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;* StrWrite - Writes ASCIIZ string to video memory at specified row/column.
;*
;* Shows:   Instructions - lodsb     stosb
;*
;* Uses:    vconfig - Video configuration structure, declared in the
;*          DEMO.INC include file. The structure must first be
;*          initialized by calling the GetVidConfig procedure.
;*
;* Params:  row - Row coordinate
;*          col - Column coordinate
;*          str - Pointer to string
;*
;* Return:  None
 
StrWrite PROC \
        USES ds di si, \
        row:WORD, col:WORD, str:PTR BYTE
 
        GetVidOffset row, col           ; Get offset in video segment
        mov     di, ax                  ; Copy to DI
        LoadPtr ds, si, str             ; DS:SI points to string
        mov     es, vconfig.sgmnt       ; ES:DI points to video RAM
loop1:  lodsb                           ; Get 1 character from string
        or      al, al                  ; Null terminator?
        jz      exit                    ; Yes?  Exit loop
        cmp     vconfig.adapter, CGA    ; CGA adapter?
        jne     pchar                   ; No?  Skip next 10 lines
 
; For CGA systems, StrWrite waits for the video to begin a horizontal
; retrace before writing a character to memory. This avoids the problem
; of video snow inherent with some (though not all) color/graphics adapters.
; It also demonstrates a somewhat different approach to the problem than the
; one taken in the WinOpen and WinClose procedures.
 
        push    ax                      ; Save character
        mov     dx, 3dah                ; Address of status register
        cli                             ; Disallow interruptions
wait1:  in      al, dx                  ; Read current video status
        test    al, 1                   ; Horizontal retrace active?
        jnz     wait1                   ; Yes?  Wait for it to end
wait2:  in      al, dx                  ; No?  Read status again
        test    al, 1                   ; Wait for retrace to start
        jz      wait2
        pop     ax                      ; Recover character
pchar:  stosb                           ; Write char to video buffer
        sti                             ; Reenable interrupts
        inc     di                      ; Skip attribute byte
        jmp     SHORT loop1             ; Loop back
exit:   ret
 
StrWrite ENDP
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