77 lines
3.8 KiB
NASM
77 lines
3.8 KiB
NASM
; unzx0_68000.s - ZX0 decompressor for 68000 - 80 bytes
|
|
; Modified to not preserve registers!
|
|
;
|
|
; in: a0 = start of compressed data
|
|
; a1 = start of decompression buffer
|
|
;
|
|
; Copyright (C) 2021 Emmanuel Marty
|
|
; ZX0 compression (c) 2021 Einar Saukas, https://github.com/einar-saukas/ZX0
|
|
;
|
|
; This software is provided 'as-is', without any express or implied
|
|
; warranty. In no event will the authors be held liable for any damages
|
|
; arising from the use of this software.
|
|
;
|
|
; Permission is granted to anyone to use this software for any purpose,
|
|
; including commercial applications, and to alter it and redistribute it
|
|
; freely, subject to the following restrictions:
|
|
;
|
|
; 1. The origin of this software must not be misrepresented; you must not
|
|
; claim that you wrote the original software. If you use this software
|
|
; in a product, an acknowledgment in the product documentation would be
|
|
; appreciated but is not required.
|
|
; 2. Altered source versions must be plainly marked as such, and must not be
|
|
; misrepresented as being the original software.
|
|
; 3. This notice may not be removed or altered from any source distribution.
|
|
|
|
zx0_decompress:
|
|
;movem.l a2/d2,-(sp) ; preserve registers
|
|
moveq #-128,d1 ; initialize empty bit queue
|
|
; plus bit to roll into carry
|
|
moveq #-1,d2 ; initialize rep-offset to 1
|
|
|
|
.literals: bsr.s .get_elias ; read number of literals to copy
|
|
subq.l #1,d0 ; dbf will loop until d0 is -1, not 0
|
|
.copy_lits: move.b (a0)+,(a1)+ ; copy literal byte
|
|
dbf d0,.copy_lits ; loop for all literal bytes
|
|
|
|
add.b d1,d1 ; read 'match or rep-match' bit
|
|
bcs.s .get_offset ; if 1: read offset, if 0: rep-match
|
|
|
|
.rep_match: bsr.s .get_elias ; read match length (starts at 1)
|
|
.do_copy: subq.l #1,d0 ; dbf will loop until d0 is -1, not 0
|
|
.do_copy_offs: move.l a1,a2 ; calculate backreference address
|
|
add.l d2,a2 ; (dest + negative match offset)
|
|
.copy_match: move.b (a2)+,(a1)+ ; copy matched byte
|
|
dbf d0,.copy_match ; loop for all matched bytes
|
|
|
|
add.b d1,d1 ; read 'literal or match' bit
|
|
bcc.s .literals ; if 0: go copy literals
|
|
|
|
.get_offset: moveq #-2,d0 ; initialize value to $fe
|
|
bsr.s .elias_loop ; read high byte of match offset
|
|
addq.b #1,d0 ; obtain negative offset high byte
|
|
beq.s .done ; exit if EOD marker
|
|
move.w d0,d2 ; transfer negative high byte into d2
|
|
lsl.w #8,d2 ; shift it to make room for low byte
|
|
|
|
moveq #1,d0 ; initialize length value to 1
|
|
move.b (a0)+,d2 ; read low byte of offset + 1 bit of len
|
|
asr.l #1,d2 ; shift len bit into carry/offset in place
|
|
bcs.s .do_copy_offs ; if len bit is set, no need for more
|
|
bsr.s .elias_bt ; read rest of elias-encoded match length
|
|
bra.s .do_copy_offs ; go copy match
|
|
|
|
.get_elias: moveq #1,d0 ; initialize value to 1
|
|
.elias_loop: add.b d1,d1 ; shift bit queue, high bit into carry
|
|
bne.s .got_bit ; queue not empty, bits remain
|
|
move.b (a0)+,d1 ; read 8 new bits
|
|
addx.b d1,d1 ; shift bit queue, high bit into carry
|
|
; and shift 1 from carry into bit queue
|
|
|
|
.got_bit: bcs.s .got_elias ; done if control bit is 1
|
|
.elias_bt: add.b d1,d1 ; read data bit
|
|
addx.l d0,d0 ; shift data bit into value in d0
|
|
bra.s .elias_loop ; keep reading
|
|
|
|
.done: ;movem.l (sp)+,a2/d2 ; restore preserved registers
|
|
.got_elias: rts |