From:	jbs30000 AT aol DOT com (Joel)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Mapping DOS base memory under 1 meg
Date:	1 Aug 2003 22:01:41 -0700
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References:	<84e4e2a9 DOT 0307311826 DOT 6a107b0c AT posting DOT google DOT com> <3f29ebe0 DOT sandmann AT clio DOT rice DOT edu>
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To:	djgpp AT delorie DOT com
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Charles Sandmann <sandmann AT clio DOT rice DOT edu> wrote in message news:<3f29ebe0 DOT sandmann AT clio DOT rice DOT edu>...
> > 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.

Yeah, while looking up dpmi functin I came across the segment to
descriptor function, but didn't like that any descriptor it made
couldn't be modified or freed, but it's nice to know that I can just
allocate a descriptor without having to map it.  Thanks.

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