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From: truett@cup.portal.com
Newsgroups: comp.dcom.lans
Subject: Re: PC Lan Comparison
Message-ID: <1559@cup.portal.com>
Date: Mon, 23-Nov-87 23:55:27 EST
Article-I.D.: cup.1559
Posted: Mon Nov 23 23:55:27 1987
Date-Received: Fri, 27-Nov-87 06:03:25 EST
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While discussing DMA versus PIO devices on PCs, let me add the following
little tidbit which I discovered working on a piece of hardware that used
DMA.

Most DMA devices designed for PCs assume an 8-bit peripheral interface, so
they assume the DMA is an 8-bit DMA, which it is on the PC or XT.  In that
case it takes 5 bus cycles, or 1.05 microseconds at 4.77 MHz, to move one
byte.  Now the 8-bit DMA channels of an AT are cascaded through one of the
16-bit DMA channels, so it takes 10 bus cycles to move one byte through an
8-bit DMA access peripheral on the AT.  Yes, a 16-bit peripheral connected
directly to the 16-bit channel only takes 5 bus cycles to move 2 bytes.
But the effect is, for 8-bit peripherals attached to the 8-bit DMA channels,
that any AT operating at less than about 9.8 MHz moves the data more slowly
than the lowly XT at 4.77 MHz!

The moral?  On an AT, PIO is faster than DMA, especially if your I/O device
can handle the OUTSW instruction at full speed.  Even OUTSB is faster.pa


As far as I know, none of the technical documentation from IBM takes the
trouble to mention this.  I only found out when a peripheral made by the
company I work for which was optimized for the XT using DMA failed miserably
to run on a true blue AT but did on some clones which had different DMA
arrangements.  We finally ended up using dual-ported memory because some
I/O circuits are very slow on some machines.

Truett Smith, Sunnyvale, CA
UUCP:  truett@cup.portal.com