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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.dcom.lans,comp.sys.ibm.pc
Subject: Re: PC LAN Comparison
Message-ID: <8977@utzoo.UUCP>
Date: Mon, 23-Nov-87 14:02:40 EST
Article-I.D.: utzoo.8977
Posted: Mon Nov 23 14:02:40 1987
Date-Received: Mon, 23-Nov-87 14:02:40 EST
References: <2070@killer.UUCP> <1020@kodak.UUCP>, <155@tic.UUCP>
Organization: U of Toronto Zoology
Lines: 20
Keywords: IBM PC, Novell, hardware

> ...I had great difficulty believing their hardware section, which
> repeatedly says that DMA interface boards are slower than others...
> DMA is much faster than interrupt driven hardware.

Well... it depends.  There are really *three* flavors of hardware here:
interrupt-per-character, DMA, and internally-buffered, to give them vaguely
accurate names.  Internally-buffered stuff often brings its internal buffer
out as dual-ported memory, although that isn't logically necessary.  The
difference between DMA and buffered is just who does the data copying,
peripheral or CPU.

Interrupt-per-character stuff *is* slow.

The tradeoffs between DMA and buffered are much less clear.  In principle
DMA can be better.  In practice, complications like bus acquisition and
just plain badly-designed hardware often make buffered competitive or even
superior.  Modern CPUs are pretty good at copying data.
-- 
Those who do not understand Unix are |  Henry Spencer @ U of Toronto Zoology
condemned to reinvent it, poorly.    | {allegra,ihnp4,decvax,utai}!utzoo!henry