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Assembly Source File | 1996-01-06 | 6.3 KB | 311 lines

;====================================================================== ; ; COMPASM.asm ; ; Copyright (c) 1995 Mark Russinovich and Bryce Cogswell ; ; This code is free to use and modify in whatever manner you wish. ; ; Highly optimized assembly language compress/decompress ; for use when running vxd. ; ; NOTE: in the mask byte, a 0 indicates a hit, while a 1 is a mis ; prediction. ; ;====================================================================== .386p NO_SEGMENTS=1 include vmm.inc include vsegment.inc .list VXD_LOCKED_DATA_SEG ; prediction table ApcTable db 1000h dup(0) VXD_LOCKED_DATA_ENDS VXD_LOCKED_CODE_SEG ;---------------------------------------------------------------------- ; ; DecompressByte ; ; Macro for decompressing. ; ; Register usage: ; EBX: previous two bytes ; DL: hit mask ; ESI: current source pointer ; EDI: current destination pointer ; ;---------------------------------------------------------------------- DecompressByte MACRO LOCAL hit, miss shl dl, 1 jnc hit ; was it a hit lodsb mov ApcTable[ebx], al ; no, update pred table jmp miss hit: mov al, ApcTable[ebx] ; get predicted value miss: shl bx, 6 ; create new pred value xor bl, al and bh, 0Fh mov [edi], al ; place in output buffer inc edi ENDM ;---------------------------------------------------------------------- ; ; Decompress ; ; Interfaces with C code to provide super-optimized decompression ; algorithm. Based highly on original FIN Dr. Dobb's decompression ; algorithm. ; ;---------------------------------------------------------------------- Public ADecompress ADecompress PROC C USES ebx ecx edx esi edi, inbuf_orig: DWORD, outbuf_len: DWORD, outbuf_orig: DWORD, init: BYTE ; is it a zero-len buffer? cld xor eax, eax cmp outbuf_len, 0 je dcexit ; zero prediction table cmp init, 0 je dcnoclear mov ecx, 1000h ; 4K bytes shr ecx, 2 ; 4 bytes cleared each loop mov edi, offset ApcTable xor eax, eax rep stosd xor ebx, ebx ; initialize prediction value jmp dcinitparams dcnoclear: xor ebx, ebx mov edx, inbuf_orig mov bl, [edx-3] ; setup prediction value shl bx, 6 xor bl, [edx-2] shl bx, 6 xor bl, [edx-1] and bh, 0Fh ; set up parameters dcinitparams: mov esi, inbuf_orig mov edi, outbuf_orig mov ecx, outbuf_len shr ecx, 3 ; do 8 byte blocks ; enter main loop DecompressBlock: lodsb ; get mask byte mov dl, al DecompressByte DecompressByte DecompressByte DecompressByte DecompressByte DecompressByte DecompressByte DecompressByte dec ecx jnz DecompressBlock ; return the amount decompressed sub esi, inbuf_orig mov eax, esi dcexit: ret ADecompress ENDP ;---------------------------------------------------------------------- ; ; CompressByte ; ; This is a macro for processing the current byte on compression. ; ; Register usage: ; EBX: previous two bytes ; DL: accumulated hit mask ; ESI: current source pointer ; EDI: current destination pointer ; ;---------------------------------------------------------------------- CompressByte MACRO curbyte:REQ, bitval: REQ LOCAL hit cmp curbyte, ApcTable[ebx] ; is it a correct prediction? je hit ; yep or dl, bitval ; no, so update table mov ApcTable[ebx], curbyte ; update prediction table mov [edi], curbyte ; copy to destination inc edi ; increment dest pointer hit: shl bx, 6 ; create new pred value xor bl, curbyte and bh, 0Fh ENDM ;---------------------------------------------------------------------- ; ; Compress ; ; Interfaces with C code to provide super-optimized compression ; algorithm. Based highly on original FIN Dr. Dobb's compression ; algorithm. ; ;---------------------------------------------------------------------- Public ACompress ACompress PROC C USES ebx ecx edx esi edi, inbuf: DWORD, inbuf_len: DWORD, outbuf_orig: DWORD, init: BYTE ; is it a zero-len buffer? cld xor eax, eax cmp inbuf_len, 0 je cexit ; first, zero prediction table cmp init, 0 jz cnoclear mov ecx, 1000h ; 4K byte table shr ecx, 2 ; 4 bytes cleared each loop mov edi, offset ApcTable xor eax, eax rep stosd xor ebx, ebx ; initialize prediction value jmp cinitparams cnoclear: xor ebx, ebx mov edx, inbuf mov bl, [edx-3] ; set-up prediction value shl bx, 6 xor bl, [edx-2] shl bx, 6 xor bl, [edx-1] and bh, 0Fh ; set up parameters cinitparams: mov esi, inbuf mov edi, outbuf_orig mov ecx, inbuf_len shr ecx, 5 ; divide by 8 for 8 byte blocks push ebp ; enter the main loop CompressBlock: mov ebp, edi ; save location of mask byte inc edi ; skip over mask byte xor dl, dl ; zero bit mask lodsw CompressByte al, 80h CompressByte ah, 40h lodsw CompressByte al, 20h CompressByte ah, 10h lodsw CompressByte al, 08h CompressByte ah, 04h lodsw CompressByte al, 02h CompressByte ah, 01h mov [ebp], dl ; save bit mask mov ebp, edi ; save location of mask byte inc edi ; skip over mask byte xor dl, dl ; zero bit mask lodsw CompressByte al, 80h CompressByte ah, 40h lodsw CompressByte al, 20h CompressByte ah, 10h lodsw CompressByte al, 08h CompressByte ah, 04h lodsw CompressByte al, 02h CompressByte ah, 01h mov [ebp], dl ; save bit mask mov ebp, edi ; save location of mask byte inc edi ; skip over mask byte xor dl, dl ; zero bit mask lodsw CompressByte al, 80h CompressByte ah, 40h lodsw CompressByte al, 20h CompressByte ah, 10h lodsw CompressByte al, 08h CompressByte ah, 04h lodsw CompressByte al, 02h CompressByte ah, 01h mov [ebp], dl ; save bit mask mov ebp, edi ; save location of mask byte inc edi ; skip over mask byte xor dl, dl ; zero bit mask lodsw CompressByte al, 80h CompressByte ah, 40h lodsw CompressByte al, 20h CompressByte ah, 10h lodsw CompressByte al, 08h CompressByte ah, 04h lodsw CompressByte al, 02h CompressByte ah, 01h mov [ebp], dl ; save bit mask dec ecx jnz CompressBlock ; go for another round ; calculate size of output pop ebp sub edi, outbuf_orig mov eax, edi ; return compressed length cexit: ret ACompress ENDP VXD_LOCKED_CODE_ENDS end