Re: [9fans] 386

[Mike Haertel <9fans@ducky.net>]

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.