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The Data Reconfiguration Service -- Compiler/Interpreter Implementation Notes.
V. Cerf, E. Harslem, J. Heafner, B. Metcalfe, J. White. July 1971.

 
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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]

   

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