;jul91c.asm
;[LISTING THREE]

; Mode X (320x240, 256 colors) read pixel routine. Works on all VGAs.
; No clipping is performed.
; C near-callable as:
;    unsigned int ReadPixelX(int X, int Y, unsigned int PageBase);

GC_INDEX equ    03ceh   ;Graphics Controller Index
READ_MAP equ    04h     ;index in GC of the Read Map register
SCREEN_SEG equ  0a000h  ;segment of display memory in mode X
SCREEN_WIDTH equ 80     ;width of screen in bytes from one scan line
                        ; to the next
parms   struc
        dw      2 dup (?) ;pushed BP and return address
X       dw      ?       ;X coordinate of pixel to read
Y       dw      ?       ;Y coordinate of pixel to read
PageBase dw     ?       ;base offset in display memory of page from
                        ; which to read pixel
parms   ends

        .model  small
        .code
        public  _ReadPixelX
_ReadPixelX     proc    near
        push    bp      ;preserve caller's stack frame
        mov     bp,sp   ;point to local stack frame

        mov     ax,SCREEN_WIDTH
        mul     [bp+Y]  ;offset of pixel's scan line in page
        mov     bx,[bp+X]
        shr     bx,1
        shr     bx,1    ;X/4 = offset of pixel in scan line
        add     bx,ax   ;offset of pixel in page
        add     bx,[bp+PageBase] ;offset of pixel in display memory
        mov     ax,SCREEN_SEG
        mov     es,ax   ;point ES:BX to the pixel's address

        mov     ah,byte ptr [bp+X]
        and     ah,011b ;AH = pixel's plane
        mov     al,READ_MAP ;AL = index in GC of the Read Map reg
        mov     dx,GC_INDEX ;set the Read Map to read the pixel's
        out     dx,ax       ; plane

        mov     al,es:[bx] ;read the pixel's color
        sub     ah,ah   ;convert it to an unsigned int

        pop     bp      ;restore caller's stack frame
        ret
_ReadPixelX     endp
        end

