From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Sun, 30 Oct 2005 13:27:15 -0800
From: Mike Haertel <9fans@ducky.net>
Message-Id: <200510302127.j9ULRFT3026074@ducky.net>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] 386
In-Reply-To: <9754e46bd89fc67ac49939b849c4e414@plan9.bell-labs.com>
Topicbox-Message-UUID: a2bc17ce-ead0-11e9-9d60-3106f5b1d025

jmk wrote:
>If you keep the 486 then there is no point in removing the 386,
>you would gain nothing.

Just to clarify exactly what the differences are, the 486 added:

* Integrated floating point (and better IEEE 754 conformance)

* CR0.WP control bit -- allows copy-on-write paging strategies
  even for accesses from ring 0  (386 didn't honor page write
  protection when CPU was in kernel mode)

* New instructions:
  * BSWAP
  * CMPXCHG
  * XADD
  * INVD
  * WBINVD

I agree with jmk that Plan 9 would be unlikely to gain much, since:

* Plan 9 on the 386 already requires a 387, and Plan 9 is not
  pedantic about IEEE 754

* Historically Plan 9 has not been concerned with the kind
  of pedal-to-the-metal optimization that would benefit
  from CR0.WP, BSWAP, CMPXCHG, or XADD.

* There is rarely any good reason to use WBINVD, and almost
  never a good reason to use INVD (except maybe in the BIOS),
  since x86 systems are cache coherent.