118 lines
4.2 KiB
ArmAsm
Executable File
118 lines
4.2 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 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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lzsa1_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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.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: O|LLL|MMMM
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mov dx,ax ; keep token in dl
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and al,070H ; isolate literals length in token (LLL)
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mov cl,4
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shr al,cl ; shift literals length into place
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cmp al,07H ; LITERALS_RUN_LEN?
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jne .got_literals ; no, we have the full literals count from the token, go copy
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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 ; if no overflow, we have the full literals count, go copy
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jne .mid_literals
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lodsw ; grab 16-bit extra length
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jmp short .got_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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.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 size in token (O bit)
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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
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dec ah
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jmp short .get_match_length
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.get_long_offset:
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lodsw ; Get 2-byte match offset
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.get_match_length:
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xchg ax,dx ; dx: match offset ax: original token
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and al,0FH ; isolate match length in token (MMMM)
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add al,3 ; add MIN_MATCH_SIZE
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cmp al,012H ; MATCH_RUN_LEN?
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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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add al,012H ; add MIN_MATCH_SIZE + MATCH_RUN_LEN
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jnc .got_matchlen ; 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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test ax,ax ; bail if we hit EOD
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je short .done_decompressing
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jmp short .got_matchlen
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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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.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,dx
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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 short .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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