Path: utzoo!attcan!uunet!husc6!purdue!spaf@cs.purdue.edu
From: spaf@cs.purdue.edu (Gene Spafford)
Newsgroups: comp.os.misc
Subject: Re: a very naive Question???
Message-ID: <5085@medusa.cs.purdue.edu>
Date: 12 Oct 88 04:42:46 GMT
References: <835@amethyst.ma.arizona.edu>
Sender: spaf@cs.purdue.EDU
Reply-To: spaf@cs.purdue.edu (Gene Spafford)
Organization: Department of Computer Science, Purdue University
Lines: 27

The definition I use in teaching OS courses is the following:

If the address presented to the system may be bound to a physical
address different from that seen by the program, you have
virtual memory.  That is, if the software generates some address
like 5 for a storage location, and if the binding to a physical
address COULD POSSIBLY be to a hard physical address different
from 5, then that implies virtual memory (let's not try to
twist this to include fault handling, etc.).

Virtual memory includes, possibly, pages, segments, overlays,
base register addressing, and some forms of swapping.

Virtual memory does not mean that the virtual address range is bigger
than the physical address range -- it could be smaller, as in
the case of old CDC machines that had megabytes of memory but could
only run program images of a few 100K.  

Swapping is not per se virtual memory.  If you swap images in and out
of the exact same addresses, and if there is no translation of the
addresses after any swap, then that is not virtual memory.

The "virtual" means that the memory looks as if it were the
range of physical addresses specified by the program.

That's how I've taught it in OS courses at both Purdue and
Georgia Tech, and that's consistent with many OS texts....