lzsa/asm/8088/decompress_small.S

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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
; ---------------------------------------------------------------------------
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; Decompress raw LZSA block
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; inputs:
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; * ds:si: raw LZSA block
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; * es:di: output buffer
; output:
; * ax: decompressed size
; ---------------------------------------------------------------------------
lzsa_decompress:
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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mov cx,ax ; copy literals length into cx
cmp al,07H ; LITERALS_RUN_LEN?
jne .copy_literals ; no, we have the full literals count from the token, go copy
call .get_varlen ; get complete literals length
.copy_literals:
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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xchg ax,cx ; clear ah - cx is zero from the rep movsb above
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lodsb
jmp short .get_match_length
.get_long_offset:
lodsw ; Get 2-byte match offset
.get_match_length:
inc ax ; the match offset is stored off-by-1, increase it
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xchg ax,dx ; dx: match offset ax: original token
and al,0FH ; isolate match length in token (MMMM)
mov cx,ax ; copy match length into cx
cmp al,0FH ; MATCH_RUN_LEN?
jne .copy_match ; no, we have the full match length from the token, go copy
call .get_varlen ; get complete match length
test cx,cx
je short .done_decompressing ; bail if we hit EOD
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.copy_match:
add cx,3 ; add MIN_MATCH_SIZE to get the final match length to copy
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
sub si,dx
rep movsb ; copy match
xchg si,ax ; restore ds:si
pop ds
jmp short .decode_token ; go decode another token
.done_decompressing:
pop ax ; retrieve the original decompression offset
xchg ax,di ; compute decompressed size
sub ax,di
ret ; done
.get_varlen:
lodsb ; grab extra length byte
cmp al,0FFH ; 3-byte extra length?
je .large_varlen ; yes, go grab it
add cx,ax ; add extra length byte to length from token
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cmp al,0FEH ; 2-byte extra length?
jne .varlen_done ; no, we have the full length now, bail
lodsb ; grab extra length byte
add cx,ax ; add to length from token
.varlen_done:
ret ; bail
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.large_varlen:
lodsw ; grab 16-bit extra length
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xchg cx,ax
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ret