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From: radford@calgary.UUCP (Radford Neal)
Newsgroups: comp.arch
Subject: Definition of "virtual memory" (was Re: Free Software Foundation)
Message-ID: <1087@vaxb.calgary.UUCP>
Date: Sun, 4-Oct-87 17:07:45 EDT
Article-I.D.: vaxb.1087
Posted: Sun Oct  4 17:07:45 1987
Date-Received: Sat, 10-Oct-87 18:48:47 EDT
References: <738@hplabsz.HPL.HP.COM> <76700006@uiucdcsp>
Organization: U. of Calgary, Calgary, Ab.
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In article <76700006@uiucdcsp>, johnson@uiucdcsp.cs.uiuc.edu writes:
> 
> >Virtual memory == ability for a task to run without its entire address space
> >residing in primary memory, in a manner that is transparent to the task
> >itself.
> 
> This answer would be marked as incorrect in the O.S. class that I teach.

I have always understood this to be the correct answer. Common, long-
established usage is the test. I think, for instance, that when IBM
introduced "virtual memory" into their 360/370 line with great fanfare,
they meant the ability to run individual programs with a process address
space larger than the physical size of memory.

> > 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.
> 
> This answer would be judged correct.

By this definition various old processors - e.g. the CDC 6600 - would
be judged to have "virtual memory" - but we didn't say they did back then.
It's a lot more consistent with established usage to call this a "memory
management" facility. Unfortunately, I've noticed a tendency to redefine
"memory management" recently so that it means anything that manages 
memory - i.e. just about any program or machine whatsoever.

> I suppose that either answer could serve as a definition for virtual memory,
> but you have to pick one and use it.  Perhaps it depends on the text book
> that you use.  The definition that you use determines whether you think
> that the PDP-11 has virtual memory or not.

By any definition, the more modern PDP-11s have the capability for
virtual memory. There's just no practical point, since the address
space is divided into only eight pages. Most processes will access
all pages most of the time, so demand-paging would be pointless.

    Radford Neal
    The University of Calgary