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;* StrFindChar - Finds first occurence of character in given ASCIIZ string,
;* searching either from beginning or end of string. See StrWrite, WinOpen,
;* WinClose, and StrCompare procedures for other examples of string
;* instructions.
;*
;* Shows:   Instructions - repne     scasb    cld     std
;*
;* Params:  ichar - Character to search for
;*          str - Pointer to ASCIIZ string in which to search
;*          direct - Direction flag:
;*                   0 = search from start to end
;*                   1 = search from end to start
;*
;* Return:  Null pointer if character not found, else pointer to string where
;*          character first encountered
 
StrFindChar PROC \
        USES ds di si, \
        ichar:BYTE, str:PTR BYTE, direct:WORD
 
        LoadPtr es, di, str             ; ES:DI points to string
        LoadPtr ds, si, str             ;   as does DS:SI
        mov     cx, -1                  ; Set scan counter to maximum
        mov     bx, cx                  ; BX = max string tail
        cld                             ; Assume head-to-tail search
        cmp     direct, 0               ; Assumption correct?
        je      loop1                   ; Yes?  Continue
        mov     bx, di                  ; No?  Set BX to byte before
        dec     bx                      ;   string head and scan
        sub     al, al                  ;   string for null terminator
        push    cx                      ;   to find string tail
        repne   scasb
        pop     cx                      ; Recover scan counter
        dec     di                      ; Backup pointer to last
        dec     di                      ;   character in string and
        mov     si, di                  ;   begin search from there
        std                             ; Set direction flag
 
loop1:  lodsb                           ; Get first char from string
        cmp     si, bx                  ; At head limit?
        je      xmatch                  ; Yes?  Then no match
        or      al, al                  ; At tail limit?
        je      xmatch                  ; Yes?  Then no match
        cmp     al, ichar               ; Character match?
        je      match                   ; Yes?  Then exit
        loop    loop1                   ; No?  Resume search
 
xmatch: sub     ax, ax                  ; Set null pointer if no match
        sub     dx, dx
        jmp     SHORT exit
 
match:  mov     ax, si                  ; If match, point to first
        dec     ax                      ;   occurence
        cmp     direct, 0               ; Head-to-tail search?
        je      exit                    ; Yes?  Then exit
        inc     ax                      ; No?  Then adjust pointer
        inc     ax                      ;   forward
        mov     dx, ds                  ; Pointer segment
exit:   ret
 
StrFindChar ENDP
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