mirror of
https://github.com/emmanuel-marty/lzsa.git
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f123a6d9df
Because the 8086's BIU is more efficient and can read a word in a single operation, we can skew LZSA1 decompression faster on 8086 by using a jump table to eliminate 2 comparions.
275 lines
12 KiB
NASM
275 lines
12 KiB
NASM
; lzsa1jmp.asm - time-efficient LZSA1 decompressor implementation for 8086.
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; Turbo Assembler IDEAL mode dialect; can also be assembled with NASM.
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;
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; Usual DOS assembler SMALL model assumptions apply. This code:
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; - Assumes it was invoked via NEAR call (change RET to RETF for FAR calls)
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; - Is interrupt-safe
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; - Is not re-entrant (do not decompress while already running decompression)
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; - Trashes all data and segment registers
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;
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; Copyright (C) 2019 Jim Leonard, Emmanuel Marty
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;
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; This software is provided 'as-is', without any express or implied
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; warranty. In no event will the authors be held liable for any damages
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; arising from the use of this software.
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;
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; Permission is granted to anyone to use this software for any purpose,
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; including commercial applications, and to alter it and redistribute it
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; freely, subject to the following restrictions:
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;
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; 1. The origin of this software must not be misrepresented; you must not
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; claim that you wrote the original software. If you use this software
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; in a product, an acknowledgment in the product documentation would be
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; appreciated but is not required.
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; 2. Altered source versions must be plainly marked as such, and must not be
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; misrepresented as being the original software.
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; 3. This notice may not be removed or altered from any source distribution.
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IDEAL
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P8086
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SEGMENT CODE para public
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ASSUME cs:CODE, ds:CODE
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PUBLIC lzsa1_decompress_speed_jumptable
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; ---------------------------------------------------------------------------
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; Decompress raw LZSA1 block
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; inputs:
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; * ds:si: raw LZSA1 block
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; * es:di: output buffer
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; output:
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; * ax: decompressed size
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; ---------------------------------------------------------------------------
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;Jump table for handling LLL bits in initial LZSA1 tokens.
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;Previous code would SHR val,4 to get a count from 0 to 7, then rep movsb.
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;We can overload the shift operation into a jump table that jumps directly
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;to optimized copying routine for 0-7 bytes. Must declare in code segment.
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;Note: If this looks strange for declaring a jump table, that's because it
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;is a workaround for the Turbo Pascal harness that tests it. Turbo Pascal
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;treats OFFSET (label) as a relocatble item and throws an error, so we fool
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;it by building the table with absolute EQU/literals instead.
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L0b EQU OFFSET check_offset_size
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L1b EQU OFFSET copy1b
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L2b EQU OFFSET copy2b
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L3b EQU OFFSET copy3b
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L4b EQU OFFSET copy4b
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L5b EQU OFFSET copy5b
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L6b EQU OFFSET copy6b
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L7b EQU OFFSET need_length_byte
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copytable DW L0b,L0b,L0b,L0b,L0b,L0b,L0b,L0b
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DW L1b,L1b,L1b,L1b,L1b,L1b,L1b,L1b
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DW L2b,L2b,L2b,L2b,L2b,L2b,L2b,L2b
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DW L3b,L3b,L3b,L3b,L3b,L3b,L3b,L3b
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DW L4b,L4b,L4b,L4b,L4b,L4b,L4b,L4b
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DW L5b,L5b,L5b,L5b,L5b,L5b,L5b,L5b
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DW L6b,L6b,L6b,L6b,L6b,L6b,L6b,L6b
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DW L7b,L7b,L7b,L7b,L7b,L7b,L7b,L7b
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PROC lzsa1_decompress_speed_jumptable NEAR
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lzsa1_start:
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push di ;remember decompression offset
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cld ;ensure string ops move forward
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xor cx,cx
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@@decode_token:
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xchg cx,ax ;clear ah (cx = 0 from match copy's rep movsb)
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lodsb ;read token byte: O|LLL|MMMM
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mov dx,ax ;copy our token to dl for later MMMM handling
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and al,070H ;isolate literals length in token (LLL)
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jz check_offset_size ;if LLL=0, we have no literals; goto match
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; Jump to short copy routine for LLL=1 though 6, need_length_byte for LLL=7
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mov bx,ax ;prep for table lookup (must copy, don't XCHG!)
