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;
; 2001-11-14, Piotr Fusik
; 2018-05-20, Christian Kruger
;
; unsigned long __fastcall__ crc32 (unsigned long crc, unsigned char* buf,
; unsigned len);
;
.export _crc32
.import compleax, incsp2, incsp4, popptr1, popeax
.importzp sreg, ptr1, ptr2, tmp1, tmp2
POLYNOMIAL = $EDB88320
make_table:
ldx #0
@L1: lda #0
sta tmp2
sta sreg
sta sreg+1
ldy #8
txa
@L2: sta tmp1
lsr a
bcc @L3
lda sreg+1
lsr a
eor #(POLYNOMIAL>>24)&$FF
sta sreg+1
lda sreg
ror a
eor #(POLYNOMIAL>>16)&$FF
sta sreg
lda tmp2
ror a
eor #(POLYNOMIAL>>8)&$FF
sta tmp2
lda tmp1
ror a
eor #POLYNOMIAL&$FF
bcs @L4 ; branch always
@L3: rol a
lsr sreg+1
ror sreg
ror tmp2
ror a
@L4: dey
bne @L2
sta table_0,x
lda tmp2
sta table_1,x
lda sreg
sta table_2,x
lda sreg+1
sta table_3,x
inx
bne @L1
inc table_initialised
RET:
rts
_crc32:
; ptr2 = (len & 0xff) == 0 ? len : len + 0x100;
tay
beq @L1
inx
@L1: sta ptr2
stx ptr2+1
; ptr1 = buf
jsr popptr1
; if (buf == NULL) return 0;
ora ptr1+1
beq @L0
; if (!tables_initialised) make_tables();
lda table_initialised
bne @dont_make
jsr make_table
@dont_make:
; eax = crc
jsr popeax
; if (len == 0) return crc;
ldy ptr2
bne @L2
ldy ptr2+1
beq RET
@L2:
; eax = ~crc
jsr compleax
stx tmp2
ldy #0
; crc = (crc >> 8) ^ table[(crc & 0xff) ^ *p++];
@L3: eor (ptr1),y
tax
lda table_0,x
eor tmp2
sta tmp1
lda table_1,x
eor sreg
sta tmp2
lda table_2,x
eor sreg+1
sta sreg
lda table_3,x
sta sreg+1
lda tmp1
iny
bne @L4
inc ptr1+1
@L4: dec ptr2
bne @L3
dec ptr2+1
bne @L3
ldx tmp2
jmp compleax
; return 0L
@L0: sta sreg
sta sreg+1
tax ; (popptr1 doesn't set .X)
; ignore crc
jmp incsp4
.data
table_initialised:
.byte 0
.bss
table_0: .res 256
table_1: .res 256
table_2: .res 256
table_3: .res 256
|
