Calculate CRC32

The following code from Kevin Horton can be used to calculate a CRC32 checksum.

Details

• Variable testcrc should be in zero page and (obviously) requires 4 bytes of space
• The "magic value" $EDB88320 (bytes $ED, $B8, $83, and $20) are the polynomial

Code

;usage:
;
;initialize by calling crc32init
;
;feed bytes to crc32
;
;finish by calling crc32end
;
;result is in testcrc0-3
;

crc32init:   ldx #3
             lda #$ff

c3il:        sta testcrc+0,x
             dex
             bpl c3il
             rts


crc32:       ldx #8
             eor testcrc+0
             sta testcrc+0

c32l:        lsr testcrc+3
             ror testcrc+2
             ror testcrc+1
             ror testcrc+0
             bcc dc32
             lda #$ed
             eor testcrc+3
             sta testcrc+3
             lda #$b8
             eor testcrc+2
             sta testcrc+2
             lda #$83
             eor testcrc+1
             sta testcrc+1
             lda #$20
             eor testcrc+0
             sta testcrc+0

dc32:        dex
             bne c32l
             rts

crc32end:    ldx #3

c3el:        lda #$ff
             eor testcrc+0,x
             sta testcrc+0,x
             dex
             bpl c3el
             rts

References

• http://mdfs.net/Info/Comp/Comms/CRC32.htm
• http://www.6502.org/source/integers/crc.htm