or tape, commonly known as double-density recording. In contrast to FM, modified frequency modulation omits all clock pulses except those between pairs of zero bits. See also FM, RLL.
Mickey
The smallest increment of motion a mouse can sense.
MIDI
(Musical Instrument Digital Interface) A standardized interface
for controlling musical instruments with a computer.
Modem
(contraction of MOdulator/DEModulator)
Multitasking
Any of a number of methods by which multiple programs may
execute concurrently, with rapid switching between the programs giving the appearance that all are executing simultaneously.
MZ
The letters M and Z appear in numerous places in DOS (memory
control blocks, .EXE header, etc.); the conventional explanation is that these are the initials of Mark Zbikowski, one of the principal architects of MSDOS 2.0.
NCB
see Network Control Block
NDIS
(Network Driver Interface Specification) A hardware-independent
network interface developed by Microsoft and 3com. See also Packet Driver, TCP/IP.
NetBIOS
One of a number of low-level device-independent network
interfaces; the other major interfaces are Novell's IPX and the Internet's IP (Internet Protocol, the lower-level portion of TCP/IP).
Network Control Block
A Network Control Block used to pass requests to NetBIOS and
receive status information from the NetBIOS handler.
Network Redirector
A program which permits access to network devices (disks,
printers, etc.) using the MSDOS kernel network redirector interface. See also Network Shell, Redirector Interface.
Network Shell
A program which permits access to network devices (disks,
printers, etc.) by intercepting DOS calls before they reach the DOS kernel and handling those operating on network devices while passing through actions on local devices. See also Network Redirector.
NMI
see Non-Maskable Interrupt
Non-Maskable Interrupt
An interrupt which can not be disabled by clearing the CPU's
interrupt enable flag, unlike most normal interrupts. Non-maskable interrupts are typically used to signal calamities which require immediate action, such as a hardware failure or imminent loss of power.
Non-Volatile RAM
Memory which can be modified like normal RAM but does not lose
its contents when the system's power is turned off. This memory may be powered by a battery when the system power if off, or it may be a type of memory which does not need electricity to maintain its contents, such as EEPROM or bubble memory.
NVRAM
see Non-Volatile RAM
ODI
(Open Data-link Interface) A hardware-independent network
interface developed by Novell, Inc. See also NDIS, Packet Driver.
Open File Table
see Job File Table
Overscan Area
The "border" between the edge of the area where graphics or text can
be displayed and the actual edge of the area the video adapter can illuminate on the monitor's screen.
Overscan Register
On a display adapter, the control register which specifies the color
to be displayed in the overscan area.
Packet Driver
Any one of the numerous drivers conforming to FTP Software's
Packet Driver Specification, which provides a hardware-independent network interface. See also NDIS, ODI.
Page Fault
A CPU-generated signal, and the operating system's reaction to it,
generated when a program accesses a page of virtual memory which is not located in RAM at the time. The operating system's response is to load in the required page, possibly writing some other page out to disk in order to make room. See also INT 0E.
Palette Register
A memory location on the video controller which specifies the actual color displayed for a particular color number.
Park
To move a hard disk's read/write heads to a position in which
it is safe to turn off the power and transport the disk drive. Many drives also lock the heads into position when they are parked, providing additional protection from sudden movement.
Pel
see Pixel
Pixel
A picture element, the smallest addressable unit of a graphical
display.
Post
Make known, either generally or to a specific handler, that a
particular event of interest has occurred.
POST
see Power-On Self-Test
Power-On Self-Test
A brief examination of the system's functionality performed
each time the system is turned on.
Print Spooler
see SPOOL.
Program Segment Prefix
The Program Segment Prefix is a 256-byte data area prepended to
a program when it is loaded. It contains the command line that the program was invoked with, and a variety of housekeeping information for DOS. See also INT 21h Function 26h.
Protected Mode
One of the operating modes of the 80286 and higher Intel
processors, in which addresses used by programs no longer correspond to physical addresses and the CPU enforces various protection mechanisms designed to prevent one program from disrupting other programs or the operating system. See also Real Mode, Virtual-86 Mode.
PSP
see Program Segment Prefix
PWORD
Six bytes. Used to hold an 80386 protected-mode "far" address,
consisting of a segment selector and a 32-bit offset, or a Turbo Pascal "real" variable. See also DWORD, QWORD.
