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From: IAP@psuvm.psu.edu (Kirk Hanawalt)
Newsgroups: comp.graphics,comp.sys.ibm.pc
Subject: VGA - direct memory access question
Message-ID: <90039.091705IAP@PSUVM.BITNET>
Date: 8 Feb 90 14:17:05 GMT
Organization: Penn State University
Lines: 44


    I'm trying to write a program (TurboC 2.0, IBM PS/2 Model 50
with VGA adapter, HP LaserJet II) to dump a bitmap image of the
high resolution VGA screen to a LaserJet printer.  Using the
TurboC getpixel function, the 640x480 graphics screen can be
dumped in about 35 seconds.  I would like to access memory
directly, and dispense with getpixel() altogether.
Unfortunately, I can't figure out how the pixels are mapped in
memory.

    Starting at address A000:0000, each byte in memory seems to
indicate the on/off state of 8 pixels on the screen.  For
example, the statement:

        *((unsigned char far *) 0xa0000000L) = 0x80;

turns the first pixel in the first row of the graphics screen
on.  I can use this method to get monochrome graphics.
Obviously, 16 color graphics requires 4 bits to specify pixel
color.

    The problem is this:  when using this method to read pixels
from the screen, I only get the odd color pixels (for the
default EGA/VGA palette 1=blue, 3=cyan,...,15=white).  The even
color pixels do not have a bit set.  The following pseudo-code
works only for odd colors, the even colors aren't printed.

     ptr=(unsigned char huge *) 0xa0000000L;
     for (j=0; j<=MaxY; j++)
       for (i=0; i < (MaxX+1)/8; i++)
         Send_To_Printer(*ptr++);  /* send byte = 8 pixels */

This method dumps the screen to the LaserJet in about 15
seconds, so I would like to use it.  However, getting only
half of the displayed colors printed is a drawback :-).

    My question is this: where in the heck are the pixel values
stored in memory for the VGA adapter in 640x480 16 color mode?
Every book I've seen covers only monochrome graphics and uses
the method above to write/read pixels.
-------
Kirk Hanawalt     |                                        |
Chem. Eng. Dept.  |  BITNET:  IAP @ PSUVM                  |
Penn. St. Univ.   |   OTHER:  ...!psuvax1!iap@psuvm.bitnet |