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From: farren@gethen.UUCP (Michael J. Farren)
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: Re: IBM PC/AT DMA loses (was Re: PC LAN Comparison)
Message-ID: <372@gethen.UUCP>
Date: Sun, 22-Nov-87 03:00:11 EST
Article-I.D.: gethen.372
Posted: Sun Nov 22 03:00:11 1987
Date-Received: Tue, 24-Nov-87 04:58:42 EST
References: <2070@killer.UUCP> <1020@kodak.UUCP> <155@tic.UUCP> <261@kaos.UUCP>
Reply-To: farren@gethen.UUCP (Michael J. Farren)
Organization: Sci-Fido - Unix in Oakland
Lines: 31
Keywords: IBM PC, hardware, DMA loses
Xref: mnetor comp.dcom.lans:952 comp.sys.ibm.pc:10403

In article <261@kaos.UUCP> romkey@kaos.UUCP (John Romkey) writes:
>
>First off, interrupts and DMA are orthogonal. It's really a choice of DMA
>versus programmed I/O. On the PC/AT, the DMA controller runs about 10%
>slower than the standard PC DMA controller. On top of this, the 80286 has
>string IN and OUT instructions in addition to the 8086 string MOV instruction.
>Both string IN/OUT and string MOV instructions push data around a lot faster
>than the DMA on that system controller can. You basically end up with the
>processor sitting in a tight loop shoveling bytes across the bus.

DMA and interrupts are NOT orthogonal.  In any interrupt-driven scheme,
there will be system overhead required for each and every byte of data
transferred.  In most systems I am aware of, this interrupt overhead so
overwhelms any one-time overhead involved in setting up DMA, and the
processor slowing associated with DMA (if any - many systems are
designed such that DMA does not affect the processor in any meaningful
way.  Most 68000 systems, for example, take full advantage of the fact
that the processor only requires the bus every other cycle, more or
less), for any transfer over a very few bytes, that considering
interrupts instead of DMA ensures great inefficiency.

On the choice between DMA and programmed I/O, much depends on the system
design.  The IBM architecture may not allow a distinct advantage for DMA
vs. programmed I/O, but many other systems do.  To make a general
statement that DMA is not as efficient as programmed I/O is wrong.

-- 
----------------
Michael J. Farren      "... if the church put in half the time on covetousness
unisoft!gethen!farren   that it does on lust, this would be a better world ..."
gethen!farren@lll-winken.arpa             Garrison Keillor, "Lake Wobegon Days"