CRC     EQU $6          ; current value of CRC
CRC8:
        EOR CRC         ; A contained the data
        STA CRC         ; XOR it with the byte
        ASL             ; current contents of A will become x^2 term
        BCC UP1         ; if b7 = 1
        EOR #$07        ; then apply polynomial with feedback
UP1     EOR CRC         ; apply x^1
        ASL             ; C contains b7 ^ b6
        BCC UP2
        EOR #$07
UP2     EOR CRC         ; apply unity term
        STA CRC         ; save result
        RTS

http://forum.6502.org/viewtopic.php?f=2&t=558&p=3519&hilit=Paul+Guertin#p3519