lzsa/asm/8088/decompress_small_v1.S

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ArmAsm
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; decompress_small.S - space-efficient decompressor implementation for 8088
;
; Copyright (C) 2019 Emmanuel Marty
;
; This software is provided 'as-is', without any express or implied
; warranty. In no event will the authors be held liable for any damages
; arising from the use of this software.
;
; Permission is granted to anyone to use this software for any purpose,
; including commercial applications, and to alter it and redistribute it
; freely, subject to the following restrictions:
;
; 1. The origin of this software must not be misrepresented; you must not
; claim that you wrote the original software. If you use this software
; in a product, an acknowledgment in the product documentation would be
; appreciated but is not required.
; 2. Altered source versions must be plainly marked as such, and must not be
; misrepresented as being the original software.
; 3. This notice may not be removed or altered from any source distribution.
segment .text
bits 16
; ---------------------------------------------------------------------------
; Decompress raw LZSA1 block
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; inputs:
; * ds:si: raw LZSA1 block
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; * es:di: output buffer
; output:
; * ax: decompressed size
; ---------------------------------------------------------------------------
lzsa1_decompress:
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push di ; remember decompression offset
cld ; make string operations (lods, movs, stos..) move forward
xor cx,cx
.decode_token:
mov ax,cx ; clear ah - cx is zero from above or from after rep movsb in .copy_match
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lodsb ; read token byte: O|LLL|MMMM
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mov dx,ax ; keep token in dl
and al,070H ; isolate literals length in token (LLL)
mov cl,4
shr al,cl ; shift literals length into place
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cmp al,07H ; LITERALS_RUN_LEN?
jne .got_literals ; no, we have the full literals count from the token, go copy
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lodsb ; grab extra length byte
add al,07H ; add LITERALS_RUN_LEN
jnc .got_literals ; if no overflow, we have the full literals count, go copy
jne .mid_literals
lodsw ; grab 16-bit extra length
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db 81H ; mask inc ah/lodsb
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; (*like jmp short .got_literals but faster)
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.mid_literals:
inc ah ; add 256
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lodsb ; grab single extra length byte
.got_literals:
xchg cx,ax
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rep movsb ; copy cx literals from ds:si to es:di
test dl,dl ; check match offset size in token (O bit)
js .get_long_offset
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dec cx
xchg cx,ax ; ah to 0xff - cx was zero from the rep movsb above
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lodsb
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db 3CH ; mask lodsw
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; (*like jmp short .get_match_length but faster)
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.get_long_offset:
lodsw ; Get 2-byte match offset
.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)
add al,3 ; add MIN_MATCH_SIZE
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cmp al,012H ; MATCH_RUN_LEN?
jne .got_matchlen ; no, we have the full match length from the token, go copy
lodsb ; grab extra length byte
add al,012H ; add MIN_MATCH_SIZE + MATCH_RUN_LEN
jnc .got_matchlen ; if no overflow, we have the entire length
jne .mid_matchlen
lodsw ; grab 16-bit length
test ax,ax ; bail if we hit EOD
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je short .done_decompressing
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db 81H ; mask inc ah/lodsb
; (*like jmp short .got_literals but faster)
.mid_matchlen:
inc ah ; add 256
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lodsb ; grab single extra length byte
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.got_matchlen:
xchg cx,ax ; copy match length into cx
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push ds ; save ds:si (current pointer to compressed data)
xchg si,ax
push es
pop ds
mov si,di ; ds:si now points at back reference in output data
add si,dx
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rep movsb ; copy match
xchg si,ax ; restore ds:si
pop ds
jmp short .decode_token ; go decode another token
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.done_decompressing:
pop ax ; retrieve the original decompression offset
xchg ax,di ; compute decompressed size
sub ax,di
ret ; done