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jmp [cs:copytable+bx]
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need_length_byte:
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lodsb ;grab extra length byte
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add al,07H ;add LITERALS_RUN_LEN
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jnc @@got_literals_exact ;if no overflow, we have full count
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je @@big_literals
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@@mid_literals:
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lodsb ;grab single extra length byte
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inc ah ;add 256
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xchg cx,ax ;with longer counts, we can save some time
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shr cx,1 ;by doing a word copy instead of a byte copy.
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rep movsw ;We don't need to account for overlap because
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adc cx,0 ;source for literals isn't the output buffer.
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rep movsb
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jmp check_offset_size
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@@big_literals:
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lodsw ;grab 16-bit extra length
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xchg cx,ax ;with longer counts, we can save some time
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shr cx,1 ;by doing a word copy instead of a byte copy.
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rep movsw
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adc cx,0
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rep movsb
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jmp check_offset_size
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; Used for counts 7-248. In test data, average value around 1Ah. YMMV.
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@@got_literals_exact:
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xchg cx,ax
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rep movsb ;copy cx literals from ds:si to es:di
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jmp check_offset_size
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;Literal copy sequence for lengths 1-6:
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copy6b: movsb
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copy5b: movsb
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copy4b: movsb
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copy3b: movsb
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copy2b: movsb
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copy1b: movsb
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;Literals done; fall through to match offset determination
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check_offset_size:
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test dl,dl ;check match offset size in token (O bit)
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js @@get_long_offset ;load absolute 16-bit match offset
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mov ah,0ffh ;set up high byte
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lodsb ;load low byte
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@@get_match_length:
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xchg dx,ax ;dx: match offset ax: original token
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and al,0FH ;isolate match length in token (MMMM)
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cmp al,0FH ;MATCH_RUN_LEN?
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jne @@got_matchlen_short ;no, we have the full match length from the token, go copy
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lodsb ;grab extra length byte
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add al,012H ;add MIN_MATCH_SIZE + MATCH_RUN_LEN
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jnc @@do_long_copy ;if no overflow, we have the entire length
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jne @@mid_matchlen
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lodsw ;grab 16-bit length
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xchg cx,ax ;get ready to do a long copy
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jcxz @@done_decompressing ;wait, is it the EOD marker? Exit if so
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jmp @@copy_len_preset ;otherwise, do the copy
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@@got_matchlen_short:
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add al,3 ;add MIN_MATCH_SIZE
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xchg cx,ax ;copy match length into cx
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mov bp,ds ;save ds
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mov ax,es
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mov ds,ax ;ds=es
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xchg ax,si ;save si
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mov si,di ;ds:si now points at back reference in output data
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add si,dx
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rep movsb ;copy match
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xchg si,ax ;restore si
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mov ds,bp ;restore ds
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jmp @@decode_token ;go decode another token
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@@done_decompressing:
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pop ax ;retrieve the original decompression offset
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xchg di,ax ;compute decompressed size
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sub ax,di
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ret ;done decompressing, exit to caller
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;These are called less often; moved here to optimize the fall-through case
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@@get_long_offset:
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lodsw ;Get 2-byte match offset
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jmp @@get_match_length
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;With a confirmed longer match length, we have an opportunity to optimize for
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;the case where a single byte is repeated long enough that we can benefit
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;from rep movsw to perform the run (instead of rep movsb).
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@@mid_matchlen:
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lodsb ;grab single extra length byte
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inc ah ;add 256
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@@do_long_copy:
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xchg cx,ax ;copy match length into cx
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@@copy_len_preset:
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push ds ;save ds
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mov bp,es
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mov ds,bp ;ds=es
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mov bp,si ;save si
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mov si,di ;ds:si now points at back reference in output data
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add si,dx
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cmp dx,-2 ;do we have a byte/word run to optimize?
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jae @@do_run ;perform a run
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;You may be tempted to change "jae" to "jge" because DX is a signed number.
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;Don't! The total window is 64k, so if you treat this as a signed comparison,
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;you will get incorrect results for offsets over 32K.
