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rfc:rfc194
                                    Vint Cerf - UCLA
                                    Eric Harslem - Rand

RFC 194 John Heafner - Rand NIC 7139 Category: D.4 Bob Metcalfe - MIT Updates: None Obsoletes: None Jim White - UCSB

                THE DATA RECONFIGURATION SERVICE --
             COMPILER/INTERPRETER IMPLEMENTATION NOTES
                  I.  NEW FEATURES OF THE LANGUAGE

1. The meaning of S(#,E,,l) is only find an arbitrary

   number  (<=256) of EBCDIC characters and store them in
   identifier S.  This descriptor is terminated only by
   an invalid EBCDIC or by exceeding maximum permissible
   character count (256).

2. The assignment (S .⇐. T) causes all attributes of

   identifier T to be given to S, i.e., length, type,
   and contents.

3. (S .⇐. T || X) concatenates X onto the right-hand

   side of T and stores the result in S.  If T and X
   are binary the resulting value has a length equal
   to the sum L(T) + L(X).

4. T(X) joins L(X) and V(X) as a built-in identifier

   function.
   T(X) = type of identifier X.
   L(X) = length of contents of X.
   V(X) = contents of X converted to binary
          (decimal - binary is presently the only
          transformation).

5. New types ED and AD are EBCDIC and ASCII encoded

   decimal, respectively.  These have been added to
   complement the V(X) function.

6. New type SB has been added as signed binary. Type B

   is a logical binary string.

7. The syntactic notation for return-from-a-form has

   been changed.  See new syntax.
                                                              [Page 1]

Data Reconfiguration Service RFC 194

II. NEW SYNTAX

form :: = rule | form rule :: = label inputstream outputstream; label :: = INTEGER | NULL inputstream :: = terms | NULL terms :: = term | terms, term outputstream :: = :terms | NULL term :: = identifier | identifier descriptor |

                     descriptor | comparator

identifier :: = <alpha followed by 0-3 alphanumerics> descriptor :: = (replicationexpr, datatype, valueexpr,

                     lengthexpr control)

comparator :: = (concatexpr connective concatexpr control) |

                     (identifier .<=. concatexpr control)

replicationexpr :: = # | arithmetricexpr | NULL datatype :: = B | O | X | E | A | ED | AD | SB | T (identifier) valueexpr :: = concatexpr | NULL lengthexpr :: = arithmeticexpr | NULL connective :: = .LE. | .LT. | .GT. | .GE. | .EQ. | .NE. concatexpr :: = value | concatexpr value value :: = literal | arithmeticexpr arithmeticexpr :: = primary | arithmeticexpr operator primary primary :: = identifier | L(identifier) | V(identifier) |

                       INTEGER

operator :: = + | - | * | / literal :: = literaltype "string"

literaltype :: = B | 0 | X | E | A | ED | AD | SB string :: = <from 0 to 256 chars> control :: = :options | NULL options :: = SFUR (arithmeticexpr) | SFUR (arithmeticexpr),

                     SFUR (arithmeticexpr)

SFUR :: = S | F | U | SR | FR | UR

                                                              [Page 2]

Data Reconfiguration Service RFC 194

III. THE FORM INTERPRETER

Interpreter Overview

   The interpreter is a simple minded machine having the virtue of

helping the compiler writer by providing a rather powerful instruction set for hard-to-compile operations. Figure 1 shows the machine configuration:

+-------------+                        +--------------+
| inputstream |                        | outputstream |
+-------------+                        +--------------+
           /\                           /
            \                          /
             \                        /
              \                     \/
              +-----------------------+
              |         CPU           |
              +-----------------------+
                     |        /\
                     |         |
                     |         |
                     \/        |
              +-----------------------+
  Storage:    | Instruction           |
              | Sequence              |
              +-----------------------+
              | Label Table           |
              +-----------------------+
              | Literal/Identifier    |
              | Pool                  |
              +-----------------------+
              | Variable length       |
              | string area           |
              +-----------------------+
              Fig. 1. Form Interpreter
                                                              [Page 3]

Data Reconfiguration Service RFC 194

  The CPU is a box full of miscellaneous parts, the most important

being the Arithmetic Logic Unit and the instruction decoding unit. The CPU also maintains a collection of state registers to keep track of what it is doing. Figure 2 shows the rough layout.

