*BSD News Article 62693


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From: ccheney@mail.utexas.edu (Chris Cheney)
Newsgroups: comp.unix.bsd.freebsd.misc,alt.comp.periphs.mainboard.asus,comp.sys.ibm.pc.hardware.systems,comp.os.linux.hardware,comp.sys.intel
Subject: Re: PCI bus freq with CPU freq at (X * 40)Mhz?
Date: Mon, 04 Mar 1996 20:07:59 GMT
Organization: The University of Texas at Austin, Austin, Texas
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stesin@elvisti.kiev.ua (Andrew V. Stesin) wrote:

>Sorry for ignorance, but what will be the frequency on a PC's PCI bus
>when the CPU runs at main frequency of 40MHz (DX2/80, DX4/120 i.e.)?

>I always thought that PCI will than run at 40MHz too.
>But was told today that it will be only 20MHz?!  Oh God.
>How can I check this for sure, without running hated benchmarks,
>but only by exploration of MB jumpers?

>I was also told that having 40MHz will probably cause
>an addition of extra wait states for cache and RAM access,
>thus making a box considerably _slower_ for real work
>than with 33MHz CPU main frequency, especially for UNIX-like OS
>(FreeBSD in my case, but who cares).  Any comments on this one?

>And, in case both statements are true, why bother looking
>at (X * 40)MHz 486 CPUs at all?

The PCI commonly used specification goes from 25 Mhz to 33 Mhz.  I
think I have heard that it is possible to go to 66 Mhz, but I think
most peripherals are only up to the 33 Mhz spec so pushing them to 40
Mhz sometimes causes problems.

Chris