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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
;*          Sptr - 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:SBYTE,
        Sptr:PBYTE,
        Direct:WORD
 
        LoadPtr es, di, Sptr            ; ES:DI points to string
        LoadPtr ds, si, Sptr            ;   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
 
        .IF     Direct != 0             ; If assumption correct:
        mov     bx, di                  ; 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
        .ENDIF
 
        .REPEAT
        lodsb                           ; Get first char from string
        .IF     (si == bx) || (al == 0) ; If at head or tail limit:
        sub     ax, ax                  ; No match
        sub     dx, dx                  ; Set null pointer
        jmp     exit
        .ENDIF
        .UNTILCXZ al == IChar
 
        mov     ax, si                  ; Match, so point to first
        dec     ax                      ;   occurence
        .IF     Direct != 0             ; If head-to-tail search:
        inc     ax                      ; Adjust pointer forward
        inc     ax
        mov     dx, ds                  ; Pointer segment
        .ENDIF
exit:
        ret
 
StrFindChar ENDP
                                    -♦-