Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!lll-crg!lll-lcc!pyramid!ncr-sd!greg
From: greg@ncr-sd.UUCP (Greg Noel)
Newsgroups: net.arch
Subject: Re: Paging
Message-ID: <1181@ncr-sd.UUCP>
Date: Tue, 23-Sep-86 19:43:17 EDT
Article-I.D.: ncr-sd.1181
Posted: Tue Sep 23 19:43:17 1986
Date-Received: Wed, 24-Sep-86 02:58:25 EDT
References: <8494@duke.duke.UUCP> <147@eneevax.UUCP> <7110@utzoo.UUCP>
Reply-To: greg@ncr-sd.UUCP (Greg Noel)
Organization: NCR Corporation, San Diego
Lines: 43

In article <7110@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:

>I assure you that the PDP-11 is *not* considered a virtual-memory machine,
>not by DEC, not by anyone who knows the machine and knows what the words
>mean.

That's an awfully snippy comment.  Just because the virtual space is smaller
than the physical doesn't mean that it isn't a virtual machine.

>Why do you think the Berkeley Unixes
>also go by the name "VMUNIX"?  The "VM" part stands for "virtual memory",
>which earlier Unixes -- on the 11, and on the VAX with address translation
>but no demand paging -- were not.

"You can call a tail a leg, but that doesn't make it one."

(Actually, that's not quite correct, either.  Indeed, "VMUNIX" is a virtual
memory operating system -- but then, so were the earlier ones.)

I will agree that an extremely strong motivation for virtual memory is so
that you are not required to have all of your program residing in physical
memory.  I will even agree that most virtual memory systems use some form
of paging (or whatnot) for memory management -- but that doesn't mean that
the concepts are the same, or that virtual memory mandates paging.  There
are other, perfectly valid reasons to have virtual memory.

I think the problem here is one of age.  Henry is probably too young to have
heard the terms used correctly and is using the (usually very reasonable)
assumption that if A and B always occur together, then A implies B.  On the
other hand, I worked on the Project Genie time-sharing system (one of the two
original ARPA-funded time-sharing projects; the other was Project Multics),
so I can claim some degree of expertise on the topic.

The fundamental concept of virtual memory is that a large name space (the
virtual memory) can be partially mapped to a smaller name space (the physical
memory) with the remainder of the space unmapped.  Uttering a name that is
not mapped causes some sort of an exception mechanism, but the semantics of
that exception are not part of the virtual memory model.  Admittedly, the
concept is not terribly interesting unless you postulate some exception
semantics, but paging is just one potential set of exception semantics, not
the \only/ one.
-- 
-- Greg Noel, NCR Rancho Bernardo    Greg@ncr-sd.UUCP or Greg@nosc.ARPA