QWORD
(quad-word) Eight bytes. See also DWORD, PWORD.
RAM
(Random Access Memory) See also DRAM, SRAM.
Real Mode
One of the operating modes of the 80286 and higher Intel
processors, and the only operating mode of the 8088, 8086, 80186, and 80188 processors. In this mode, all addresses used by programs correspond directly to real physical addresses (thus the full name, Real Address Mode) and there are no CPU-imposed protections between programs. See also Protected Mode, Virtual-86 Mode.
Real-Time Clock
A battery-powered clock which continues to maintain its time even
while the system is powered down. On PCs, the real-time clock contains a small amount of battery-powered memory (set CMOS RAM).
Redirector Interface
The set of device-independent INT 2Fh function calls invoked by
the MSDOS kernel to operate on devices it recognizes as network devices. These function calls provide a lower-level interface than the INT 21h calls made to DOS, allowing a program intercepting these functions to be simpler than one intercepting INT 21h calls. See INT 2Fh Functions 1100h through 1130h.
Refresh
The process of periodically rewriting the contents of a DRAM
memory chip to keep it from fading. The term "refresh" is also commonly applied to redrawing the image on a CRT's phosphors. See also DRAM.
RGB
(Red-Green-Blue) The color specification mechanism normally
used in computer displays, where colors are separated into their primary-color components. See also YUV.
RLL
(Run-Length Limited) A method of encoding data as a series of
magnetic flux reversals on disk or tape. RLL can achieve higher data densities than MFM recording because it encodes the data such that (on average), fewer than one flux reversal per data bit is required (however, timing becomes more critical). RLL is actually an entire family of encoding methods, specified with two numbers indicating the minimum and maximum distances between one bits (flux transitions). The variant normally called RLL is RLL-2,7; RLL-1,7 and RLL-3,9 are also in use. MFM is in effect RLL-1,3. See also FM, MFM.
ROM
(Read-Only Memory) A memory for program storage which may not be
changed by the program as it runs.
RTC
see Real-Time Clock
Scan Code
The actual key number sent by the keyboard, which differs from the
key codes seen by application programs. The enhanced (101/102-key) keyboard actually sends different scan codes than the original (83/84-key) IBM keyboard, but these are normally translated by the keyboard controller into the scan codes used by the original keyboard before they become visible to programs.
Scatter/Gather
A technique in which the contiguous data of a disk sector or sectors
is transferred to or from multiple non-contiguous areas of memory. When reading into multiple areas of memory, this is called a scatter-read; the opposing operation is called gather-write.
Scatter-Read
see Scatter/Gather
SCSI
(Small Computer Systems Interface) A system-independent
expansion bus typically used to connect hard disks, tape drives, and CD-ROMs to a computer. A host adapter connects the SCSI bus to the computer's own bus. See also ESDI, IDE.
SDA
see Swappable Data Area
Sector
The smallest addressable unit of data on a disk; under MS-DOS,
this is normally 512 bytes. See also Track.
SFT
see System File Table
SPOOL
(Simultaneous Peripheral Operation OnLine) The process of
performing output to a slow peripheral such as a printer while other tasks continue running on the CPU. This term dates back to mainframe days before the invention of timesharing.
SQL
(Structured Query Language)
SRAM
(Static Random Access Memory) RAM which typically consists of one
flip-flop per bit of memory. Unlike DRAMs, static RAM retains its contents as long as power is applied. Because there is no need to refresh the contents of memory addresses which are read, SRAM is faster than DRAM, but it is more expensive and typically is available in much smaller sizes than DRAM because each bit occupies more space on the chip. See also DRAM.
SVGA
(Super VGA) A video adapter capable of higher resolution
(pixels and/or colors) than the 320x200x256 and 640x480x16 which IBM's VGA adapter is capable of producing. See also VESA.
Swappable Data Area
The portion of the DOS data segment containing all of the
variables used internally by DOS to record the state of a function call in progress. See also INT 21h Function 5D06h and INT 21h Function 5D0Bh.
System File Table
A System File Table is a DOS-internal data structure used to
maintain the state of an open file for the DOS 2+ handle functions, just as an FCB maintains the state for DOS 1.x functions. See also INT 21h Function 52h.
TCP
(Transmission Control Protocol) A higher level (session layer)
of the TCP/IP protocol suite. See also IP, TCP/IP.
