L47-3.ASM

; 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