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From: bzs@bu-cs.UUCP
Newsgroups: comp.arch
Subject: Using a DMA chip in strange ways
Message-ID: <4872@bu-cs.BU.EDU>
Date: Tue, 3-Mar-87 22:20:50 EST
Article-I.D.: bu-cs.4872
Posted: Tue Mar  3 22:20:50 1987
Date-Received: Fri, 6-Mar-87 04:07:09 EST
Organization: Boston U. Comp. Sci.
Lines: 22


>    Since the DMA chip on your favorite disk/tape controller works by stealing
>bus cycles when the CPU is busy with other things (like arithmetic), would
>there be any advantage in having a DMA chip which would simply be used for
>memory to memory copies (from user to kernel space, or from one user space to
>another)?

I remember suggesting this on our LSI-11 systems using the DMA buffer
in the RX02 floppy drive as you could write/read it w/o any I/O going
to the disk (load/unload buffer [DMA] and xfer to/from disk were
different operations.) I figured we could buy an extra RX02 controller
if this brilliant idea worked (this was around '78-'79 I guess.)

Just an anecdote, we never tried it because the Mini-Unix was too busy
swapping to the RX02 to be used for this...I think we figured out it
wouldn't be very fast either.

Anyhow, maybe you already have a device to try it with, sure this wouldn't
work on your disk controller or some such (hey, no warranties expressed
or implied!)

	-Barry Shein, Boston University