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Graphics Programming Black Book (Special Edition)
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BlackBook.bin
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source
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chapter9
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l9-2.asm
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Assembly Source File
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1997-06-18
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4KB
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97 lines
; Searches a text buffer for a text string. Uses REPNZ SCASB to scan
; the buffer for locations that match a specified character of the
; searched-for string, then uses REPZ CMPS to check fully only those
; locations that REPNZ SCASB has identified as potential matches.
;
; C small model-callable as:
; unsigned char * FindString(unsigned char * Buffer,
; unsigned int BufferLength, unsigned char * SearchString,
; unsigned int SearchStringLength,
; unsigned int ScanCharOffset);
;
; Returns a pointer to the first match for SearchString in Buffer, or
; a NULL pointer if no match is found. Buffer should not start at
; offset 0 in the data segment to avoid confusing a match at 0 with
; no match found.
Parms struc
dw 2 dup(?) ;pushed BP/return address
Buffer dw ? ;pointer to buffer to search
BufferLength dw ? ;length of buffer to search
SearchString dw ? ;pointer to string for which to search
SearchStringLength dw ? ;length of string for which to search
ScanCharOffset dw ? ;offset in string of character for
; which to scan
Parms ends
.model small
.code
public _FindString
_FindString proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to our stack frame
push si ;preserve caller's register variables
push di
cld ;make string instructions increment pointers
mov si,[bp+SearchString] ;pointer to string to search for
mov cx,[bp+SearchStringLength] ;length of string
jcxz FindStringNotFound ;no match if string is 0 length
mov dx,[bp+BufferLength] ;length of buffer
sub dx,cx ;difference between buffer and search lengths
jc FindStringNotFound ;no match if search string is
; longer than buffer
inc dx ;difference between buffer and search string
; lengths, plus 1 (# of possible string start
; locations to check in the buffer)
mov di,ds
mov es,di
mov di,[bp+Buffer] ;point ES:DI to buffer to search thru
mov bx,[bp+ScanCharOffset] ;offset in string of character
; on which to scan
add di,bx ;point ES:DI to first buffer byte to scan
mov al,[si+bx] ;put the scan character in AL
inc bx ;set BX to the offset back to the start of the
; potential full match after a scan match,
; accounting for the 1-byte overrun of
; REPNZ SCASB
FindStringLoop:
mov cx,dx ;put remaining buffer search length in CX
repnz scasb ;scan for the scan byte
jnz FindStringNotFound ;not found, so there's no match
;found, so we have a potential match-check the
; rest of this candidate location
push di ;remember the address of the next byte to scan
mov dx,cx ;set aside the remaining length to search in
; the buffer
sub di,bx ;point back to the potential start of the
; match in the buffer
mov si,[bp+SearchString] ;point to the start of the string
mov cx,[bp+SearchStringLength] ;string length
shr cx,1 ;convert to word for faster search
jnc FindStringWord ;do word search if no odd byte
cmpsb ;compare the odd byte
jnz FindStringNoMatch ;odd byte doesn't match, so we
; haven't found the search string here
FindStringWord:
jcxz FindStringFound ;if the string is only 1 byte long,
; we've found a match
repz cmpsw ;check the rest of the string a word at a time
jz FindStringFound ;it's a match
FindStringNoMatch:
pop di ;get back pointer to the next byte to scan
and dx,dx ;is there anything left to check?
jnz FindStringLoop ;yes-check next byte
FindStringNotFound:
sub ax,ax ;return a NULL pointer indicating that the
jmp FindStringDone ; string was not found
FindStringFound:
pop ax ;point to the buffer location at which the
sub ax,bx ; string was found (earlier we pushed the
; address of the byte after the scan match)
FindStringDone:
pop di ;restore caller's register variables
pop si
pop bp ;restore caller's stack frame
ret
_FindString endp
end