220 lines
4.9 KiB
ArmAsm
220 lines
4.9 KiB
ArmAsm
; Stepwise LZ4 data decompressor for Apple II
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; Only decodes X tokens before returning
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; Code by Peter Ferrie (qkumba) (peter.ferrie@gmail.com)
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; "LZ4 unpacker in 143 bytes (6502 version) (2013)"
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; http://pferrie.host22.com/misc/appleii.htm
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; This is that code, but with comments and labels added for clarity.
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; I also found a bug when decoding with runs of multiples of 256
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; which has since been fixed upstream.
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; For LZ4 reference see
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; https://github.com/lz4/lz4/wiki/lz4_Frame_format.md
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; LZ4 summary:
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;
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; HEADER:
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; Should: check for magic number 04 22 4d 18
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; FLG: 64 in our case (01=version, block.index=1, block.checksum=0
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; size=0, checksum=1, reserved
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; MAX Blocksize: 40 (64kB)
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; HEADER CHECKSUM: a7
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; BLOCK HEADER: 4 bytes (le) If highest bit set, uncompressed!
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; BLOCKS:
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; Token byte. High 4-bits literal length, low 4-bits copy length
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; + If literal length==15, then following byte gets added to length
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; If that byte was 255, then keep adding bytes until not 255
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; + The literal bytes follow. There may be zero of them
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; + Next is block copy info. little-endian 2-byte offset to
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; be subtracted from current read position indicating source
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; + The low 4-bits of the token are the copy length, which needs
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; 4 added to it. As with the literal length, if it is 15 then
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; you read a byte and add (and if that byte is 255, keep adding)
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;LZ4_SRC EQU $00
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;LZ4_DST EQU $02
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;LZ4_END EQU $04
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;COUNT EQU $06
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;DELTA EQU $08
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;UNPACK_BUFFER EQU $5E00 ; offset of first unpacked byte
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;======================
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; LZ4 decode
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;======================
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; input buffer in LZ4_SRC
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; output buffer hardcoded still
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; size in ENDH:ENDL
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lz4_decode_setup:
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lda LZ4_SRC ; packed data offset
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clc
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adc LZ4_END
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sta LZ4_END
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lda LZ4_SRC+1
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adc LZ4_END+1
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sta LZ4_END+1
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lda #>UNPACK_BUFFER ; original unpacked data offset
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sta LZ4_DST+1
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lda #<UNPACK_BUFFER
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sta LZ4_DST
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rts
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lz4_decode_step:
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ldy #0 ; used to index, always zero
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parsetoken:
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jsr getsrc ; get next token
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pha ; save for later (need bottom 4 bits)
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lsr ; number of literals in top 4 bits
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lsr ; so shift into place
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lsr
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lsr
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beq copymatches ; if zero, then no literals
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; jump ahead and copy
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jsr buildcount ; add up all the literal sizes
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; result is in ram[count+1]-1:A
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tax ; now in ram[count+1]-1:X
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jsr docopy ; copy the literals
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lda LZ4_SRC ; 16-bit compare
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cmp LZ4_END ; to see if we have reached the end
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lda LZ4_SRC+1
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sbc LZ4_END+1
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bcs done
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copymatches:
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jsr getsrc ; get 16-bit delta value
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sta DELTA
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jsr getsrc
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sta DELTA+1
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pla ; restore token
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and #$0f ; get bottom 4 bits
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; match count. 0 means 4
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; 15 means 19+, must be calculated
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jsr buildcount ; add up count bits, in ram[count+1]-:A
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clc
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adc #4 ; adjust count by 4 (minmatch)
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tax ; now in ramp[count+1]-1:X
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beq copy_no_adjust ; BUGFIX, don't increment if
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; exactly a multiple of 0x100
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bcc copy_no_adjust
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inc COUNT+1 ; increment if we overflowed
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copy_no_adjust:
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lda LZ4_SRC+1 ; save src on stack
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pha
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lda LZ4_SRC
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pha
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sec ; subtract delta
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lda LZ4_DST ; from destination, make new src
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sbc DELTA
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sta LZ4_SRC
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lda LZ4_DST+1
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sbc DELTA+1
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sta LZ4_SRC+1
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jsr docopy ; do the copy
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pla ; restore the src
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sta LZ4_SRC
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pla
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sta LZ4_SRC+1
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clc ; c==0 means not done
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rts
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; jmp parsetoken ; back to parsing tokens
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done:
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pla
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sec ; c=1 means done
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rts
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;=========
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; getsrc
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;=========
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; gets byte from src into A, increments pointer
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getsrc:
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lda (LZ4_SRC), Y ; get a byte from src
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inc LZ4_SRC ; increment pointer
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bne done_getsrc ; update 16-bit pointer
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inc LZ4_SRC+1 ; on 8-bit overflow
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done_getsrc:
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rts
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;============
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; buildcount
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;============
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buildcount:
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ldx #1 ; high count starts at 1
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stx COUNT+1 ; (loops at zero?)
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cmp #$0f ; if LITERAL_COUNT < 15, we are done
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bne done_buildcount
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buildcount_loop:
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sta COUNT ; save LITERAL_COUNT (15)
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jsr getsrc ; get the next byte
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tax ; put in X
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clc
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adc COUNT ; add new byte to old value
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bcc bc_8bit_oflow ; if overflow, increment high byte
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inc COUNT+1
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bc_8bit_oflow:
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inx ; check if read value was 255
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beq buildcount_loop ; if it was, keep looping and adding
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done_buildcount:
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rts
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;============
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; getput
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;============
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; gets a byte, then puts the byte
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getput:
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jsr getsrc
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; fallthrough to putdst
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;=============
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; putdst
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;=============
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; store A into destination
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putdst:
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sta (LZ4_DST), Y ; store A into destination
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inc LZ4_DST ; increment 16-bit pointer
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bne putdst_end ; if overflow, increment top byte
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inc LZ4_DST+1
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putdst_end:
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rts
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;=============================
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; docopy
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;=============================
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; copies ram[count+1]-1:X bytes
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; from src to dst
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docopy:
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docopy_loop:
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jsr getput ; get/put byte
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dex ; decrement count
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bne docopy_loop ; if not zero, loop
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dec COUNT+1 ; if zero, decrement high byte
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bne docopy_loop ; if not zero, loop
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rts
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