lzsa/asm/8088/decompress_small_v2.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
; ---------------------------------------------------------------------------
; Decompress raw LZSA2 block
; inputs:
; * ds:si: raw LZSA2 block
; * es:di: output buffer
; output:
; * ax: decompressed size
; ---------------------------------------------------------------------------
lzsa2_decompress:
push di ; remember decompression offset
cld ; make string operations (lods, movs, stos..) move forward
xor cx,cx
mov bx,0100H
xor bp,bp
.decode_token:
mov ax,cx ; clear ah - cx is zero from above or from after rep movsb in .copy_match
lodsb ; read token byte: XYZ|LL|MMMM
mov dx,ax ; keep token in dl
and al,018H ; isolate literals length in token (LL)
mov cl,3
shr al,cl ; shift literals length into place
cmp al,03H ; LITERALS_RUN_LEN_V2?
jne .got_literals ; no, we have the full literals count from the token, go copy
call .get_nibble ; get extra literals length nibble
add al,cl ; add len from token to nibble
cmp al,012H ; LITERALS_RUN_LEN_V2 + 15 ?
jne .got_literals ; if not, we have the full literals count, go copy
lodsb ; grab extra length byte
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add al,012H ; overflow?
jnc .got_literals ; if not, we have the full literals count, go copy
lodsw ; grab 16-bit extra length
.got_literals:
xchg cx,ax
rep movsb ; copy cx literals from ds:si to es:di
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test dl,0C0h ; check match offset mode in token (X bit)
js .rep_match_or_large_offset
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;;cmp dl,040H ; check if this is a 5 or 9-bit offset (Y bit)
; discovered via the test with bit 6 set
xchg cx,ax ; clear ah - cx is zero from the rep movsb above
jne .offset_9_bit
; 5 bit offset
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cmp dl,020H ; test bit 5
call .get_nibble_x
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jmp short .dec_offset_top
.offset_9_bit: ; 9 bit offset
lodsb ; get 8 bit offset from stream in A
dec ah ; set offset bits 15-8 to 1
test dl,020H ; test bit Z (offset bit 8)
je .get_match_length
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.dec_offset_top:
dec ah ; clear bit 8 if Z bit is clear
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; or set offset bits 15-8 to 1
jmp short .get_match_length
.rep_match_or_large_offset:
cmp dl,0c0H ; check if this is a 13-bit offset or a 16-bit offset/rep match (Y bit)
jnb .rep_match_or_16_bit
; 13 bit offset
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cmp dl,0A0H ; test bit 5 (knowing that bit 7 is also set)
xchg ah,al
call .get_nibble_x
sub al,2 ; substract 512
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jmp short .get_match_length_1
.rep_match_or_16_bit:
test dl,020H ; test bit Z (offset bit 8)
jne .repeat_match ; rep-match
; 16 bit offset
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lodsb ; Get 2-byte match offset
.get_match_length_1:
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xchg ah,al
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lodsb ; load match offset bits 0-7
.get_match_length:
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xchg bp,ax ; bp: offset
.repeat_match:
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xchg ax,dx ; ax: original token
and al,07H ; isolate match length in token (MMM)
add al,2 ; add MIN_MATCH_SIZE_V2
cmp al,09H ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2?
jne .got_matchlen ; no, we have the full match length from the token, go copy
call .get_nibble ; get extra literals length nibble
add al,cl ; add len from token to nibble
cmp al,018H ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2 + 15?
jne .got_matchlen ; no, we have the full match length from the token, go copy
lodsb ; grab extra length byte
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add al,018H ; overflow?
jnc .got_matchlen ; if not, we have the entire length
je short .done_decompressing ; detect EOD code
lodsw ; grab 16-bit length
.got_matchlen:
xchg cx,ax ; copy match length into cx
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,bp
rep movsb ; copy match
xchg si,ax ; restore ds:si
pop ds
jmp .decode_token ; go decode another token
.done_decompressing:
pop ax ; retrieve the original decompression offset
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xchg di,ax ; compute decompressed size
sub ax,di
ret ; done
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.get_nibble_x:
cmc ; carry set if bit 4 was set
rcr al,1
call .get_nibble ; get nibble for offset bits 0-3
or al,cl ; merge nibble
rol al,1
xor al,0E1H ; set offset bits 7-5 to 1
ret
.get_nibble:
neg bh ; nibble ready?
jns .has_nibble
xchg bx,ax
lodsb ; load two nibbles
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xchg bx,ax
.has_nibble:
mov cl,4 ; swap 4 high and low bits of nibble
ror bl,cl
mov cl,0FH
and cl,bl
ret