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unzx0_68000.S
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; unzx0_68000.s - ZX0 decompressor for 68000
;
; platon42: Modified to not preserve registers and to not use long word operations with
; unlikely and unsupported block lengths > 64 KB. get_elias inlined for speed and other
; optimizations.
;
; in: a0 = start of compressed data
; a1 = start of decompression buffer
;
; out: a0 = end of compressed data
; a1 = end of decompression buffer
;
; trashes: d0-d2/a2
;
; Copyright (C) 2021 Emmanuel Marty
; Copyright (C) 2023 Emmanuel Marty, Chris Hodges
; 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:
moveq.l #-128,d1 ; initialize empty bit queue
; plus bit to roll into carry
moveq.l #-1,d2 ; initialize rep-offset to 1
bra.s .literals
.do_copy_offs2
move.b (a1,d2.l),(a1)+
move.b (a1,d2.l),(a1)+
add.b d1,d1 ; read 'literal or match' bit
bcs.s .get_offset ; if 0: go copy literals
.literals
; read number of literals to copy
moveq.l #1,d0 ; initialize value to 1
.elias_loop1
add.b d1,d1 ; shift bit queue, high bit into carry
bne.s .got_bit1 ; 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_bit1
bcs.s .got_elias1 ; done if control bit is 1
add.b d1,d1 ; read data bit
addx.w d0,d0 ; shift data bit into value in d0
bra.s .elias_loop1 ; keep reading
.got_elias1
subq.w #1,d0 ; dbf will loop until d0 is -1, not 0
.copy_lits
move.b (a0)+,(a1)+ ; copy literal byte
dbra 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
; read match length (starts at 1)
moveq.l #1,d0 ; initialize value to 1
.elias_loop2
add.b d1,d1 ; shift bit queue, high bit into carry
bne.s .got_bit2 ; 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_bit2
bcs.s .got_elias2 ; done if control bit is 1
add.b d1,d1 ; read data bit
addx.w d0,d0 ; shift data bit into value in d0
bra.s .elias_loop2 ; keep reading
.got_elias2
subq.w #1,d0 ; dbra 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
dbra 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.l #-2,d0 ; initialize value to $fe
; read high byte of match offset
.elias_loop3
add.b d1,d1 ; shift bit queue, high bit into carry
bne.s .got_bit3 ; 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_bit3
bcs.s .got_elias3 ; done if control bit is 1
add.b d1,d1 ; read data bit
addx.w d0,d0 ; shift data bit into value in d0
bra.s .elias_loop3 ; keep reading
.got_elias3
addq.b #1,d0 ; obtain negative offset high byte
beq.s .done ; exit if EOD marker
move.b d0,-(sp) ; transfer negative high byte to stack
move.w (sp)+,d2 ; shift it to make room for low byte
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_offs2 ; if len bit is set, no need for more
moveq.l #1,d0 ; initialize length value to 1
; read rest of elias-encoded match length
add.b d1,d1 ; read data bit
addx.w d0,d0 ; shift data bit into value in d0
.elias_loop4
add.b d1,d1 ; shift bit queue, high bit into carry
bne.s .got_bit4 ; 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_bit4
bcs.s .do_copy_offs ; done if control bit is 1
add.b d1,d1 ; read data bit
addx.w d0,d0 ; shift data bit into value in d0
bra.s .elias_loop4 ; keep reading
.done
rts