Date:	Sun, 11 Feb 96 12:44:02 GMT
From:	Neil Jarvis <Neil DOT Jarvis AT proteon DOT com>
To:	ulfg AT groundhog DOT phy DOT nist DOT gov
Cc:	djgpp AT delorie DOT com
Subject:	Re: how to allocate DMA pages ?
Reply-To:	Neil DOT Jarvis AT proteon DOT com

>>>>> "Ulf" == Ulf Griesmann <ulfg AT groundhog DOT phy DOT nist DOT gov> writes:

    Ulf> Hello All,
    Ulf> The 8237 DMA controllers in a PC can only perform DMA
    Ulf> transfers to the lower 16MB of physical memory. This makes it
    Ulf> awkward to allocate buffers for DMA transfers on machines
    Ulf> with >16MB of memory because a simple 'malloc' is no longer
    Ulf> guaranteed to allocate DMAable memory.  Does anyone out there
    Ulf> know of a good strategy for allocating DMA buffers which
    Ulf> works on large memory machines ? (Will take source code...)

Ulf,

It is actually worse than this - the 8237 DMA controller takes
physical addresses, malloc returns virtual addresses. There is
currently no mechanism to translate malloc'ed virtual addresses into
physical addresses with which to program the DMA controller.

Having said that, I came up with a solution that uses XMS to allocate
and lock a region of memory above 1Mb - XMS will tell you the physical
address of this region, allowing you to program DMA transfers. I then
used DPMI to map a region of linear memory to this XMS region. The
application can then access the data using the <sys/farptr.h>
functions.

I posted example code to this mailing list a couple of months
ago. Check out in the DJGPP mail archives.

   http://www.delorie.com/djgpp/mail-archives/djgpp/1995/12/13/17:04:41

*NOTE* There are problem with this approach :-( . Windows 95 does not
let me lock the XMS memory (I've not yet experimented with not locking
the XMS region - it may not be necessary). You also require some DPMI
1.0 functions, CWSDPMI (supplies them, my version of QDPMI.COM does
not. I have been using this method in a large application for the last
few months with no problems (I never run under Windows 95).

Hope this help.

-- Neil Jarvis, Proteon International, York R&D (Neil DOT Jarvis AT proteon DOT com)

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