mirror of
https://github.com/Pecusx/jataricart.git
synced 2026-05-20 22:33:22 +02:00
Jakub Husak
58 lines
1.1 KiB
NASM
58 lines
1.1 KiB
NASM
/*
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org $2000
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mwa #$ffff crc16.crc
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lda #'B'
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jsr crc16.updCRC
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lda #'N'
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jsr crc16.updCRC
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lda #'E'
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jsr crc16.updCRC
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jmp *
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*/
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.proc crc16
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crc equ $80
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tmpx equ $82
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tmpy equ $83
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updCRC
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stx tmpx
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sty tmpy
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EOR CRC+1 ; A contained the data
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STA CRC+1 ; XOR it into high byte
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LSR @ ; right shift A 4 bits
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LSR @ ; to make top of x^12 term
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LSR @ ; ($1...)
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LSR @
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TAX ; save it
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ASL @ ; then make top of x^5 term
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EOR CRC ; and XOR that with low byte
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STA CRC ; and save
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TXA ; restore partial term
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EOR CRC+1 ; and update high byte
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STA CRC+1 ; and save
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ASL @ ; left shift three
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ASL @ ; the rest of the terms
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ASL @ ; have feedback from x^12
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TAX ; save bottom of x^12
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ASL @ ; left shift two more
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ASL @ ; watch the carry flag
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EOR CRC+1 ; bottom of x^5 ($..2.)
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TAY ; save high byte
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TXA ; fetch temp value
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ROL @ ; bottom of x^12, middle of x^5!
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EOR CRC ; finally update low byte
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STA CRC+1 ; then swap high and low bytes
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STY CRC
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ldx tmpx
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ldy tmpy
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RTS
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.endp
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