Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!murtoa.cs.mu.oz.au!munnari.oz.au!otc!softway!chris
From: chris@softway.oz (Chris Maltby)
Newsgroups: comp.arch
Subject: Re: DMA on RISC-based systems
Message-ID: <1620@softway.oz>
Date: 6 Jun 89 00:36:35 GMT
References: <46500067@uxe.cso.uiuc.edu> <1989May26.170247.1165@utzoo.uucp> <1552@softway.oz> <182@dg.dg.com>
Organization: Softway Pty Ltd, Sydney, Australia
Lines: 25

In article <182@dg.dg.com> rec@dg.UUCP (Robert Cousins) writes:
> However, just remember that you are throwing MIPS away doing the copying.
> I would rather have a $5 DMA controller spending the time than my high
> powered CPU.  Sure, it works to use the CPU to do the copying, but when
> you realize the amount of time the CPU may be forced to spend because of 
> the copy (including extra interrupt service, context switches, polling
> loops, cache flushes, etc.), it often turns out that a DMA controller
> can provide the user with VERY CHEAP MIPS by freeing up the CPU.  It
> is this logic which allows people to avoid using graphics processors
> in workstations by saying "the CPU is fast therefore I don't need one."

Without rejecting anything you said, let me point out that the opposite
logic can also apply though. Why install special purpose I/O intelligence
if you can ony use it for I/O. A general purpose (extra perhaps) CPU
can do all that I/O nonsense as well as other good things.  I guess it
all depends on what you want the machine to do best. Select the
criteria - then design the machine.

At this point we should adopt Mr Mashey's approach... measure then
draw conclusions on actual data.
-- 
Chris Maltby - Softway Pty Ltd	(chris@softway.sw.oz)

PHONE:	+61-2-698-2322		UUCP:		uunet!softway.sw.oz.au!chris
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