 +-----------------+                +---------------+
 | Instruction     |                | Instruction   |
 | Counter         |                | Register      |
 +-----------------+                +---------------+
                                         |
                                         |
                                         V
                                  +----------------+
                                  | Operation Code |
                                  | Decoding       |
   Run Time Stack                 +----------------+
 +------------------+                 /     |    \
 |   Operands       |                /      |     \
 +------------------+              \/       V     \/
 |                  |               +-----------------+
 +------------------+              /   Instruction     \
 |                  |              |   Interpreter     |
 +------------------+              |   Routines        |
 |                  |               \                 /
 +------------------+                +---------------+
 |                  |                    |      /\
 +------------------+                    |       |
 |                  |                    |       |
 +------------------+                    V       |
 |                  |                +---------------+
 +------------------+ <------------- | Arithmetic    |
 |                  | -------------> | Logic Unit    |
 +------------------+                +---------------+
 |                  |
 +------------------+
 |                  |
 +------------------+
 +------------------+                +------------------+
 |Initial Input Ptr.|                | Output pointer   |
 +------------------+                +------------------+
 +------------------+                +------------------+
 |Current Input Ptr.|                | True/False Flag  |
 +------------------+                +------------------+
                                                              [Page 4]

Data Reconfiguration Service RFC 194

Fig. 2. The Central Processor

   The CPU is a stack machine driven by a Polish postfix instruction

sequence. Operands placed on the Run Time Stack are used for arithmetic expression evaluation and for parameter passing between the interpreter and the built-in functions.

   The Current Input Pointer and the Output Pointer keep track of the

two data streams. Two input pointers are needed because of the backup requirement in the event of rule failure. All of these pointers are bit pointers into the two streams.

   Various implementations of the Run Time Stack are independent of

the interpretation of the DRS machine's instruction set. It is suggested that the stack will contain instruction operands from the instruction stream.

   The format of a compiled instruction sequence for a form is shown

in Fig. 3.

                      16 bits
               +--------/\---------+
              /                     \
              +---------------------+
              |  length n in bytes  |
         +--  +---------------------+
         |    |                     |
         |    |     compiled        |
         |    |     16-bit          |
      n <     |     instructions    |
         |    |                     |
         |    |                     |
         |    |                     |
         +--  +---------------------+
     Fig. 3.  Compiled Instruction Sequence Format
                                                              [Page 5]

Data Reconfiguration Service RFC 194

 The format of the compiled Label Table is shown in Fig. 4.
                                     16 bits
                                +-----/\-------+
                               /                \
                               +-----------------+
                               |  length n       |
                               |  in bytes       |
      +--   +------------------+-----------------+
      |     | numeric value of |  byte offset    |
      |     | statement number |  in inst. seq.  |
      |     +------------------+-----------------+
      |     |        :                :          |
   n <      |        :                :          |
      |     |        :                :          |
      |     |                                    |
      |     |                                    |
      |     |                                    |
      +--   +------------------------------------+
            \_________________ _________________/
                              V
                            32 bits
                  Fig. 4.  Compiled Label Table
                                                              [Page 6]

Data Reconfiguration Service RFC 194

 Literals and Identifiers are compiled as shown in fig. 5.
                                 2              2
                            +----/\----+   +----/\----+
                           /            \ /            \
                           +-------------+--------------+
          1         1      | length n    |    length n  |
       ___/\____ ___/\____ | in bytes    |    in bytes  |
      +---------+----------+-------------+--------------+
   /  |         |//////////|             |              |
   |  | Type    |//////////| bit length  |  byte offset |
   |  |         |//////////|             |              |
   |  +---------+----------+-------------+--------------+

