forked from chrisly42/Hamazing
77 lines
3.8 KiB
NASM
77 lines
3.8 KiB
NASM
; unzx0_68000.s - ZX0 decompressor for 68000 - 80 bytes
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; Modified to not preserve registers!
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;
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; in: a0 = start of compressed data
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; a1 = start of decompression buffer
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;
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; Copyright (C) 2021 Emmanuel Marty
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; ZX0 compression (c) 2021 Einar Saukas, https://github.com/einar-saukas/ZX0
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;
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; This software is provided 'as-is', without any express or implied
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; warranty. In no event will the authors be held liable for any damages
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; arising from the use of this software.
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;
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; Permission is granted to anyone to use this software for any purpose,
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; including commercial applications, and to alter it and redistribute it
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; freely, subject to the following restrictions:
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;
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; 1. The origin of this software must not be misrepresented; you must not
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; claim that you wrote the original software. If you use this software
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; in a product, an acknowledgment in the product documentation would be
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; appreciated but is not required.
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; 2. Altered source versions must be plainly marked as such, and must not be
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; misrepresented as being the original software.
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; 3. This notice may not be removed or altered from any source distribution.
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zx0_decompress:
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;movem.l a2/d2,-(sp) ; preserve registers
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moveq #-128,d1 ; initialize empty bit queue
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; plus bit to roll into carry
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moveq #-1,d2 ; initialize rep-offset to 1
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.literals: bsr.s .get_elias ; read number of literals to copy
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subq.l #1,d0 ; dbf will loop until d0 is -1, not 0
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.copy_lits: move.b (a0)+,(a1)+ ; copy literal byte
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dbf d0,.copy_lits ; loop for all literal bytes
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add.b d1,d1 ; read 'match or rep-match' bit
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bcs.s .get_offset ; if 1: read offset, if 0: rep-match
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.rep_match: bsr.s .get_elias ; read match length (starts at 1)
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.do_copy: subq.l #1,d0 ; dbf will loop until d0 is -1, not 0
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.do_copy_offs: move.l a1,a2 ; calculate backreference address
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add.l d2,a2 ; (dest + negative match offset)
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.copy_match: move.b (a2)+,(a1)+ ; copy matched byte
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dbf d0,.copy_match ; loop for all matched bytes
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add.b d1,d1 ; read 'literal or match' bit
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bcc.s .literals ; if 0: go copy literals
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.get_offset: moveq #-2,d0 ; initialize value to $fe
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bsr.s .elias_loop ; read high byte of match offset
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addq.b #1,d0 ; obtain negative offset high byte
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beq.s .done ; exit if EOD marker
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move.w d0,d2 ; transfer negative high byte into d2
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lsl.w #8,d2 ; shift it to make room for low byte
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moveq #1,d0 ; initialize length value to 1
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move.b (a0)+,d2 ; read low byte of offset + 1 bit of len
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asr.l #1,d2 ; shift len bit into carry/offset in place
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bcs.s .do_copy_offs ; if len bit is set, no need for more
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bsr.s .elias_bt ; read rest of elias-encoded match length
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bra.s .do_copy_offs ; go copy match
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.get_elias: moveq #1,d0 ; initialize value to 1
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.elias_loop: add.b d1,d1 ; shift bit queue, high bit into carry
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bne.s .got_bit ; queue not empty, bits remain
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move.b (a0)+,d1 ; read 8 new bits
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addx.b d1,d1 ; shift bit queue, high bit into carry
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; and shift 1 from carry into bit queue
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.got_bit: bcs.s .got_elias ; done if control bit is 1
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.elias_bt: add.b d1,d1 ; read data bit
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addx.l d0,d0 ; shift data bit into value in d0
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bra.s .elias_loop ; keep reading
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.done: ;movem.l (sp)+,a2/d2 ; restore preserved registers
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.got_elias: rts |