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;* WriteFile - Write ASCIIZ string to file. If handle = 0, the string is
;* written to STDOUT (console). See the CopyFile procedure for another
;* example of using DOS Function 40h to write to files.
;*
;* Shows:   DOS Function - 40h (Write File or Device)
;*
;* Params:  handle - File handle
;*          str - Pointer to ASCIIZ string
;*
;* Return:  Short integer with error code
;*          0 if successful
;*          1 if write error
;*          2 if number of bytes written not equal to string length
 
WriteFile PROC \
        USES ds di, \
        handle:WORD, str:PTR BYTE
 
        LoadPtr es, di, str             ; Point ES:DI to string
        push    di                      ; Hold on to string pointer
        mov     cx, -1                  ; Set CX to maximum
        sub     al, al                  ; AL = 0
        repne   scasb                   ; Scan string for NULL
        pop     dx                      ; Recover string pointer
        dec     di
        sub     di, dx                  ; Get string length (w/o NULL)
        mov     cx, di                  ; Put it into CX
        mov     bx, handle              ; Load BX with handle
        push    es                      ; Set DS to ES to ensure
        pop     ds                      ;   DS:DX points to string
        mov     ah, 40h                 ; Request DOS write
        int     21h                     ; Write File or Device
        mov     bx, ax                  ; Get number of bytes written
        mov     ax, 0                   ; Set error code, preserve CY
        jc      e_exit                  ; If error, exit
        cmp     bx, cx                  ; All bytes written?
        je      exit                    ; Yes?  Exit, error code = 0
        inc     ax                      ; Else inc error code twice
e_exit: inc     ax                      ; Increment error code once
exit:   ret
 
WriteFile ENDP
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