5*n < | : |

   |  |                           :                     |
   |  |                           :                     | Identifiers
   |  |                                                 |
   \  |                                                 |
      +-------------------------------------------------+
   /  |                                                 |
   |  |            literals are                         |
   |  |            byte-aligned                         | Literals
m <   |                                                 |
   |  |                                                 |
   |  |                                                 |
   \  +-------------------------------------------------+
   Legend:
   Type 0 = undefined
        1 = B (binary)
        2 = 0 (octal)
        3 = X (hexadecimal)
        4 = E (EBCDIC)
        5 = A (ASCII)
        6 = ED (EBCDIC encoded decimal)
        7 = AD (ASCII encoded decimal)
        8 = SB (signed binary, two's complement)
           Fig. 5.  Compiled Literals and Identifiers
                                                              [Page 7]

Data Reconfiguration Service RFC 194

 Types B, 0, X, AD, ED, and SB point to 32-bit word- aligned data
 shown below.

+—+—+—–+——-+ +——————-+ word-aligned, | T |/| L | —+—–> | | 32-bit right- +—+—+—–+——-+ +——————-+ justified Types E and A point to byte-aligned symbol streams as shown below. byte-aligned, L ⇐ 256 +—+—+—–+——-+ +————————+ | T |/| L | —+—–> | | +—+—+—–+——-+ +————————+

                                                              [Page 8]

Data Reconfiguration Service RFC 194

Instruction Format

   Since literals and identifiers will be stored in the same data

area, more than 256 literals plus identifiers might be encountered so more than 8 bits are needed to reference literal/id pool. Furthermore, such references must be distinguished from operators in the instruction stream, so a 16-bit instruction will be used, as shown below.

   +--------+------------------------+
   |    4   |          12            |
   +--------+------------------------+
       |
      /
     /
    /
   |
   V
   LD = 0  literal or identifier reference (12-bit positive integer)
   IC = 1  12-bit two's complement integer constant
   OP = 2  operator
   AD = 3  address (12-bit positive integer)
  ARB = 4  indefinite replication factor
 NULL = 5  missing attribute of term
   The operation code decoder picks up types 0, 1, 3, 4,

and 5 and deposits them on top of the stack (TOS). LD is an index into the literal/identifier table, and AD is an index into the instruction sequence.

   The decoder examines OP elements further:
       4        4            8
   +--------+--------+----------------+
   |  0010  |        |////////////////|
   +--------+--------+----------------+
      OP        |
                +----------> 0 = binary operator
                             1 = unary operator
                             2 = special operator
                                                              [Page 9]

Data Reconfiguration Service RFC 194

Binary Operators (*)

   Let the TOS contain y and the next level, x.  The binary operators

compute x <bop> y, popping both x, y from stack, and put the result back on top of the stack.

                   +---+ <-- TOS  +-----+ <-- TOS
                   | y |          | x-y |
   e.g.     x-y => +---+     ===> +-----+
                   | x |          |/////|
                   +---+          +-----+

Binary Operator Encoding

                   4        4        4        4
              +--------+--------+--------+--------+
              |  0010  |  0000  |        |////////|
              +--------+--------+--------+--------+
                                     |
          +--------------------------+
          |
          V
   0 = integer +
   1 = integer -
   2 = integer x
   3 = integer : (or /), no remainder
   4 = concatenate ||
   All binary operations except concatenate expect the top

two elements on the stack to describe type B, 0, X, or SB. The result is always a 32-bit type B element. The concatenate operator fails unless both types are identical. For example:


   (*) As suggested above, the stack really contains instruction

operands that describe data; for convenience in illustrations the data rather than their descriptors are shown on the stack.

                                                             [Page 10]

Data Reconfiguration Service RFC 194

      type     L    value             T       L     V
     +------+------+------+        +------+------+------+

TOS → | B | 32 | 4 | | B | 32 | 12 | ← TOS

     +------+------+------+   ==>  +------+------+------+
     |  B   |  8   |  16  |        |//////|//////|//////|
     +------+------+------+        +------+------+------+
     Before-operation                after-operation
     +------+------+------+        +------+------+------+

TOS → | A | 2 | DE | | A | 5 |ABCDE | ← TOS

     +------+------+------+   ==>  +------+------+------+
     |  A   |  3   | ABC  |        |//////|//////|//////|
     +------+------+------+        +------+------+------+
     Before || operation             after || operation

No binary operator has any effect on the TRUE/FALSE flag.

