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From: greg@ncr-sd.SanDiego.NCR.COM (Greg Noel)
Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <1755@ncr-sd.SanDiego.NCR.COM>
Date: Fri, 25-Sep-87 21:52:27 EDT
Article-I.D.: ncr-sd.1755
Posted: Fri Sep 25 21:52:27 1987
Date-Received: Sun, 27-Sep-87 07:02:44 EDT
References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP>
Organization: NCR Corporation, Rancho Bernardo
Lines: 52
Xref: mnetor comp.arch:2362 comp.unix.wizards:4498 comp.os.minix:1723

>In article <1745@ncr-sd>, greg@ncr-sd (Greg Noel) writes:
>> ... the PDP-11 \does/ have virtual
>> memory.  It's just that, for various technical reasons, the original Unix
>> implementation for it chose to use swapping instead of paging as its virtual
>> memory technique.

In article <819@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes:
>And neither did any other operating system for the PDP-11 (RSX, RSTS, RT-11),
>probably because it didn't in fact have the capability of supporting VM.

You don't give the criteria by which you make this rather bald claim, but I'll
try to respond to it anyway.

The test for virtual memory is whether the name space of the process is
independent of the name space of the processor; that is, the memory seen by the
process is the same, no matter where it is located in physical (real) memory.
In other words, if the process can be moved to a new place in memory and is
unaware that it has been moved, then the process memory is virtual, and the
architecture supports virtual memory.  The PDP-11 passes this test.

If you are trying to say that the PDP-11 didn't permit a process to be run
without all of its image mapped, that is indeed true of some processor models.
(But not all models -- I've seen experimental versions of Unix on the PDP-11
that were demand paged.)  But this isn't a requirement for virtual memory; it
just makes alternative virtual memory schemes (like paging) more attractive.

>Why do you think DEC developed the Virtual Address Extension (VAX) in the
>first place?

Yes, DEC did indeed extend the amount of virtual memory available, from a
maximum of two 64k pieces to ~2G.  I'll even agree that that's quite an
extension -- over four orders of magnitude.  But they extended the amount,
not the concept.

>..... Sometimes
>virtual memory means virtual performance, as a good many PDP-11 fans have
>pointed out. You can run way more users on and get way better real-time
>response from a PDP 11/70 than any VAX you care to name.

There's some truth in that.  I suspect that it's more of a function of
the increasing size of programs (and the disk cost to read them in) than
a function of the virtual memory architecture (although it \is/ due to
virtual memory that programs have been able to bloat so much).  But that
is currently being mooted elsewhere, so I won't get on that soap box here.

BTW, I'm assuming that you meant "interactive" when you said "real-time,"
since in an actual hard-real-time environment, I would want my tasks running
with the memory management turned off and in a soft-real-time environment, I
would want my tasks locked down.  In either case, the overhead for virtual
memory would be similar and the faster processor would win.
-- 
-- Greg Noel, NCR Rancho Bernardo     Greg.Noel@SanDiego.NCR.COM