From: Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Mapping DOS base memory under 1 meg
Date: Thu, 31 Jul 2003 23:26:08 CDT
Organization: Rice University, Houston, TX
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> I'm writting some SVGA code which uses banked modes if a linear mode
> is not availible.  Now, I had an idea of, when using _farpokeb instead
> of adding segment << 4 to the offset, which is usually A0000 + offset,
> I would make a selector for the A000 - AFFF graphics memory buffer. 

SNIP.  No need to map items in first megabyte.

> So, is there a reliable way to make a base memory selector other than
> just using the __djgpp_nearptr_enable() ?

First allocate an ldt descriptor (__dpmi_allocate_ldt_descriptors(1))
Set the size to be 64K.
Set the base to be A0000

You can now access it directly - no need to map the memory in 1st MB.

Alternately, you can call __dpmi_segment_to_descriptor which is easier.