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;* WinOpen - Saves portion of screen to allocated memory, then opens a window
;* with specified fill attribute. See also the WinClose procedure.
;*
;* Shows:   DOS Function - 48h (Allocate Memory Block)
;*          Instructions - movsw      stosw     rep
;*
;* Uses:    vconfig - Video configuration structure (initialized
;*          by calling the GetVidConfig procedure)
;*
;* Params:  Row1 - Row at top of window
;*          Col1 - Column at left edge of window
;*          Row2 - Row at bottom of window
;*          Col2 - Column at right edge of window
;*          Attr - Fill attribute for window
;*
;* Return:  Short integer with segment address of allocated buffer, or
;*          0 if unable to allocate memory
 
WinOpen PROC USES ds di si,
        Row1:WORD, Col1:WORD, Row2:WORD, Col2:WORD, Attr:WORD
 
        GetVidOffset Row1, Col1         ; Get offset in video segment
        mov     si, ax                  ; SI = video offset for window
        mov     bx, Row2
        sub     bx, Row1
        inc     bx                      ; BX = number of window rows
        mov     cx, Col2
        sub     cx, Col1
        inc     cx                      ; CX = number of columns
 
        mov     ax, cx                  ; Compute number of video
        mul     bl                      ;   cells in window
        add     ax, 3                   ; Plus 3 additional entries
        shr     ax, 1                   ; Shift right 3 times to
        shr     ax, 1                   ;   multiply by 2 bytes/cell,
        shr     ax, 1                   ;   divide by 16 bytes/para
        inc     ax                      ; Add a paragraph
        push    bx                      ; Save number of rows
        mov     bx, ax                  ; BX = number of paragraphs
        mov     ah, 48h                 ; Request DOS Function 48h
        int     21h                     ; Allocate Memory Block
        pop     bx
 
        .IF     carry?                  ; If unsuccessful:
        sub     ax, ax                  ; Return null pointer
        .ELSE
        mov     es, ax                  ; Point ES:DI to allocated
        sub     di, di                  ;   buffer
        mov     ax, si
        stosw                           ; Copy video offset to buffer
        mov     ax, bx
        stosw                           ; Number of rows to buffer
        mov     ax, cx
        stosw                           ; Number of cols to buffer
        mov     ax, 160                 ; Number of video cells/row
        mov     ds, vconfig.sgmnt       ; DS = video segment
 
        .REPEAT
        push    si                      ; Save ptr to start of line
        push    cx                      ;   and number of columns
 
; For CGA adapters, WinOpen avoids screen "snow" by disabling the video prior
; to block memory moves, then reenabling it. Although this technique can
; result in brief flickering, it demonstrates the fastest way to access a
; block in the CGA video buffer without causing display snow. See also the
; StrWrite procedure for another solution to the problem of CGA snow.
 
        .IF     vconfig.adapter == CGA  ; If not CGA adapter,
        INVOKE  DisableCga              ;   disable video
        .ENDIF
 
        rep     movsw                   ; Copy one row to buffer
 
        .IF     vconfig.adapter == CGA  ; If CGA adapter,
        INVOKE  EnableCga               ;   reenable CGA video
        .ENDIF
        pop     cx                      ; Recover number of columns
        pop     si                      ;   and start of line
        add     si, ax                  ; Point to start of next line
        dec     bx                      ; Decrement row counter
        .UNTIL  zero?                   ; Until no rows remain
 
; Screen contents (including display attributes) are now copied to buffer.
; Next open window, overwriting the screen portion just saved.
 
        mov     ax, 0600h               ; Scroll service
        mov     bh, BYTE PTR Attr       ; Fill attribute
        mov     cx, Col1                ; CX = row/col for upper left
        mov     ch, BYTE PTR Row1
        mov     dx, Col2                ; DX = row/col for lower right
        mov     dh, BYTE PTR Row2
        int     10h                     ; Blank window area on screen
        mov     ax, es                  ; Return address of allocated
        .ENDIF                          ;   segment
        ret
 
WinOpen ENDP
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