From owner-freebsd-current  Wed Jan 17 12: 6:42 2001
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Date: Thu, 18 Jan 2001 07:06:09 +1100
From: Peter Jeremy <peter.jeremy@alcatel.com.au>
Subject: Re: Atomic breakage?
In-reply-to: <200101171543.KAA18055@khavrinen.lcs.mit.edu>; from
 wollman@khavrinen.lcs.mit.edu on Wed, Jan 17, 2001 at 10:43:10AM -0500
To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
Cc: current@FreeBSD.ORG
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On 2001-Jan-17 10:43:10 -0500, Garrett Wollman <wollman@khavrinen.lcs.mit.edu> wrote:
><<On Wed, 17 Jan 2001 14:26:54 +1100, Peter Jeremy <peter.jeremy@alcatel.com.au> said:
>
>> To support multiple masters, you need proper locks.
>
>On older processors, yes.  On processors with the CX8 feature bit set,
>you can do it without any sort of locking (indeed, this is a primitive
>that semaphores can be built upon).

This particular sub-thread was specifically discussing the 80386 - as I
thought was clear from the context.  I am aware of the CMPXCHG8B insn,
but it's not relevant to the 386 or 486.

>On pre-Pentium processors (which lack the CX8 feature) this sort of
>sequence is impossible.  OTOH, I don't think SMP works on any
>pre-Pentium processor, so again this degenerates to:

There are SMP machines using both 386 and 486 processors.  There is
no support in FreeBSD for SMP on pre-Pentium processors.

Peter


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