From:	"Charles Sandmann" <sandmann AT clio DOT rice DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: XMS with HIMEM? (and DMA)
Date:	Wed, 28 Nov 2001 23:48:46
Organization:	Aspen Technology, Inc.
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> I am happy to share my code but I must note that what I use was
> originally posted by Neil Jarvis and at this point I have not 
> substantially added to it.
> I am also trying to build some large buffer DMA code handling as many 
> environments as possible. In this context I noticed that this is not so
> simple under Windows (only considering 9x DOS box). Having allocated
> some XMS it seems impossible to lock it in Windows. 

I noticed this also in testing.   You are limited to DOS memory or 
trying to use the VDS functions.

> I have not even been lucky
> with DOS memory. It seems that Windows does not map it over the first 
> physical MB. Makes sense insofar that there can be multiple dos boxes, 
> VMs that could not possibly all be in the first MB. 

If I remember correctly it captures the pokes to the DMA I/O ports and
re-maps it for you for each DOS machine.  But last time I messed with it
was under Win95 beta when we were prototyping stuff for Quake.  You
might look at a lot of code which has been written since then for 
examples.

> Nevertheless, what about the DMPI spec? 

DPMI says nothing about physical addresses or DMA - you have to use the
VDS spec for that.  It turns out that most VDS implementations are next
to useless, but you can see what's available.

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