From: "Charles Sandmann" <sandmann AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Thoughts on physical address and DMA
Date: Mon, 20 Aug 2001 16:58:13
Organization: Aspen Technology, Inc.
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> Since you add `__djgpp_conventional_base' to a physical address to
> convert it to a linear address, could you possibly _subtract_ it from
> a linear address to convert it to a physical address? If so, could the
> physical address be used for DMA?

Doing DMA under DPMI is a somewhat specialized operation.  To make this
note short, you would need to know what platforms you plan to support
(DOS/CWSDPMI, Windows, etc).  The traditional way is to allocate a
DOS memory buffer which has 1:1 address mapping.  If you need a large
buffer, and run only under CWSDPMI, you can grab a work in progress
http://clio.rice.edu/djgpp/cwsdma2.zip for an example of how to get
the real address mapping.