Unary Operators

       4        4        4        4
   +--------+--------+--------+--------+
   |  0010  |  0001  |        |        |
   +--------+--------+--------+--------+
                        |         |
         +--------------+         |
         |                        |
         V                        |
  0 = integer minus               V
  1 = load identifier          0 = evaluated contents
                                   (after dec - binary
                                    conversion)
                               1 = length field
                               2 = type field
  2 = Label Table Reference
                                                             [Page 11]

Data Reconfiguration Service RFC 194

   For the unary minus operator the data described by the top of the

stack is replaced with its 2's complement. The form fails if the TOS type is not SB, B, 0, or X.

   The Load identifier expects the TOS to describe an index into the

literal/identifier pool (that is, an LD instruction) . The TOS described data is replaced by 32-bit type B values. The operation fails if the contents cannot be converted from encoded decimal to binary. B, 0, and X types are treated as unsigned integers, SB is treated as 2's complement.

   The Label Table Reference operator expects a 32-bit type B value

described by TOS and searches for this label in the label Table. If found, the TOS described data is replaced by the relative address in the instruction sequence of the label (in the form of an AD instruction). If not found, the form fails. No Unary operator has any effect on the TRUE/FALSE flag.

Special Operators

              4        4        4        4
          +--------+--------+--------+--------+
          |  0010  |  0010  |        |        |
          +--------+--------+--------+--------+
                               |         |
       +-----------------------+        /
       |                               /
       V                              /
 0 = store TOS                        |
 1 = return                           V
 2 = branch               0 = true, 1 = false, 2 = unconditional
 3 = compare              0 = .EQ.  2 = .LE.   4 = .GE.
                          1 = .NE.  3 = .LT.   5 = .GT.
 4 = move input ptr       0 = store current into initial
                          1 = store initial into current
 5 = input call           0 = no compare
                          1 = compare
 6 = output call
                                                             [Page 12]

Data Reconfiguration Service RFC 194

Store TOS

  The TOS describes an index into the ID table and the next lower

element in the stack describes a value to be stored. After execution, both elements are popped off the stack.

Return

   The TOS describes a value to be returned to the routine which

initiated the FORM MACHINE. The actual mechanism will be implementation dependent, but the FORM MACHINE will relin- quish control after this instruction completes execution.

Branch

   The TOS describes an index into the instruction sequence to be used

as the new instruction counter (IC) if the branch conditions are satisfied. The branch instruction checks the state of the TRUE/FALSE flag register and either increments the IC by 1 or replaces it with the TOS described element. In any case, the TOS is popped.

Compare

   The compare operator takes the two elements described by the two

top stack entries and compares them (.EQ.,.LT.,etc.). If n is at the top of the stack, and m is just below, then m.xx.n is performed, and the TRUE/False flag is set accordingly. For .xx. = .EQ. or .NE. we must have identical type, length, and content for equality to hold.

   The other boolean comparators will not be attempted if types are

different (i.e., form fails), but for same types, B, 0, X cause binary- justified compares, and A, E, AD, ED cause left-justified string compares with the shorter string padded with blanks.

Move Input Pointer

   This operator (no operands) replaces the Current Input Pointer with

the Initial Input Pointer (back-up), or the Initial Input Pointer with the current one (entry to rule).

Input Call

  This is the most complex operator thus far encountered.  It requires

four operands from the run-time stack:

                                                             [Page 13]

Data Reconfiguration Service RFC 194

TOS +—————————-+

   | binary or null             |  length to find
   +----------------------------+
   | LD to literal or null      |  value (literal)
   +----------------------------+
   | binary code                |  input data type
   +----------------------------+
   | binary, arbitrary, or null |  replication count
   +----------------------------+
   The input call operator can be invoked with the "no compare" flag

set, in which case the value expression parameter is ignored and only the input type and length expressions are used. In this case, the input routine tries to find in the input stream as many characters of the required type (bits, digits, etc.) as needed to fill the length expression requirement. If successful, the TRUE/FALSE flag is set TRUE, the stack is popped to remove the input parameters, and the string obtained is described by the TOS. If the input stream cannot be matched then the parameters are popped off the stack, and the TRUE/FALSE flag is set FALSE.

