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Fast 6502 LZSA2 depacker: smaller size, same speed
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Emmanuel Marty
GitHub
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8721c11041
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410544f4e6
@ -5,10 +5,12 @@
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;
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; NMOS 6502 decompressor for data stored in Emmanuel Marty's LZSA2 format.
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;
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; This code is written for the ACME assembler.
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;
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; Optional code is presented for two minor 6502 optimizations that break
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; compatibility with the current LZSA2 format standard.
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;
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; This code is written for the ACME assembler.
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; The code is 241 bytes for the small version, and 256 bytes for the normal.
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;
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; Copyright John Brandwood 2019.
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;
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@ -46,12 +48,34 @@ LZSA_SWAP_LEN16 = 0
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LZSA_SWAP_XZY = 0
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;
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; Remove code inlining to save space?
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;
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; This saves 15 bytes of code, but decompression is 7% slower.
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; Choose size over space (within sane limits)?
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;
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LZSA_BEST_SIZE = 0
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LZSA_SMALL_SIZE = 0
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;
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; Remove code inlining to save space?
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;
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; This saves 15 bytes of code at the cost of 7% speed.
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;
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!if LZSA_SMALL_SIZE {
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LZSA_NO_INLINE = 1
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} else {
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LZSA_NO_INLINE = 0
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}
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;
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; Use smaller code for copying literals?
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;
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; This saves 11 bytes of code at the cost of 5% speed.
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;
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!if LZSA_SMALL_SIZE {
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LZSA_SHORT_CP = 1
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} else {
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LZSA_SHORT_CP = 1
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}
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;
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; Assume that we're decompessing from a large multi-bank
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@ -66,27 +90,23 @@ LZSA_FROM_BANK = 0
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;
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!if LZSA_FROM_BANK {
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!macro LZSA_INC_PAGE {
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jsr .next_page
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}
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} else {
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!macro LZSA_INC_PAGE {
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inc <lzsa_srcptr + 1
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}
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}
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;
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; Macro to read a byte from the compressed source data.
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;
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!if LZSA_BEST_SIZE {
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!if LZSA_NO_INLINE {
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!macro LZSA_GET_SRC {
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jsr .get_byte
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jsr lzsa2_get_byte
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}
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} else {
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@ -104,10 +124,13 @@ LZSA_FROM_BANK = 0
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;
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; Macro to speed up reading 50% of nibbles.
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;
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; This seems to save very few cycles compared to the
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; increase in code size, and it isn't recommended.
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;
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LZSA_SLOW_NIBL = 1
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!if (LZSA_SLOW_NIBL + LZSA_BEST_SIZE) {
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!if (LZSA_SLOW_NIBL + LZSA_SMALL_SIZE) {
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!macro LZSA_GET_NIBL {
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jsr lzsa2_get_nibble ; Always call a function.
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@ -118,9 +141,9 @@ LZSA_SLOW_NIBL = 1
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!macro LZSA_GET_NIBL {
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lsr <lzsa_nibflg ; Is there a nibble waiting?
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lda <lzsa_nibble ; Extract the lo-nibble.
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bcs .skip\@
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jsr .new_nibble ; Extract the hi-nibble.
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.skip\@: ora #$F0
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bcs .skip
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jsr lzsa2_new_nibble ; Extract the hi-nibble.
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.skip: ora #$F0
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}
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}
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@ -141,15 +164,19 @@ lzsa_winptr = $FA ; 1 word.
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lzsa_srcptr = $FC ; 1 word.
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lzsa_dstptr = $FE ; 1 word.
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lzsa_length = lzsa_winptr ; 1 word.
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LZSA_SRC_LO = $FC
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LZSA_SRC_HI = $FD
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LZSA_DST_LO = $FE
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LZSA_DST_HI = $FF
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; ***************************************************************************
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; ***************************************************************************
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;
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; lzsa2_unpack - Decompress data stored in Emmanuel Marty's LZSA2b format.
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; lzsa2_unpack - Decompress data stored in Emmanuel Marty's LZSA2 format.
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;
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; Args: lzsa_srcptr = ptr to compessed data
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; Args: lzsa_dstptr = ptr to output buffer
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@ -182,20 +209,45 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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jsr .get_length ; X=0 table index for literals.
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!if LZSA_SHORT_CP {
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.got_cp_len: cmp #0 ; Check the lo-byte of length.
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beq .put_cp_len
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inx ; Increment # of pages to copy.
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.put_cp_len: stx <lzsa_length
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tax
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.cp_page: lda (lzsa_srcptr),y
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sta (lzsa_dstptr),y
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inc <lzsa_srcptr + 0
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bne .skip1
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inc <lzsa_srcptr + 1
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.skip1: inc <lzsa_dstptr + 0
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bne .skip2
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inc <lzsa_dstptr + 1
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.skip2: dex
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bne .cp_page
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dec <lzsa_length ; Any full pages left to copy?
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bne .cp_page
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} else {
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.got_cp_len: tay ; Check the lo-byte of length.
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beq .cp_page
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inx ; Increment # of pages to copy.
