From:	Chris Holmes <cholmes AT surfsouth DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: color pallete for 256 bits
Date:	Tue, 10 Aug 1999 07:32:11 -0400
Organization:	Georgia Institute of Technology, Atlanta GA, USA
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zidharta AT yahoo DOT com wrote:
> 
> when we use color pallete technique in 13h graphics mode ...
> (where we set RGB using outport or inport )
> are we really get more colors?... it still 256 bits anyway isnt it?
> so why are we using it? ...

  256 bits?
  256 color modes compose colors by using 3 6 bit (not 8 bit) values
(red, green, and blue).
  Here's the port breakdown for changing the palette:

  0x3c6 --> send this 0xff to tell it "hey, I'm using all 256 colors"
  0x3c7 --> send this the palette entry (0 to 255) that you want to 
	    WRITE
  0x3c8 --> send this the palette entry (0 to 255) that you want to
	    READ
  0x3c9 --> this is the ONLY color port.  You HAVE to either write to
	    it 3 times or read from it 3 times.  

  i.e. to set color 44 to bright white, 
  outportb(0x3c6, 255);
  outportb(0x3c7, 44);
  outportb(0x3c9, 63); // set red to highest
  outportb(0x3c9, 63); // set green to highest
  outportb(0x3c9, 63); // set blue to highest

  to read the red, green, and blue values from color 57:
  outportb(0x3c6, 255); 
  outportb(0x3c8, 57);
  red = inportb(0x3c9);
  green = inportb(0x3c9);
  blue = inportb(0x3c9);

BTW, the outportb(0x3c6, 255) isn't totally necessary, but if something 
swaps out your program from control of the screen, this could get 
screwed and it's not that many cycles, so unless you're doing 
something really palette intensive, it shouldn't cost much time and 
you won't have to worry about your palette just not working right on
some machines.  

  Hope this helps,
    Chris

-- 
I know that I will never be politically correct,
and I don't give a damn about my lack of etiquette!
-- Meatloaf

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