From: Jeff Weeks <pweeks AT execulink DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: vga 16 colour (4 bit graphics)
Date: 30 Apr 1997 02:03:14 GMT
Organization: Code X Software
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To: djgpp AT delorie DOT com
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Hi everybody,

I already know a fair bit about graphics programming, but one of the
areas I really lack are in 16 colour modes.  I've started reading
Michael Abrash's book which starts out with 16 colour modes, but I find
it difficult to follow some of his examples.  I've written a tiny
program (excert follows) which will change the BYTES of VGA memory, but
I'm interested in how I would go about actually affecting the BITS in
those bytes.

I'm under the impression that I would just use the bit mask register. 
But that doesn't seem to work.  Here's some code to illustrate what I'm
doing:

  int y, x;
  int colour;
  for(y = 0; y < 350; y++)
    for(x = 0; x < 640; x++) {
      colour = (int)rand() % 16;
      //_farpeekb(_dos_ds,0xA000+y*80+(x/4));    // load the latches
      set_vga_reg(SC_INDEX,MAP_MASK, 0xF);
      //set_vga_reg(GC_INDEX,GC_BIT_MASK, x % 8);
      set_vga_reg(GC_INDEX,GC_ENABLE_SET_RESET, 0xF);
      set_vga_reg(GC_INDEX,GC_SET_RESET,colour);
      _farpokeb(_dos_ds,0xA0000+y*80+(x/4), 0xff);
    }              

The above code changes the bytes fine.  I attempted to affect the bits
individually by using the GC_BIT_MASK line, but that didn't work.

My logic behind it was:
- The remainder of the x value divided by 8 (x % 8) should be the bit
we're working on right?
- Therefore if I set the bit mask to that value I should only affect
that bit right?



I have a feeling that I have some things TOTALLY WRONG.  So I would hope
you guys would put me in my place :)  I appreciate any help on this
topic.  I know 256 colour+ modes, but they're relatively easy compared
to this stuff.  16 colour modes present more of a challenge so I'd like
to say I can do them! :)

Thanks a lot,
Jeff

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