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From: romkey@kaos.UUCP (John Romkey)
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: Re: IBM PC/AT DMA loses (was Re: PC LAN Comparison)
Message-ID: <267@kaos.UUCP>
Date: Tue, 24-Nov-87 03:02:05 EST
Article-I.D.: kaos.267
Posted: Tue Nov 24 03:02:05 1987
Date-Received: Fri, 27-Nov-87 21:49:10 EST
References: <2070@killer.UUCP> <1020@kodak.UUCP> <155@tic.UUCP> <261@kaos.UUCP> <372@gethen.UUCP>
Reply-To: romkey@kaos.UUCP (John Romkey)
Organization: Chaos; Somerville, MA
Lines: 36
Keywords: IBM PC, hardware, DMA loses
Xref: mnetor comp.dcom.lans:964 comp.sys.ibm.pc:10469

In article <372@gethen.UUCP> farren@gethen.UUCP (Michael J. Farren) writes:
>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.
>
>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.

It was my impression that the original article meant to say "programmed I/O"
instead of "interrupts", which is why I launched off into my discussion of
programmed I/O.

It isn't rational to discuss DMA vs. interrupts for any PC or PC/AT
bus network interface that I've encountered, and I've written or seen
drivers for most of them (the list would double the length of this
message). None of them give you an option of taking an interrupt per
byte while transferring data. You either do or do not take one
interrupt on receive or transmit completion (or DMA completion if you
use DMA), and you either use DMA to transfer data or you use
programmed I/O. The two are independent and therefore *orthogonal*. If
you want any responsiveness out of your network code (at least in a
TCP/IP implementation), you'll want to use interrupts regardless of
whether or not you use DMA. You'll decide whether to use DMA based on
the network interface's architecture (many memory-mapped interfaces don't
support it) and your bus. 

>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"

-- 
			- john romkey
		...mit-eddie!blblbl!kaos!romkey
		    romkey@xx.lcs.mit.edu