nesemu/tests/cpu_exec_space/source/common/crc.s

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2023-12-03 00:27:07 -05:00
; CRC-32 checksum calculation
zp_res checksum,4
zp_byte checksum_temp
zp_byte checksum_off_
; Turns CRC updating on/off. Allows nesting.
; Preserved: A, X, Y
crc_off:
dec checksum_off_
rts
crc_on: inc checksum_off_
beq :+
jpl internal_error ; catch unbalanced crc calls
: rts
; Initializes checksum module. Might initialize tables
; in the future.
init_crc:
jmp reset_crc
; Clears checksum and turns it on
; Preserved: X, Y
reset_crc:
lda #0
sta checksum_off_
lda #$FF
sta checksum
sta checksum + 1
sta checksum + 2
sta checksum + 3
rts
; Updates checksum with byte in A (unless disabled via crc_off)
; Preserved: A, X, Y
; Time: 357 clocks average
update_crc:
bit checksum_off_
bmi update_crc_off
update_crc_:
pha
stx checksum_temp
eor checksum
ldx #8
@bit: lsr checksum+3
ror checksum+2
ror checksum+1
ror a
bcc :+
sta checksum
lda checksum+3
eor #$ED
sta checksum+3
lda checksum+2
eor #$B8
sta checksum+2
lda checksum+1
eor #$83
sta checksum+1
lda checksum
eor #$20
: dex
bne @bit
sta checksum
ldx checksum_temp
pla
update_crc_off:
rts
; Prints checksum as 8-character hex value
print_crc:
jsr crc_off
; Print complement
ldx #3
: lda checksum,x
eor #$FF
jsr print_hex
dex
bpl :-
jmp crc_on
; EQ if checksum matches CRC
; Out: A=0 and EQ if match, A>0 and NE if different
; Preserved: X, Y
.macro is_crc crc
jsr_with_addr is_crc_,{.dword crc}
.endmacro
is_crc_:
tya
pha
; Compare with complemented checksum
ldy #3
: lda (ptr),y
sec
adc checksum,y
bne @wrong
dey
bpl :-
pla
tay
lda #0
rts
@wrong:
pla
tay
lda #1
rts