Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpda!hpcupt1!hprnd!pyt
From: pyt@hprnd.HP.COM (Pierre-Yves Thoulon)
Newsgroups: comp.sys.ibm.pc
Subject: VGA and Bit Mask Register: HEEEEEELP !
Message-ID: <1040010@hprnd.HP.COM>
Date: 5 Mar 89 23:21:43 GMT
Organization: HP Roseville Networks Division
Lines: 51

I've been trying to play with my VGA card and I'm trying to use it in
write mode 2 and to write ot a given pixel after setting the bit mask
correctly.
From I understand, if the bit mask register is set to 0, nothing should
be happening when I write to the display memory, but every time I write
a byte, I see an nice 8 pixel long line appearing on my screen, half
green, half blue.
What am I doing wrong ? Is there any register other than the Mode and
Bit Mask registers that I need to program ?

My test program looks like this:

#define START	(char far *)0xa0000000
#define END	(char far *)0xa0009600
void main(void)
{
	union REGS regs;
	char far *Address;

	regs.h.ah = 0;
	regs.h.al = 0x12;
	int86(0x10, &regs, &regs);	/* set display mode 0x12 */

	outp(0x3ce, 5);	/* Mode register */
	outp(0x3cf, 2); /* Write Mode 2 */

	outp(0x3ce, 8); /* Bit Mask register */
	outp(0x3cf, 0); /* all bits masked */

	for (Address=START; Address < END; Address++)
		*Address = 4;	/* paint pixel Red */
}

I set the video mode to 0x12, but the same behavior occurs in mode 0x10.
Instead of a nice black-remaining screen, I get vertical green and blue
stripes.
If I set the bit mask to something else than 0, the unmasked pixels are
correctly painted, but the masked one are painted green or blue, depending
on where they are in the byte.

In a nutshell, I am unable to update a single pixel without trashing its
7 neighbors. What am I doing wrong ?

Any help VERY MUCH appreciated.

Desperately,
Pyt "starting to see green and blue stripes in bed at night...".
hplabs!hprnd!pyt

BTW: I noticed that if I use write mode 0 rather than write mode 2 in
the small program above, I still get these stupid green and blue stripes.