NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/sys/ibm/pc/hardware/22002

Text File | 1992-08-17 | 1.7 KB | 37 lines

Newsgroups: comp.sys.ibm.pc.hardware
Path: sparky!uunet!pipex!warwick!mrccrc!doc.ic.ac.uk!rhbnc!csqx.cs.rhbnc.ac.uk!adrian
From: [e-mail] (A Johnstone)
Subject: DMA on AT as opposed to XT
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Reply-To: [e-mail] (A Johnstone)
Organization: Dept of Comp Sci, Royal Holloway & Bedford New College Uni London
Date: Mon, 17 Aug 1992 09:49:55 GMT
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A while back I posted a call for any info on PC DMA. most reponses were of the
me-too variety so perhaps I touched a nerve...

I've been on holiday a while and it is possible that I've missed some discussion.

As an update:
I now have DMA to and from my homebrew SCSI card working on an XT (ie with 20-bit
addressing). Now on an AT (which of course has 24-bit addressing) an extra
four bits were added to the page registers that supply address bits above A15.

I don't know what to put in these extended page registers, because it depends
on what MS-DOS, Windows, Extended memory managers etc are doing. In fact even
under vanilla MS-DOS V5 with no cleverness about loading drivers high I
can't make it work. So, my next question is simple: how do you do virtual to
physical address mapping under MS-DOS on an AT, in other words how do I find out
what the real absolute address is of a buffer decalared in a normal non-high
memory program?

I'm happy to tell people how I got XT DMA going but I probably need to respond
to specific enquiries, so if you have a similar problem mail me and I'll try and
help. When (and if!) I've got it running on an AT I will post a short tutorial
document for your comments.

                           Adrian