175 lines
6.3 KiB
ArmAsm
Executable File
175 lines
6.3 KiB
ArmAsm
Executable File
; decompress_small.S - space-efficient decompressor implementation for 8088
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;
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; Copyright (C) 2019 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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segment .text
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bits 16
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; ---------------------------------------------------------------------------
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; Decompress raw LZSA2 block
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; inputs:
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; * ds:si: raw LZSA2 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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lzsa2_decompress:
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push di ; remember decompression offset
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cld ; make string operations (lods, movs, stos..) move forward
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xor cx,cx
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xor bx,bx
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xor bp,bp
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.decode_token:
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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: XYZ|LL|MMMM
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mov dx,ax ; keep token in dl
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and al,018H ; isolate literals length in token (LL)
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mov cl,3
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shr al,cl ; shift literals length into place
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cmp al,03H ; LITERALS_RUN_LEN_V2?
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jne .got_literals ; no, we have the full literals count from the token, go copy
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call .get_nibble ; get extra literals length nibble
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add al,cl ; add len from token to nibble
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cmp al,012H ; LITERALS_RUN_LEN_V2 + 15 ?
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jne .got_literals ; if not, we have the full literals count, go copy
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lodsb ; grab extra length byte
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test al,al ; zero?
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jne .got_literals ; if not, we have the full literals count, go copy
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lodsw ; grab 16-bit extra length
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.got_literals:
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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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test dl,dl ; check match offset mode in token (X bit)
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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)
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jnb .offset_9_bit
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; 5 bit offset
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xchg ax,cx ; clear ah - cx is zero from the rep movsb above
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mov al,020H ; shift Z (offset bit 4) in place
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and al,dl
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shr al,1
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call .get_nibble ; get nibble for offset bits 0-3
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or al,cl ; merge nibble
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or al,0E0H ; set offset bits 7-5 to 1
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dec ah ; set offset bits 15-8 to 1
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jmp short .get_match_length
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.offset_9_bit: ; 9 bit offset
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xchg ax,cx ; clear ah - cx is zero from the rep movsb above
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lodsb ; get 8 bit offset from stream in A
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dec ah ; set offset bits 15-8 to 1
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test dl,020H ; test bit Z (offset bit 8)
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jne .get_match_length
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dec ah ; clear bit 8 if Z bit is clear
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jmp short .get_match_length
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.rep_match_or_large_offset:
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cmp dl,0c0H ; check if this is a 13-bit offset or a 16-bit offset/rep match (Y bit)
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jnb .rep_match_or_16_bit
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; 13 bit offset
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lodsb ; load match offset bits 0-7
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mov ah,020H ; shift Z (offset bit 12) in place
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and ah,dl
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shr ah,1
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call .get_nibble ; get nibble for offset bits 8-11
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or ah,cl ; merge nibble
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or ah,0E0H ; set offset bits 15-13 to 1
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sub ah,2 ; substract 512
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jmp short .get_match_length
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.rep_match_or_16_bit:
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test dl,020H ; test bit Z (offset bit 8)
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jne .repeat_match ; rep-match
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; 16 bit offset
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lodsw ; Get 2-byte match offset
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.get_match_length:
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mov bp,ax ; bp: offset
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.repeat_match:
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mov ax,dx ; ax: original token
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and al,07H ; isolate match length in token (MMM)
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add al,2 ; add MIN_MATCH_SIZE_V2
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cmp al,09H ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2?
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jne .got_matchlen ; no, we have the full match length from the token, go copy
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call .get_nibble ; get extra literals length nibble
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add al,cl ; add len from token to nibble
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cmp al,018H ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2 + 15?
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jne .got_matchlen ; 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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test al,al ; zero?
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jne .got_matchlen ; if not, we have the entire length
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lodsw ; grab 16-bit length
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test ax,ax ; bail if we hit EOD
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je short .done_decompressing
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.got_matchlen:
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xchg cx,ax ; copy match length into cx
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push ds ; save ds:si (current pointer to compressed data)
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xchg si,ax
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push es
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pop ds
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mov si,di ; ds:si now points at back reference in output data
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add si,bp
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rep movsb ; copy match
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xchg si,ax ; restore ds:si
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pop 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 ax,di ; compute decompressed size
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sub ax,di
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ret ; done
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.get_nibble:
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dec bh ; nibble ready?
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jns .has_nibble
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mov cx,ax
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lodsb ; load two nibbles
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mov bl,al
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mov bh,1
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mov ax,cx
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.has_nibble:
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mov cl,4 ; swap 4 high and low bits of nibble
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ror bl,cl
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mov cl,0FH
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and cl,bl
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ret
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