Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!pasteur!ames!killer!chasm
From: chasm@killer.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: VGA and Bit Mask Register: HEEEEEELP !
Summary: READ the byte first to load the latches
Message-ID: <7467@killer.DALLAS.TX.US>
Date: 7 Mar 89 14:10:34 GMT
References: <1040010@hprnd.HP.COM>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 46

In article <1040010@hprnd.HP.COM>, pyt@hprnd.HP.COM (Pierre-Yves Thoulon) writes:
:> 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.
:> . . .
:> 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++)

To load the 32-bit latch register, insert a read operation here:
                junk = *Address; /* loads the latches so unmodified bits are OK */

:> 		*Address = 4;	/* paint pixel Red */
:> }
:> 

The effect you were seeing was the contents of the latches (left over from
long ago) being copied into the current byte in memory (bye-bye other 7 bits).

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

Charles Marslett
===========================================================================
Charles Marslett
STB Systems, Inc.  <== Apply all standard disclaimers
Wordmark Systems   <== No disclaimers required -- that's just me
chasm@killer.dallas.tx.us