; ; SYSLIB Module Name: SCRC ; Author: Richard Conn ; SYSLIB Version Number: 2.3 ; Module Version Number: 1.0 ; Module Entry Points: ; CRCCLR CRCDONE CRCUPD ; Module External References: ; None ; ; ; 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) ; ; These routines are typically used as follows: ; CRC$MAKE: ; ROUTINE TO ESTABLISH CRC VALUE ; CALL CRCCLR ; CLEAR CRC ; ; ACQUIRE VALUES ; CALL CRCDONE ; GET VALUE ; SHLD CRCVAL ; SAVE VALUE ; CRC$CHECK: ; ROUTINE TO CHECK CRC VALUE ; CALL CRCCLR ; CLEAR CRC ; ; ACQUIRE VALUES ; CALL CRCDONE ; DONE ; XCHG ; VALUE IN DE ; LHLD CRCVAL ; FROM BEFORE ; CALL COMPHD ; COMPARE HL TO DE (SYSLIB ROUTINE) ; JNZ ERROR ; ERROR IF NOT SAME ; ; ; CRCCLR - Clear the CRC accumulator ; This routine must be called at the start of each byte stream. ; ; Input Parameters: None ; ; Output Parameters: None ; CRCCLR:: PUSH H LXI H,0 ;SET CRC TO ZERO SHLD CRCVAL POP H RET ; ; BUFFER FOR CRC VALUE ; CRCVAL: DS 2 ; ; CRCUPD - Update the CRC accumulator ; This routine must be called once for each byte in the ; byte stream for which the CRC is being calculated. ; ; Input Parameters: A = byte to be included in CRC ; ; Output Parameters: None ; CRCUPD:: PUSH PSW ;SAVE ALL REGS PUSH B PUSH H MVI B,8 ;ROTATE 8 BITS MOV C,A ;BYTE IN C LHLD CRCVAL ;HL=OLD CRC VALUE UPDLOOP: MOV A,C ;ROTATE HLC AS A 24-BIT ACC LEFT 1 BIT 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: ; and XOR 05H for XOR 21H in the adjacent code. DCR B ;COUNT DOWN 8 BITS JNZ UPDLOOP SHLD CRCVAL ;SAVE NEW CRC VALUE POP H ;RESTORE ALL POP B POP PSW RET ; ; CRCDONE - Complete the CRC calculation ; This routine is called after the last byte of the byte stream ; has passed thru CRCUPD, and it returns the calculated ; CRC bytes, which must be transmitted as the final ; two bytes of the message (first H, then L). ; ; Input Parameters: None ; ; Output Parameters: HL = calculated CRC bytes ; CRCDONE:: PUSH PSW ;SAVE A XRA A ;SEND OUT 2 ZEROES CALL CRCUPD CALL CRCUPD LHLD CRCVAL ;RETURN CRC VALUE IN HL POP PSW RET END