TCP/IP
The protocol suite originally developed by DARPA for use on its
ARPAnet network, which is now known as the Internet. See also IP, TCP.
TSR
(Terminate and Stay Resident) A program which remains in memory
after terminating in order to provide services to other programs or the user. The name comes from the name of the DOS function call used to remain in memory after termination.
Track
One of multiple concentric circular rings of data on a single
data-bearing surface of a disk. Tracks at the same location on different surfaces form a cylinder.
UMB
see Upper Memory Block
UNC
(Universal Naming Convention) The standard way of describing
network servers and their directories under MS-DOS and Windows NT. A name in UNC format consists of two backslashes followed by the server name, optionally followed by another backslash and a list of backslash-separated fields; for example \\SERVER1\SHARED-DIR\SUBDIR1\SUBDIR2\FILENAME.EXT.
Upper Memory Block
A noncontiguous section of allocatable memory located between
the 640K and 1024K addresses. See also INT 21h Function 52h.
V86
see Virtual-86 Mode
VCPI
see Virtual Control Program Interface
VDM
see Virtual DOS Machine
VDS
see Virtual DMA Specification
VDU
(Video Display Unit) Mainframe-speak for computer monitor.
Vertical Retrace
When a monitor has finished displaying an image by sweeping its
electron beam(s) over the face of the CRT, it has to move the beam back up to the top of the display. During the time this takes, the beam is turned off. The vertical retrace interval is a good time to change the displayed picture for smooth animation. See also Horizontal Retrace.
VESA
(Video Electronics Standards Association) An industry group
which sets both hardware and software standards and recommendations. The term VESA is also used to denote compliance with the VESA SuperVGA BIOS Extensions, a standard set of video BIOS functions for accessing video modes of higher resolution than those defined by IBM.
VGA
(Video Graphics Array) The video adapter introduced with the
IBM PS/2 series of computers.
Virtual-86 Mode
One of the operating modes of the 80386 and 80486 processors in
which user programs run as if the CPU were in Real Mode, while providing the protection and address-mapping capabilities of Protected Mode to a supervisor program which oversees the virtual machine on which the user programs are running. This mode is called Virtual-86 because one or more virtual 8086 environments are run on a single CPU. See also Protected Mode, Real Mode, Virtual Machine.
Virtual Control Program Interface
A simple API for protected-mode programs to allocate memory and
switch into or out of protected mode. See also DOS Protected-Mode Interface.
Virtual DMA Specification
A set of interrupt calls which permit the use of DMA even on
systems running in protected or Virtual-86 mode with their address remapping, or systems such as Micro Channel PS/2s with multiple bus masters independently performing DMA operations.
Virtual DOS Machine
A special type of virtual machine provided by OS/2 version 2.0,
in which a copy of MS-DOS or an MS-DOS compatible operating system (or even an incompatible 8086 operating system) is run and appears to have full control of the system. See also Virtual Machine, INT 21h Function 64h.
Virtual Machine
One method for multitasking programs is to virtualize the CPU
and other hardware, giving the appearance of sole possession of the system to each program being run. Such a virtualized environment is called a virtual machine. See also Virtual-86 Mode, Virtual DOS Machine.
virus
A program which attaches itself to other programs for the purpose
of duplicating itself. Viruses often (but not always) contain harmful code which is triggered by some event, after a certain number of reproductions, or on a specific date. See also worm.
VM
see Virtual Machine
VxD
A virtual device driver for Windows 3.0 or 3.1. See also
device driver.
WORM
(Write Once, Read Many) A storage medium which may be written
exactly once, but may not be altered once data is stored.
worm
A program which duplicates itself, typically across networks.
In contrast to a virus, a worm does not attach itself to other programs, but can reproduce itself independently. See also virus.
Write-Through
One of two main types of caches, the write-through cache immediately
writes any new information to the medium it is caching, so that the cache never contains information which is not already present on the cached device. See also cache,
XBDA
see Extended BIOS Data Area
XDI
see External Device Interface
XGA
(Extended Graphics Array)
XMS
see Extended Memory Specification
YIQ
see YUV
YUV
A color specification mechanism used in NTSC-type color
television signals. Y represents luminance (overall brightness, the only part of the signal used by black-and-white televisions), while U and V are chrominance (color) information. Also called YIQ. See also RGB.