Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ukma!mailrus!ulowell!apollo!nawaf
From: nawaf@apollo.COM (Nawaf Bitar)
Newsgroups: comp.arch
Subject: Re: Criteria ... [really: are N designs better than 1?]
Message-ID: <436b1bb9.1456d@apollo.COM>
Date: 24 May 89 19:41:00 GMT
References: <19088@winchester.mips.COM> <230@ross.UUCP> <3390@orca.WV.TEK.COM>
Reply-To: nawaf@apollo.COM (Nawaf Bitar)
Organization: Apollo Computer, Chelmsford, MA
Lines: 25

In article <3390@orca.WV.TEK.COM> andrew@frip.WV.TEK.COM (Andrew Klossner) writes:
>
>	"The reference MMU is different from the MMU that Sun has been
>	using in most of its SPARC based products.  I don't quite see
>	the problem with this.  A well written operating system only
>	requires a change to a single driver to port it to a new MMU."
>
>An MMU is not a "device," to be "driven" easily by a single module.
>Big MMU differences cause ripples throughout the kernel, and in extreme
>cases can affect user code.


In actual fact it does not hurt to think of an MMU as a device that is being
driven by a single module.  The Mach operating system does precisely that,
and experience has show that porting to differing MMU architectures has only
required changes to the single MMU driver (pmap module).  In order to accomplish 
this successfully the virtual memory system has to be carefully structured
such that no assumptions about a specific underlying MMU architecture are made.
This is certainly not the case in BSD, VMS, SunOs 3.* and, no doubt, others.

-- 
Nawaf Bitar                                                nawaf@apollo.com
Apollo Computer, Inc.
330 Billerica Road
Chelmsford, Massachusetts 01824