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.get_cp_src: clc ; Calc source for partial
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adc <lzsa_srcptr + 0 ; page.
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.get_cp_src: clc ; Calc address of partial page.
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adc <lzsa_srcptr + 0
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sta <lzsa_srcptr + 0
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bcs .get_cp_dst
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dec <lzsa_srcptr + 1
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.get_cp_dst: tya
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clc ; Calc destination for partial
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adc <lzsa_dstptr + 0 ; page.
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clc ; Calc address of partial page.
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adc <lzsa_dstptr + 0
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sta <lzsa_dstptr + 0
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bcs .get_cp_idx
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dec <lzsa_dstptr + 1
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@ -214,6 +266,8 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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dex ; Any full pages left to copy?
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bne .cp_page
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}
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!if LZSA_SWAP_XZY {
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;
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@ -240,10 +294,10 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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bne .get_9_bits
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.get_5_bits: dex ; X=$FF
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.get_13_bits: LZSA_GET_NIBL ; Always returns with CS.
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.get_13_bits: +LZSA_GET_NIBL ; Always returns with CS.
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bvc .get_5_skip
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clc
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.get_5_skip: rol a ; Shift into position, set C.
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.get_5_skip: rol ; Shift into position, set C.
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cpx #$00 ; X=$FF for a 5-bit offset.
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bne .set_offset
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sbc #2 ; Subtract 512 because 13-bit
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@ -264,7 +318,7 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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;
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; Slower and longer path with STD order of bits.
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;
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; Z80 NES
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; STD NEW
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; ================================
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; xyz xzy
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; 00z 0z0 5-bit offset
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@ -274,6 +328,10 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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; 111 111 repeat offset
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; NVZ for a BIT instruction
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;
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; N.B. Costs 3 bytes in code length.
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; get5 and get13 are 8 cycles slower.
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; get9, get16, and rep are 4 cycles slower.
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;
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.lz_offset: lda <lzsa_cmdbuf
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asl
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@ -313,7 +371,7 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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; N.B. X=0 is expected and guaranteed when we get here.
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;
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.get_16_bits: jsr .get_byte ; Get hi-byte of offset.
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.get_16_bits: jsr lzsa2_get_byte ; Get hi-byte of offset.
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tax
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.get_low8: +LZSA_GET_SRC ; Get lo-byte of offset.
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@ -379,8 +437,6 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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;
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; Get 16-bit length in X:A register pair.
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;
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; N.B. Requires reversal of bytes in 16-bit length.
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;
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.get_length: +LZSA_GET_NIBL
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cmp #$FF ; Extended length?
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@ -390,28 +446,29 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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.got_length: ldx #$00 ; Set hi-byte of 4 & 8 bit
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rts ; lengths.
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.byte_length: jsr .get_byte ; So rare, this can be slow!
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.byte_length: jsr lzsa2_get_byte ; So rare, this can be slow!
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adc .byte_len_tbl,x ; Always CS from previous CMP.
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bcc .got_length
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beq .finished
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!if LZSA_SWAP_LEN16 {
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.word_length: jsr .get_byte ; So rare, this can be slow!
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.word_length: jsr lzsa2_get_byte ; So rare, this can be slow!
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tax
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} else {
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.word_length: jsr .get_byte ; So rare, this can be slow!
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.word_length: jsr lzsa2_get_byte ; So rare, this can be slow!
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pha
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jsr .get_byte ; So rare, this can be slow!
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jsr lzsa2_get_byte ; So rare, this can be slow!
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tax
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pla
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rts
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}
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.get_byte: lda (lzsa_srcptr),y ; Subroutine version for when
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lzsa2_get_byte:
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lda (lzsa_srcptr),y ; Subroutine version for when
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inc <lzsa_srcptr + 0 ; inlining isn't advantageous.
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beq .next_page
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rts
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@ -430,9 +487,10 @@ lzsa2_unpack: ldy #0 ; Initialize source index.
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; Get a nibble value from compressed data in A.
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;
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!if (LZSA_SLOW_NIBL + LZSA_BEST_SIZE) {
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!if (LZSA_SLOW_NIBL | LZSA_SMALL_SIZE) {
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lzsa2_get_nibble: lsr <lzsa_nibflg ; Is there a nibble waiting?
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lzsa2_get_nibble:
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lsr <lzsa_nibflg ; Is there a nibble waiting?
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lda <lzsa_nibble ; Extract the lo-nibble.
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bcs .got_nibble
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@ -453,13 +511,14 @@ lzsa2_get_nibble: lsr <lzsa_nibflg ; Is there a nibble waiting
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} else {
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.new_nibble: inc <lzsa_nibflg ; Reset the flag.
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LZSA_GET_SRC
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lzsa2_new_nibble:
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inc <lzsa_nibflg ; Reset the flag.
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+LZSA_GET_SRC
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sta <lzsa_nibble ; Preserve for next time.
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lsr a ; Extract the hi-nibble.
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lsr a
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lsr a
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lsr a
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lsr ; Extract the hi-nibble.
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lsr
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lsr
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lsr
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!if LZSA_SWAP_XZY {
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sec ; Offset code relies on CS.
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