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;* StrCompare - Compares two strings for equality. See StrWrite, StrFindChar,
;* WinOpen, and WinClose procedures for other examples of string instructions.
;*
;* Shows:   Instructions - cmpsb     cmpsw     repe     jcxz
;*
;* Params:  Sptr1 - Pointer to first string
;*          Sptr2 - Pointer to second string
;*          Len  - Length in bytes for comparison. Strings need not be of
;*                 equal length; however if len is an even number, comparison
;*                 is made on a word-by-word basis and thus is more efficient.
;*
;* Return:  Null pointer if strings match; else pointer to string #1 where
;*          match failed.
 
StrCompare PROC USES ds di si,
        Sptr1:PBYTE,
        Sptr2:PBYTE,
        Len:WORD
 
        LoadPtr es, di, Sptr1           ; ES:DI points to string #1
        LoadPtr ds, si, Sptr2           ; DS:SI points to string #2
        mov     cx, Len                 ; Length of search in bytes
        and     al, 0                   ; Set ZR flag in case CX = 0
        .IF     cx != 0                 ; If length is not 0:
        .IF     !(cl & 1)               ; If not even number:
        repe    cmpsb                   ; Compare byte-by-byte
        .ELSE                           ; Else compare word-by-word
        shr     cx, 1                   ; Decrease count by half
        repe    cmpsw                   ; Compare word-by-word
        sub     di, 2                   ; Back up 2 characters
        sub     si, 2
        cmpsb                           ; Match?
        .IF     zero?                   ; No?  Then failure
        cmpsb                           ; Compare last characters
        .ENDIF  ; zero
        .ENDIF  ; cl & 1
        .ENDIF  ; cx != 0
 
        mov     ax, 0                   ; Set null pointer without
        mov     dx, 0                   ;   disturbing flags
        .IF     !zero?                  ; If no match:
        dec     di                      ; Point to failure
        mov     ax, di
        mov     dx, es
        .ENDIF
        ret
 
StrCompare ENDP
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