Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!cornell!batcomputer!sun!naughton
From: naughton@sun.soe.clarkson.edu (Patrick Naughton)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Direct Graphics with the EGA
Message-ID: <439@sun.soe.clarkson.edu>
Date: 19 Feb 88 07:50:28 GMT
References: <702@uthub.toronto.edu>
Organization: Clarkson University, Potsdam, NY
Lines: 74

From article <702@uthub.toronto.edu>, by kevin@uthub.toronto.edu (Kevin A. Stoodley):
> 
> I'm looking for information on how the Tatung Supreme EGA's 350 by 640 16 
> colour graphics mode can be used directly from an assembly language program
> without using BIOS calls on an AT.  Specifically, I'd like to know how the EGA
> stores the colours of each pixel.  All I can find out from my card's
> documentation is that the mode is 16 and that the screen memory starts at
> a000:0h.  I would really appreciate some information, if anyone knows this kind
> of stuff.  Failing that, or if it's too involved to conveniently explain in a
> reasonably short note, I would like a good reference to the material.
> Thanks in advance.
> -- 
> 
> 		    Kevin A. Stoodley	University of Toronto, Dept. EE
> 		    {linus,ihnp4,uw-beaver,floyd,utzoo}!utcsri!uthub!kevin


  For one thing it is generally referred to as 640x350 (x then y).
That understood, let's try to exaplain how to use it a little.

Basically the EGA does 16 colors by using 4 video planes mapped to the
same address space.  This is why the RAM onboard is 256Kx4 as opposed
to normal memory being 256Kx1...

              -----------------------
A000:0000---->|00000001         RED
              | ----------------------
              | |00000001       GREEN
              | | ---------------------
              | | |00000001     BLUE
              | | | --------------------
              | | | |00000001   INTENSITY
              | | | |
              | | | |
                | | |
                  | |
                    |


To plot a point in a color c at the coords x,y on the EGA do this.

the byte of memory you need to modify is at this location

A000:((y * 80) + (x div 8))

You must first read from this location, ignore what you read.

set the mask register to 100000000b (80h) shifted right (x mod 8)

set write mode 2

write the color (0..15) you want to the memory location above.


The three mysteries for you are:
  How to set the mask register...
   It is register 8 of the graphics controller whose address aludes me.
  How to set write mode 2.
   Again... register 5 of the graphics controller (see above)

I'll check my references and post a code fragment later...

I'm on my sun 3 not near my AT with all of the EGA code and docs....

till then...

-Patrick

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