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|
; ***************************************************************************
; ***************************************************************************
;
; aplib_6502.s
;
; NMOS 6502 decompressor for data stored in Jorgen Ibsen's aPLib format.
;
; Includes support for Emmanuel Marty's enhancements to the aPLib format.
;
; The code is 252 bytes long for standard format, 270 for enhanced format.
;
; This code is written for the ACME assembler.
;
; Copyright John Brandwood 2019.
;
; Distributed under the Boost Software License, Version 1.0.
; (See accompanying file LICENSE_1_0.txt or copy at
; http://www.boost.org/LICENSE_1_0.txt)
;
; ***************************************************************************
; ***************************************************************************
; ***************************************************************************
; ***************************************************************************
;
; Decompression Macros
;
;
; Macro to increment the source pointer to the next page.
;
!macro APL_INC_PAGE {
inc <apl_srcptr + 1
}
;
; Macro to read a byte from the compressed source data.
;
!macro APL_GET_SRC {
lda (apl_srcptr),y
inc <apl_srcptr + 0
bne .skip
+APL_INC_PAGE
.skip:
}
; ***************************************************************************
; ***************************************************************************
;
; Data usage is last 12 bytes of zero-page.
;
apl_bitbuf = $F7 ; 1 byte.
apl_offset = $F8 ; 1 word.
apl_winptr = $FA ; 1 word.
apl_srcptr = $FC ; 1 word.
apl_dstptr = $FE ; 1 word.
apl_length = apl_winptr
; ***************************************************************************
; ***************************************************************************
;
; apl_decompress - Decompress data stored in Jorgen Ibsen's aPLib format.
;
; Args: apl_srcptr = ptr to compessed data
; Args: apl_dstptr = ptr to output buffer
; Uses: lots!
;
; As an optimization, the code to handle window offsets > 64768 bytes has
; been removed, since these don't occur with a 16-bit address range.
;
; As an optimization, the code to handle window offsets > 32000 bytes can
; be commented-out, since these don't occur in typical 8-bit computer usage.
;
apl_decompress: ldy #0 ; Initialize source index.
lda #$80 ; Initialize an empty
sta <apl_bitbuf ; bit-buffer.
;
; 0 bbbbbbbb - One byte from compressed data, i.e. a "literal".
;
.literal: +APL_GET_SRC
.write_byte: ldx #0 ; LWM=0.
sta (apl_dstptr),y ; Write the byte directly to
inc <apl_dstptr + 0 ; the output.
bne .next_tag
inc <apl_dstptr + 1
.next_tag: asl <apl_bitbuf ; 0 bbbbbbbb
bne .skip0
jsr .load_bit
.skip0: bcc .literal
.skip1: asl <apl_bitbuf ; 1 0 <offset> <length>
bne .skip2
jsr .load_bit
.skip2: bcc .copy_large
asl <apl_bitbuf ; 1 1 0 dddddddn
bne .skip3
jsr .load_bit
.skip3: bcc .copy_normal
; 1 1 1 dddd - Copy 1 byte within 15 bytes (or zero).
.copy_short: lda #$10
.nibble_loop: asl <apl_bitbuf
bne .skip4
pha
jsr .load_bit
pla
.skip4: rol
bcc .nibble_loop
beq .write_byte ; Offset=0 means write zero.
eor #$FF ; Read the byte directly from
tay ; the destination window.
iny
dec <apl_dstptr + 1
lda (apl_dstptr),y
inc <apl_dstptr + 1
ldy #0
beq .write_byte
;
; 1 1 0 dddddddn - Copy 2 or 3 within 128 bytes.
;
.copy_normal: +APL_GET_SRC ; 1 1 0 dddddddn
lsr
beq .finished ; Offset 0 == EOF.
sta <apl_offset + 0 ; Preserve offset.
sty <apl_offset + 1
tya ; Y == 0.
tax ; Bits 8..15 of length.
adc #2 ; Bits 0...7 of length.
bne .do_match ; NZ from previous ADC.
;
; Subroutines for byte & bit handling.
;
.get_gamma: lda #1 ; Get a gamma-coded value.
.gamma_loop: asl <apl_bitbuf
bne .skip5
pha
jsr .load_bit
pla
.skip5: rol
rol <apl_length + 1
asl <apl_bitbuf
bne .skip6
pha
jsr .load_bit
pla
.skip6: bcs .gamma_loop
.finished: rts ; All decompressed!
;
; 1 0 <offset> <length> - gamma-coded LZSS pair.
;
.copy_large: jsr .get_gamma ; Bits 8..15 of offset (min 2).
sty <apl_length + 1 ; Clear hi-byte of length.
cpx #1 ; CC if LWM==0, CS if LWM==1.
sbc #2 ; -3 if LWM==0, -2 if LWM==1.
bcs .normal_pair ; CC if LWM==0 && offset==2.
jsr .get_gamma ; Get length (A=lo-byte & CC).
ldx <apl_length + 1
bcc .do_match ; Use previous Offset.
.normal_pair: sta <apl_offset + 1 ; Save bits 8..15 of offset.
+APL_GET_SRC
sta <apl_offset + 0 ; Save bits 0...7 of offset.
jsr .get_gamma ; Get length (A=lo-byte & CC).
ldx <apl_length + 1
ldy <apl_offset + 1 ; If offset < 256.
beq .lt256
cpy #$7D ; If offset >= 32000, length += 2.
bcs .match_plus2
cpy #$05 ; If offset >= 1280, length += 1.
bcs .match_plus1
bcc .do_match
.lt256: ldy <apl_offset + 0 ; If offset < 128, length += 2.
bmi .do_match
sec ; aPLib gamma returns with CC.
.match_plus2: adc #1 ; CS, so ADC #2.
bcs .match_plus256
.match_plus1: adc #0 ; CS, so ADC #1, or CC if fall
bcc .do_match ; through from .match_plus2.
.match_plus256: inx
.do_match: eor #$FF ; Negate the lo-byte of length
tay ; and check for zero.
iny
beq .calc_addr
eor #$FF
inx ; Increment # of pages to copy.
clc ; Calc destination for partial
adc <apl_dstptr + 0 ; page.
sta <apl_dstptr + 0
bcs .calc_addr
dec <apl_dstptr + 1
.calc_addr: sec ; Calc address of match.
lda <apl_dstptr + 0
sbc <apl_offset + 0
sta <apl_winptr + 0
lda <apl_dstptr + 1
sbc <apl_offset + 1
sta <apl_winptr + 1
.copy_page: lda (apl_winptr),y
sta (apl_dstptr),y
iny
bne .copy_page
inc <apl_winptr + 1
inc <apl_dstptr + 1
dex ; Any full pages left to copy?
bne .copy_page
inx ; LWM=1.
jmp .next_tag
;
; Subroutines for byte & bit handling.
;
.load_bit: +APL_GET_SRC ; Reload an empty bit-buffer
rol ; from the compressed source.
sta <apl_bitbuf
rts
|
