dos33fsprogs/still_alive/lz4_decode.s

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; LZ4 data decompressor for Apple II
; Code by Peter Ferrie (qkumba) (peter.ferrie@gmail.com)
; "LZ4 unpacker in 143 bytes (6502 version) (2013)"
; http://pferrie.host22.com/misc/appleii.htm
; This is that code, but with comments and labels added for clarity.
; I also found a bug when decoding with runs of multiples of 256
; which has since been fixed upstream.
; For LZ4 reference see
; https://github.com/lz4/lz4/wiki/lz4_Frame_format.md
; LZ4 summary:
;
; HEADER:
; Should: check for magic number 04 22 4d 18
; FLG: 64 in our case (01=version, block.index=1, block.checksum=0
; size=0, checksum=1, reserved
; MAX Blocksize: 40 (64kB)
; HEADER CHECKSUM: a7
; BLOCK HEADER: 4 bytes (le) If highest bit set, uncompressed!
; BLOCKS:
; Token byte. High 4-bits literal length, low 4-bits copy length
; + If literal length==15, then following byte gets added to length
; If that byte was 255, then keep adding bytes until not 255
; + The literal bytes follow. There may be zero of them
; + Next is block copy info. little-endian 2-byte offset to
; be subtracted from current read position indicating source
; + The low 4-bits of the token are the copy length, which needs
; 4 added to it. As with the literal length, if it is 15 then
; you read a byte and add (and if that byte is 255, keep adding)
;LZ4_SRC EQU $00
;LZ4_DST EQU $02
;LZ4_END EQU $04
;COUNT EQU $06
;DELTA EQU $08
;UNPACK_BUFFER EQU $5E00 ; offset of first unpacked byte
;======================
; LZ4 decode
;======================
; input buffer in LZ4_SRC
; output buffer hardcoded still
; size in ENDH:ENDL
lz4_decode:
lda LZ4_SRC ; packed data offset
clc
adc LZ4_END
sta LZ4_END
lda LZ4_SRC+1
adc LZ4_END+1
sta LZ4_END+1
lda #>UNPACK_BUFFER ; original unpacked data offset
sta LZ4_DST+1
lda #<UNPACK_BUFFER
sta LZ4_DST
unpmain:
ldy #0 ; used to index, always zero
parsetoken:
jsr getsrc ; get next token
pha ; save for later (need bottom 4 bits)
lsr ; number of literals in top 4 bits
lsr ; so shift into place
lsr
lsr
beq copymatches ; if zero, then no literals
; jump ahead and copy
jsr buildcount ; add up all the literal sizes
; result is in ram[count+1]-1:A
tax ; now in ram[count+1]-1:X
jsr docopy ; copy the literals
lda LZ4_SRC ; 16-bit compare
cmp LZ4_END ; to see if we have reached the end
lda LZ4_SRC+1
sbc LZ4_END+1
bcs done
copymatches:
jsr getsrc ; get 16-bit delta value
sta DELTA
jsr getsrc
sta DELTA+1
pla ; restore token
and #$0f ; get bottom 4 bits
; match count. 0 means 4
; 15 means 19+, must be calculated
jsr buildcount ; add up count bits, in ram[count+1]-:A
clc
adc #4 ; adjust count by 4 (minmatch)
tax ; now in ramp[count+1]-1:X
beq copy_no_adjust ; BUGFIX, don't increment if
; exactly a multiple of 0x100
bcc copy_no_adjust
inc COUNT+1 ; increment if we overflowed
copy_no_adjust:
lda LZ4_SRC+1 ; save src on stack
pha
lda LZ4_SRC
pha
sec ; subtract delta
lda LZ4_DST ; from destination, make new src
sbc DELTA
sta LZ4_SRC
lda LZ4_DST+1
sbc DELTA+1
sta LZ4_SRC+1
jsr docopy ; do the copy
pla ; restore the src
sta LZ4_SRC
pla
sta LZ4_SRC+1
jmp parsetoken ; back to parsing tokens
done:
pla
rts
;=========
; getsrc
;=========
; gets byte from src into A, increments pointer
getsrc:
lda (LZ4_SRC), Y ; get a byte from src
inc LZ4_SRC ; increment pointer
bne done_getsrc ; update 16-bit pointer
inc LZ4_SRC+1 ; on 8-bit overflow
done_getsrc:
rts
;============
; buildcount
;============
buildcount:
ldx #1 ; high count starts at 1
stx COUNT+1 ; (loops at zero?)
cmp #$0f ; if LITERAL_COUNT < 15, we are done
bne done_buildcount
buildcount_loop:
sta COUNT ; save LITERAL_COUNT (15)
jsr getsrc ; get the next byte
tax ; put in X
clc
adc COUNT ; add new byte to old value
bcc bc_8bit_oflow ; if overflow, increment high byte
inc COUNT+1
bc_8bit_oflow:
inx ; check if read value was 255
beq buildcount_loop ; if it was, keep looping and adding
done_buildcount:
rts
;============
; getput
;============
; gets a byte, then puts the byte
getput:
jsr getsrc
; fallthrough to putdst
;=============
; putdst
;=============
; store A into destination
putdst:
sta (LZ4_DST), Y ; store A into destination
inc LZ4_DST ; increment 16-bit pointer
bne putdst_end ; if overflow, increment top byte
inc LZ4_DST+1
putdst_end:
rts
;=============================
; docopy
;=============================
; copies ram[count+1]-1:X bytes
; from src to dst
docopy:
docopy_loop:
jsr getput ; get/put byte
dex ; decrement count
bne docopy_loop ; if not zero, loop
dec COUNT+1 ; if zero, decrement high byte
bne docopy_loop ; if not zero, loop
rts