From: Charles Sandmann <sandmann AT praline DOT no DOT NeoSoft DOT com>
Subject: Re: DJGPP
To: bb07779 AT bingsuns DOT cc DOT binghamton DOT edu (Lars Damerow)
Date: Tue, 2 May 1995 20:17:39 -0500 (CDT)
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu

> >Just say NO to 0xe0000000 addressing!
> 
> Does that mean that direct video graphics will not be allowed?  If it is 
> not possible to use a pointer directed to video RAM to put pixels on the 
> screen, can we set up a segment that gives us that kind of access without 
> causing a segmentation fault?  My entire optimized graphics library is at 
> stake if we can't!  :(

With the current V1.x and V2.0 alpha, you cannot just *ptr=0x32; and write
to video memory, no.  You can write it with the farptr functions, yes, a
selector is already set up for this purpose.

There is a way on *SOME* DPMI providers to set up DS so that you could use
a direct pointer to the video memory.  To do this you would need:
  1) To sacrifice the ability to run under some DPMI environments; you
     must depend on 2GB segment limits and memory wrap.  Not all allow this.
  2) To replace the supplied malloc() with a custom version (I can provide
     the details)
  3) Never use sbrk in any code
  4) Compute the pointer offset (it will *NOT* be a constant).

It is possible to detect at runtime if the DPMI provider will support this;
but you would need the farptr functions as a fallback method if not supported.

The above code is on my LONG list of things to do, but it is not near the top.