dos33fsprogs/asm_routines/lz4_decode.s
2018-02-20 23:18:28 -05:00

195 lines
3.4 KiB
ArmAsm

; LZ4 data decompressor for Apple II
; Based heavily on code by Peter Ferrie (peter.ferrie@gmail.com)
; For LZ4 reference see
; https://github.com/lz4/lz4/wiki/lz4_Frame_format.md
src EQU $00
dst EQU $02
end EQU $04
count EQU $06
delta EQU $08
A1L EQU $3c
A1H EQU $3d
A2L EQU $3e
A2H EQU $3f
A4L EQU $42
A4H EQU $43
size EQU 794
orgoff EQU $6000 ; offset of first unpacked byte
paksize EQU size-$b-8
; size of packed data
; minus 4 for checksum at end
; not sure what other 4 is from?
; block checksum? though had that disabled?
pakoff EQU $400b ; 11 byte offset to data?
lz4_decode:
lda #<pakoff ; packed data offset
sta src
lda #<(pakoff+paksize) ; packed data size
sta end
lda #>pakoff
sta src+1
lda #>(pakoff+paksize)
sta end+1
lda #>orgoff ; original unpacked data offset
sta dst+1
lda #<orgoff
sta dst
; 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!
unpmain:
ldy #0 ; used to index
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
jsr buildcount
tax
jsr docopy
lda src
cmp end
lda src+1
sbc 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
clc
adc #4 ; adjust count by 4 (minmatch)
tax
bcc copy_skip
inc count+1
copy_skip:
lda src+1
pha
lda src
pha
sec
lda dst
sbc delta
sta src
lda dst+1
sbc delta+1
sta src+1
jsr docopy
pla
sta src
pla
sta src+1
jmp parsetoken
done:
pla
rts
;=========
; getsrc
;=========
getsrc:
lda (src), Y ; get a byte from src
inc src ; increment pointer
bne done_getsrc ; update 16-bit pointer
inc 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
;=============
putdst:
sta (dst), Y
inc dst
bne putdst_end
inc dst+1
putdst_end:
rts
;=============================
; docopy
;=============================
; copies ram[count+1]-1:X bytes
; from src to dst
docopy:
; working around bug in original code that would loop 256 too
; many times if incoming X was equal to 0
cpx #$0
bne docopy_loop
jsr getput
dex
bne docopy_hack
docopy_loop:
jsr getput ; get/put byte
dex ; decrement count
bne docopy_loop ; if not zero, loop
docopy_hack:
dec count+1 ; if zero, decrement high byte
bne docopy_loop ; if not zero, loop
rts