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;If we're here, we have a long copy and it isn't byte-overlapping (if it
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;overlapped, we'd be in @@do_run_1) So, let's copy faster with REP MOVSW.
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;This won't affect 8088 that much, but it speeds up 8086 and higher.
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shr cx,1
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rep movsw
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adc cx,0
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rep movsb
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mov si,bp ;restore si
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pop ds
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jmp @@decode_token ;go decode another token
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@@do_run:
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je @@do_run_2 ;fall through to byte (common) if not word run
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@@do_run_1:
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lodsb ;load first byte of run into al
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mov ah,al
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shr cx,1
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rep stosw ;perform word run
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adc cx,0
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rep stosb ;finish word run
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mov si,bp ;restore si
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pop ds
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jmp @@decode_token ;go decode another token
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@@do_run_2:
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lodsw ;load first word of run
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shr cx,1
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rep stosw ;perform word run
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adc cx,0 ;despite 2-byte offset, compressor might
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rep stosb ;output odd length. better safe than sorry.
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mov si,bp ;restore si
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pop ds
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jmp @@decode_token ;go decode another token
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ENDP lzsa1_decompress_speed_jumptable
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ENDS CODE
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END
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;Speed optimization history (decompression times in microseconds @ 4.77 MHz):
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; original E. Marty code shuttle 123208 alice 65660 robotron 407338 ***
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; table for shr al,4 shuttle 120964 alice 63230 robotron 394733 +++
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; push/pop to mov/mov shuttle 118176 alice 61835 robotron 386762 +++
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; movsw for literalcpys shuttle 124102 alice 64908 robotron 400220 --- rb
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; stosw for byte runs shuttle 118897 alice 65040 robotron 403518 --- rb
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; better stosw for runs shuttle 117712 alice 65040 robotron 403343 +--
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; disable RLE by default shuttle 116924 alice 60783 robotron 381226 +++
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; optimize got_matchlen shuttle 115294 alice 59588 robotron 374330 +++
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; fall through to getML shuttle 113258 alice 59572 robotron 372004 +++
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; fall through to midLI shuttle 113258 alice 59572 robotron 375060 ..- rb
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; fall through midMaLen shuttle 113247 alice 59572 robotron 372004 +.+
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; movsw for litlen > 255 shuttle 113247 alice 59572 robotron 371612 ..+
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; rep stosw for long runs shuttle 113247 alice 59572 robotron 371612 ...
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; rep movsw for long cpys shuttle 113247 alice 59572 robotron 371035 ..+
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; xchg/dec ah -> mov ah,val shuttle 112575 alice 59272 robotron 369198 +++
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; force >12h len.to longcpy shuttle 101998 alice 59266 robotron 364459 +.+
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; more efficient run branch shuttle 102239 alice 59297 robotron 364716 --- rb
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; even more eff. run branch shuttle 101998 alice 59266 robotron 364459 ***
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; BUGFIX - bad sign compare shuttle 101955 alice 59225 robotron 364117 +++
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; reverse 16-bit len compar shuttle 102000 alice 59263 robotron 364460 --- rb
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; jcxz for EOD detection no change to speed, but is 1 byte shorter +++
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; force movsw for literals shuttle 107183 alice 62555 robotron 379524 --- rb
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; defer shr4 until necessry shuttle 102069 alice 60236 robotron 364096 ---
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; skip literals if LLL=0 shuttle 98655 alice 57849 robotron 363358 ---
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; fall through to mid_liter shuttle 98595 alice 57789 robotron 361998 +++
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; == jumptable experiments begin ==
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; jumptable for small copys shuttle 101594 alice 61078 robotron 386018 ---
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; start:xchg instead of mov shuttle 100948 alice 60467 robotron 381112 +++
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; use table for LLL=0 check shuttle 106972 alice 63333 robotron 388304 --- rb
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; jmptbl to fallthrough mov shuttle 102532 alice 60760 robotron 383070 ---
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; cpy fallthrough check_ofs shuttle 98939 alice 58917 robotron 371019 +**
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; single jumptable jump shuttle 97528 alice 57264 robotron 362194 ++*
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; conditional check for L=7 shuttle 98610 alice 58521 robotron 368153 --- rb
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; defer add MIN_MATCH_SIZE shuttle 97207 alice 57200 robotron 362884 ++*
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