Date: Tue, 1 Aug 1995 08:35:32 +0300 (IDT)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: Steve Havelka <shavelk AT agora DOT rdrop DOT com>
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Video question


On Mon, 31 Jul 1995, Steve Havelka wrote:

> Hi.  I have written a few graphics programs (mode 13h) for DJGPP,
> and they access video memory at 0xd0000000.  If I try to run
> them under a DPMI server, though, they GPF when they try to
> access the video memory.  Is there another way to access video
> memory so the programs don't crap out under DPMI?

Yes.  It's all explained in the DJGPP FAQ list (available as faq102.zip 
from the same place you get DJGPP), section 10.6:

10.6  Q: I try to access the video memory at 0xa0000, but get Segmentation
         violation...
      A: Absolute addresses of certain memory-mapped devices are mapped
         differently under DJGPP, which is protected-mode environment.
         You can't just poke any address, that's what protected mode is
         all about.  In non-DPMI mode, the entire graphics video memory is
         mapped 1:1 starting at 0xD0000000 in the program's address space;
         the DJGPP paging mechanism  understands how SuperVGA's map their
         memory onto the AT bus and automatically swaps pages as the
         program tries to access them.  The program sees a linear range
         from 0xD0000000 to 0xD0100000 that corresponds to each pixel in
         the 256-color modes of SuperVGAs.  For this to work correctly,
         you will have to set the GO32 environment variable to the
         graphics driver suitable for your SuperVGA card, like this:

          SET GO32=driver c:\djgpp\drivers\ati.grd gw 640 gh 480 tw 132 th 43

         In DPMI mode this won't work.  As DJGPP v2.0 will be DPMI-only
         environment, this means that, after GRX 2.0 arrives, the above
         method should be used only as last resort.  If you want to write
         a program which will compile and run unchanged in v2.0, use
         functions described in <sys/farptr.h> (see chapter 18 below for
         details).