;************************************************************************
;* CRCSUBS (Cyclic Redundancy Code Subroutines) version 1.20 *
;* 8080 Mnemonics *
;* *
;* These subroutines will compute and check a true 16-bit *
;* Cyclic Redundancy Code for a message of arbitrary length. *
;* *
;* The use of this scheme will guarantee detection of all *
;* single and double bit errors, all errors with an odd *
;* number of error bits, all burst errors of length 16 or *
;* less, 99.9969% of all 17-bit error bursts, and 99.9984% *
;* of all possible longer error bursts. (Ref: Computer *
;* Networks, Andrew S. Tanenbaum, Prentiss-Hall, 1981) *
;* *
;* *
;* There are four entry points, which are used as follows: *
;* *
;* CLRCRC - A call to this entry resets the CRC accumulator. *
;* It must be called at the start of each message. *
;* *
;* Entry Parameters: None. *
;* *
;* Exit Conditions: CRC accumulator cleared. *
;* All registers preserved. *
;* *
;* *
;* UPDCRC - A call to this entry updates the CRC accumulator. *
;* It must be called once for each byte in the *
;* message for which the CRC is being calculated. *
;* *
;* Entry Parameters: (A) = a byte to be included *
;* in the CRC calculation. *
;* *
;* Exit Conditions: CRC accumulator updated. *
;* All registers preserved. *
;* *
;* *
;* FINCRC - A call to this entry finishes the CRC calculation *
;* for a message which is to be TRANSMITTED. It must *
;* be called after the last byte of the message has *
;* been passed thru UPDCRC. It returns the calculated *
;* CRC bytes, which must be transmitted as the final *
;* two bytes of the message (first D, then E). *
;* *
;* Entry Parameters: None. *
;* *
;* Exit Conditions: (DE) = calculated CRC bytes. *
;* All other registers preserved. *
;* *
;* *
;* CHKCRC - A call to this routine checks the CRC bytes of *
;* a RECEIVED message and returns a code to indicate *
;* whether the message was received correctly. It must *
;* be called after the message AND the two CRC bytes *
;* have been received AND passed thru UPDCRC. *
;* *
;* Entry Parameters: None. *
;* *
;* Exit Conditions: (A) = 0 if message ok. *
;* (A) = -1 if message garbled. *
;* All other registers preserved. *
;* *
;************************************************************************
;* *
;* Designed & coded by Paul Hansknecht, June 13, 1981 *
;* *
;* *
;* Copyright (c) 1981, Carpenter Associates *
;* Box 451 *
;* Bloomfield Hills, MI 48013 *
;* 313/855-3074 *
;* *
;* This program may be freely reproduced for non-profit use. *
;* *
;************************************************************************
;
; ENTRY CLRCRC,UPDCRC,FINCRC,CHKCRC
;
CLRCRC: EQU $ ; Reset CRC Accumulator for a new message.
PUSH H
LXI H,0
SHLD CRCVAL
POP H
RET
;
UPDCRC: EQU $ ; Update CRC Accumulator using byte in (A).
PUSH PSW
PUSH B
PUSH H
MVI B,8
MOV C,A
LHLD CRCVAL
UPDLOOP:MOV A,C
RLC
MOV C,A
MOV A,L
RAL
MOV L,A
MOV A,H
RAL
MOV H,A
JNC SKIPIT
MOV A,H ; The generator is X^16 + X^12 + X^5 + 1
XRI 10H ; as recommended by CCITT.
MOV H,A ; An alternate generator which is often
MOV A,L ; used in synchronous transmission protocols
XRI 21H ; is X^16 + X^15 + X^2 + 1. This may be
MOV L,A ; used by substituting XOR 80H for XOR 10H
SKIPIT: DCR B ; and XOR 05H for XOR 21H in the adjacent code.
JNZ UPDLOOP
SHLD CRCVAL
POP H
POP B
POP PSW
RET
;
FINCRC: EQU $ ; Finish CRC calc for outbound message.
PUSH PSW
XRA A
CALL UPDCRC
CALL UPDCRC
PUSH H
LHLD CRCVAL
MOV D,H
MOV E,L
POP H
POP PSW
RET
;
CHKCRC: EQU $ ; Check CRC bytes of received message.
PUSH H
LHLD CRCVAL
MOV A,H
ORA L
POP H
RZ
MVI A,-1
RET
;
;
CRCVAL DW 0
;
END