   If the "compare" flag is set, the input stream must be searched for

the value expression. However, we must take some care here to be sure we know what to look for. There are several cases:

a) The length expression parameter is greater than the

  length of the value expression but the type of input de-
  sired is the same as the value expression type.  For B, 0
  and X types, right-justify value expression in length-
  expression field, sign bit is extended left if type BS.
  If type A, E, AD, or ED pad on the right with blanks.  b)  Same as

a) but length is too small. B, 0, and X type strings

  are truncated on the left.  A, E, AD and ED are truncated
  on the right.  c)  The type of the value expression and the type

parameter

  differ.  This case is deferred for discussion and pre-
  sently is considered an error causing form failure.
   If the input string matches, then the TRUE/FALSE flag is set true,

the parameters are popped from the stack, and the resulting string is described by the TOS. Otherwise, the FALSE flag is set and the parameters are popped.

   When a successful match is found the input subroutine always

advances the Current Input Pointer by the appropriate amount. Since we are dealing at the bit level this pointer must be maintained as a bit pointer!

                                                             [Page 14]

Data Reconfiguration Service RFC 194

Output Call

   This routine utilizes the same parameters as the input call, but

operates on the output stream. The TRUE/FALSE flag is not distributed by this operator. As for input, there are four parameters on top of the stack, the length expression value, the value expression value, the desired output type, and the replication expression value. When there is a mis- match between the output type and the value expression type, a conversion must take place. The value expression is trans- formed into the desired output type and fitted into the field length specified by the length expression.

Truncation and Padding Rules

a) Character → character (A,E,AD,ED → A,E,AD,ED) conversion

  is left-justified and truncated or padded with blanks
  on the right.  b)  Character -> numeric and numeric -> character

conversion is

  right-justified and truncated or padded on the left with
  zeros.  Beware!  Two's complement numbers may be bollixed
  by this.  c)  Numeric -> character conversion is right-justified and
  left padded with blanks or left-truncated.  As for the unary

operators, a numeric bit-string is treated as unsigned, except SB which is treated as two's complement. Thus we have:

              (1,ED,X"FF",3) = E'255'
             (1,ED,X"100",3) = E'256'
   but (1,ED,SB"10000000",4) = E'-256'
   If the output routine is able to perform the desired action, it

advances the Output Stream Pointer, and pops all parameters from the run-time stack.

                                                             [Page 15]

Data Reconfiguration Service RFC 194

V. INSTRUCTION SET

it/id ref LD <num> Literal or identifier

                                           reference -> TOS

int const IC <num> small 2's comp. integer

                                           constant -> TOS

address AD <num> Address → TOS null parameter NULL missing term attribute add ADD TOS = x,y x + y → TOS subtract SUB TOS = x,y x - y → TOS multiply MUL TOS = x,y x * y → TOS divide DIV TOS = x,y x/y → TOS concatenate CON TOS = x,y x||y → TOS unary minus UNIN TOS = x -x → TOS load id value LIV TOS = LD x V(LD x) → TOS load id length LIL TOS = LD x V(LD x) → TOS load id type LIT TOS = LD x V(LD x) → TOS look up label LVL TOS = x AD x → TOS sto STO TOS = x,y y → x return RET TOS = x return to

                                                         caller with x

branch true BT TOS = AD x AD x → Instr.

                                                                 counter

branch false BF TOS = AD x AD x → Instr.

                                                                 counter

branch BU TOS = AD x AD x → Instr.

