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From: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Message-Id: <199603072141.WAA11583@labinfo.iet.unipi.it>
Subject: Re: Bad Ethernet cards
To: terry@lambert.org (Terry Lambert)
Date: Thu, 7 Mar 1996 22:41:03 +0100 (MET)
Cc: terry@lambert.org, wollman@lcs.mit.edu, questions@FreeBSD.ORG
In-Reply-To: <199603072047.NAA14745@phaeton.artisoft.com> from "Terry Lambert" at Mar 7, 96 01:47:01 pm
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> > I did my computation as follows: assume the CPU has to transfer 1MB/s
> > from the board. Say the available bandwidth on the PCI bus, doing
> > programmed I/O, is X MB/s. Then the CPU uses 1/X of its time just to
> > transfer that 1MB/s. For X=20, that makes 5% of the CPU time
> > unavailable for other things. Anything wrong ?
> 
> The CPU spends 1/X * clock_differential_for_bus_access of its time;
> it will be in "bus wait" on clock_differential_for_bus_access - 1
> of those clocks that it ordinarily would be using to run instructions
> in its L1 cache.

The bandwidth I call "X" is the available bandwidth in accessing that
particular device, which varies from device to device.
What you call X is probably something diffetent.

> I intensely dislike clock multiplied chips for precisely this reason;

If you have to drive the I/O pins at 200 MHz instead of 66, then
your CPU will likely consume 100W or more, provided you are able
to make it work reliably and cool it down to a reasonable temperature.
I suppose the little daemon would feel very comfortable on such a
CPU :)

	Luigi
====================================================================
Luigi Rizzo                     Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it       Universita' di Pisa
tel: +39-50-568533              via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522              http://www.iet.unipi.it/~luigi/
====================================================================