164 lines
3.9 KiB
ArmAsm
164 lines
3.9 KiB
ArmAsm
; De-compressor for ZX02 files
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; ----------------------------
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;
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; Decompress ZX02 data (6502 optimized format), optimized for speed and size
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; 138 bytes code, 58.0 cycles/byte in test file.
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;
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; Compress with:
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; zx02 input.bin output.zx0
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;
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; (c) 2022 DMSC
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; Code under MIT license, see LICENSE file.
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;ZP=$80
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;offset = ZP+0
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;ZX0_src = ZP+2
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;ZX0_dst = ZP+4
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;bitr = ZP+6
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;pntr = ZP+7
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; Initial values for offset, source, destination and bitr
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;zx0_ini_block:
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; .byte $00, $00 ; offset
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;comp_data:
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; .byte $0, $0 ; zx0_src
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;out_addr:
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; .byte $0, $0 ; zx0_dst
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; .byte $80 ; bitr
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;--------------------------------------------------
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; Decompress ZX0 data (6502 optimized format)
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; destination page in A
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full_decomp:
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sta ZX0_dst+1
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ldy #$80
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sty bitr
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ldy #0 ; always on page boundary
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sty ZX0_dst
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sty offset
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sty offset+1
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; Y needs to be 0 here
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; Get initialization block
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; ldy #7
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;copy_init: lda zx0_ini_block-1, y
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; sta offset-1, y
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; dey
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; bne copy_init
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; Decode literal: Ccopy next N bytes from compressed file
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; Elias(length) byte[1] byte[2] ... byte[N]
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decode_literal:
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jsr get_elias
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cop0: lda (ZX0_src), y
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inc ZX0_src
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bne plus1
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inc ZX0_src+1
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plus1: sta (ZX0_dst),y
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inc ZX0_dst
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bne plus2
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inc ZX0_dst+1
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plus2: dex
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bne cop0
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asl bitr
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bcs dzx0s_new_offset
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; Copy from last offset (repeat N bytes from last offset)
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; Elias(length)
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jsr get_elias
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dzx0s_copy:
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lda ZX0_dst
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sbc offset ; C=0 from get_elias
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sta pntr
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lda ZX0_dst+1
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sbc offset+1
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sta pntr+1
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cop1:
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lda (pntr), y
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inc pntr
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bne plus3
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inc pntr+1
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plus3: sta (ZX0_dst),y
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inc ZX0_dst
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bne plus4
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inc ZX0_dst+1
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plus4: dex
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bne cop1
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asl bitr
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bcc decode_literal
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; Copy from new offset (repeat N bytes from new offset)
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; Elias(MSB(offset)) LSB(offset) Elias(length-1)
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dzx0s_new_offset:
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; Read elias code for high part of offset
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jsr get_elias
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beq exit ; Read a 0, signals the end
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; Decrease and divide by 2
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dex
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txa
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lsr ; @
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sta offset+1
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; Get low part of offset, a literal 7 bits
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lda (ZX0_src), y
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inc ZX0_src
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bne plus5
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inc ZX0_src+1
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plus5:
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; Divide by 2
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ror ; @
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sta offset
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; And get the copy length.
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; Start elias reading with the bit already in carry:
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ldx #1
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jsr elias_skip1
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inx
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bcc dzx0s_copy
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; Read an elias-gamma interlaced code.
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; ------------------------------------
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get_elias:
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; Initialize return value to #1
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ldx #1
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bne elias_start
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elias_get: ; Read next data bit to result
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asl bitr
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rol ; @
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tax
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elias_start:
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; Get one bit
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asl bitr
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bne elias_skip1
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; Read new bit from stream
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lda (ZX0_src), y
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inc ZX0_src
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bne plus6
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inc ZX0_src+1
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plus6: ;sec ; not needed, C=1 guaranteed from last bit
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rol ;@
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sta bitr
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elias_skip1:
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txa
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bcs elias_get
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; Got ending bit, stop reading
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exit:
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rts
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