                                                                 counter

compare equal CEQ TOS = x,y (y.EQ.x) →

                                                                 TRUE/FALSE
                                                                 flag

compare not equal CNE TOS = x,y (y.NE.x) → T/FF compare ⇐ CLE TOS = x,y (y.LE.x) → T/FF call output OUT TOS = r,t,v,l (r,t,v,l) → output call input IN ( INC = compare TOS = r,t,v,l (r,t,v,l) → TOS

                       INN = no compare )

current → initial SCIP CIP → IIP (store current input

                                                          ptr - initial IP)

initial → current SICP IIP → CIP (store initial input

                                                          ptr - CIP)
                                                             [Page 16]

Data Reconfiguration Service RFC 194

VI. EXAMPLE COMPILATION

FORM SOURCE GENERATED POLISH INSTRUCTION SEQUENCE

                   ADDR. INSTR.     COMMENTS

(NUMB.⇐.1); 0 SICP RULE PRELUDE

                   1     IC   1
                   2     LD   0   REFERENCE TO NUMB
                   3     STO      STORE IN NUMB
                   4     SCIP     RULE POSTLUDE

1 CC(,E,,1:FR(99)), 5 SICP RULE PRELUDE

                   6     NULL     NO REPLICATION EXPRESSION
                   7     IC   4   TYPE EBCDIC
                   8     NULL     NO VALUE EXPRESSION
                   9     IC   1   LENGTH
                   10    INN      INPUT CALL WITH NO COMPARE
                   11    AD   15
                   12    BT       SKIP RETURN IF INN SUCCEEDS
                   13    IC   99  RETURN CODE
                   14    RET      RETURN TO CALLER IF FAILED
                   15    LD   1   REFERENCE TO CC
                   16    STO      STORE INPUT DATA IN CC

LINE(,E,,121: 17 NULL NO REPLICATION EXPRESSION

       FR(99)),    18    IC   4   TYPE IS EBCDIC
                   19    NULL     NO VALUE EXPRESSION
                   20    IC   121 LENGTH
                   21    INN      INPUT WITH NO COMPARE
                   22    AD   26
                   23    BT       SKIP RETURN IF OK
                   24    IC   98  RETURN CODE
                   25    RET      RETURN TO CALLER IF FAILED
                   26    LD   2   REFERENCE TO LINE
                   27    STO      STORE INPUT IN LINE

:CC, 28 SCIP SUCCESSFUL INPUT

                   29    NULL     NO REPLICATION FACTOR
                   30    LD   1   REFERENCE TO CC
                   31    LIT      TYPE OF CC
                   32    LD   1   REFERENCE TO VALUE OF CC
                   33    LD   1   CC AGAIN
                   34    LIL      LENGTH OF CC
                   35    OUT      OUTPUT CC

(,ED,NUMB,2), 36 NULL NO REPLICATION

                   37    IC   6   TYPE IS ED
                   38    LD   0   REFERENCE TO VALUE OF NUMB
                   39    IC   2   LENGTH OF OUTPUT FIELD
                   40    OUT      OUTPUT NUMB AS EBCDIC DEC.

(,E,E".",1), 41 NULL NO REPLICATION

                   42    IC   4   TYPE IS EBCDIC
                                                             [Page 17]

Data Reconfiguration Service RFC 194

                   43    LD   3   REFERENCE TO E"."
                   44    IC   1   LENGTH TO OUTPUT
                   45    OUT      OUTPUT THE PERIOD

(,E,LINE,117), 46 NULL NO REPLICATION

                   47    IC   4   TYPE IS EBCDIC
                   48    LD   2   REFERENCE TO LINE
                   49    IC   117 LENGTH TO OUTPUT
                   50    OUT      PUT OUT CONTENTS OF LINE

(NUMB.⇐.NUMB+1: 51 LD 0 REFERENCE TO NUMB

       U(1));      52    IC   1   AMOUNT TO ADD
                   53    ADD      ADD TO NUMB
                   54    LD   0   REFERENCE TO NUMB
                   55    STO      STORE BACK INTO NUMB
                   56    AD   5   PLACE TO GO
                   57    B        UNCONDITIONAL BRANCH BACK
              LITERAL/IDENTIFIER TABLE
                   0     NUMB
                   1     CC
                   2     LINE
                   3     E"."
                   LABEL TABLE
                LABEL     OFFSET
                  1         5
     [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by Simone Demmel 6/97 ]
                                                             [Page 18]
/data/webs/external/dokuwiki/data/pages/rfc/rfc194.txt · Last modified: 1997/06/27 17:08 (external edit)