From: Jim Brain (brain_at_jbrain.com)
Date: 2004-04-12 08:54:54
Well, interesting to me. I'm not a whiz at ML, and I found the
following code
It's from the QLink disk (yes, that disk...) I know it is a checksum,
but I can;t figure out what math it is trying to do.
The sequence 7f,7f,23,05,09 generates checksum with bcff=82,bd00=3e
I was wondering if someone could help me with this. The end result will
be in C, but I can do that myself if I can just figure out the algorithm.
.C:bccf 8D FE BC STA $BCFE
.C:bcd2 98 TYA
.C:bcd3 48 PHA
.C:bcd4 A2 08 LDX #$08
.C:bcd6 4E FE BC LSR $BCFE
.C:bcd9 2A ROL A
.C:bcda 29 01 AND #$01
.C:bcdc 4D 00 BD EOR $BD00
.C:bcdf 4E FF BC LSR $BCFF
.C:bce2 6A ROR A
.C:bce3 90 0E BCC $BCF3
.C:bce5 A8 TAY
.C:bce6 AD FF BC LDA $BCFF
.C:bce9 4D FC BC EOR $BCFC
.C:bcec 8D FF BC STA $BCFF
.C:bcef 98 TYA
.C:bcf0 4D FD BC EOR $BCFD
.C:bcf3 8D 00 BD STA $BD00
.C:bcf6 CA DEX
.C:bcf7 D0 DD BNE $BCD6
.C:bcf9 68 PLA
.C:bcfa A8 TAY
.C:bcfb 60 RTS
.C:bcfc A0 01 LDY #$01
.C:bcfe 00 BRK
.C:bcff 00 BRK
.C:bd00 00 BRK
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