Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!killer!tness7!tness1!sugar!peter
From: peter@sugar.UUCP (Peter da Silva)
Newsgroups: comp.sys.amiga
Subject: Re: Amiga UNIX
Message-ID: <2106@sugar.UUCP>
Date: 12 Jun 88 04:19:08 GMT
References: <211@laic.UUCP> <3663@cbmvax.UUCP> <1872@sugar.UUCP> <134@ssdis.UUCP> <217@toylnd.UUCP>
Organization: Sugar Land UNIX - Houston, TX
Lines: 55

In article <217@toylnd.UUCP>, dca@toylnd.UUCP (David C. Albrecht) writes:
> > 	1) The 68000 supports an MMU just fine.
> A single by itself 68000 does not to my knowledge 'support' an MMU.  Having
> a second 68000 which takes over on a page fault hardly constitutes 'support'.

For the 14th time, an MMU does not imply demand paged virtual memory. There
are plenty of reasons for having an MMU even if you can't handle page faults.
Memory protection, for one, and providing a logical zero. Even if you can
handle page faults you might not want to do more than kill the process with
a SIGSEGV. Look at the PDP-11.

> > 	2) UNIX supports swapping just fine.
> Ok.

> > 	3) Virtual memory often gives you virtual performance.
> I've seen this kind of assertion many times on this newgroup and I still
> think it is a crock.

Comparing a PDP-11 running Berkeley UNIX (3BSD) with a VAX running Berkeley
UNIX (4BSD). The PDP-11 gave a lot better response time, and supported more
users, with less real memory than the VAX. And *this* was with an overlaid
kernel in the '11!

> > 	4) You can have a damn good UNIX that isn't VM.
> I agree.  I don't, however, think that you can have a damn good UNIX that
> doesn't have process protection which is usually part and parcel with
> an MMU and thus VM.

If you can find any indication that I even implied that (1) you can have a
good UNIX without memory management, or (2) that an MMU and VM are equivalent.

> Apples and Oranges.  It has been a long time, but last time I used a PDP 11
> a process maxed out a 128K.  Supporting many users each in a 128K
> hunk is a lot easier than a bunch of users running in arbitrary size areas.
> You can almost 'swap' a 128K hunk and get reasonable response for a fair
> set of users.  Kind of limits you to wenie processes though don't it?

By the time 3BSD faded out, it had some amazing overlay support. You could
take your VAX program with huge code and just compile and link it and it
would automagically become overlaid. This is the system I'm talking about.
About the only thing the VAX had that we PDP-11 weenies missed was Berkeley
Job Control, and they got that after I left.

As a side comment, the 128K limit *is* the reason paging wasn't implemented on
the '11... even though the hardware supported it.

> Of course virtual memory isn't free but neither is it that expensive
> given that you have a reasonable amount of physical memory to back it up.

I'm not saying that VM is undesirable, just that if you have enough memory
that you never page (which is basically what you're talking about) you might
as well not have the page table overhead.
-- 
-- Peter da Silva      `-_-'      ...!hoptoad!academ!uhnix1!sugar!peter
-- "Have you hugged your U wolf today?" ...!bellcore!tness1!sugar!peter
-- Disclaimer: These may be the official opinions of Hackercorp.