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From: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Newsgroups: comp.sys.amiga
Subject: Re: Disk Controllers
Message-ID: <4063@videovax.Tek.COM>
Date: Tue, 25-Nov-86 16:31:47 EST
Article-I.D.: videovax.4063
Posted: Tue Nov 25 16:31:47 1986
Date-Received: Wed, 26-Nov-86 09:49:22 EST
References: <2043@well.UUCP> <758@ulowell.UUCP> <1986Nov19.104846.321@utcs.uucp> <1242@masscomp.UUCP> <2111@well.UUCP>
Reply-To: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Organization: Tektronix Television Systems, Beaverton, Oregon
Lines: 28
Summary: DMA enhances overall throughput!

In article <2111@well.UUCP>, Perry S. Kivolowitz (perry@well.UUCP) writes:

> In article <1242@masscomp.UUCP>, garyo@masscomp.UUCP (Gary Oberbrunner)
> writes:
>> Most of this is true.  Normally, disk rate shouldn't be much improved
>> by using DMA, since most of the time is spent seeking.  However, with a
>> non-DMA controller the overhead during the actual transfer is MUCH higher,
>> since the device must do CPU interrupts per byte transferred.  Thus not
>> only is the transfer slowed by using the CPU, it's doubly slowed
>> because other computation cannot proceed while all these interrupts are
>> happening.
> 
> But the transfers don't  HAVE  TO  NECESSITATE (sp?) interrupts. The
> processor in a tight loop will run competitively to the overhead of
> handling interrupts.  If you were to do a non dma driven disk controller,
> that is.

The problem is, it eats the entire CPU!  The overwhelming advantage of DMA
is that it frees the processor to do other things.  In a multitasking
system (like the Amiga) the CPU can execute some other task while the
first one is blocked waiting for I/O.  Or you can run asynchronous I/O,
as Matt does in his asynclib.  Either way, the DMA buys a big gain in
throughput!

					Steve Rice

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