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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.arch
Subject: Re: Paging
Message-ID: <7110@utzoo.UUCP>
Date: Thu, 11-Sep-86 13:32:38 EDT
Article-I.D.: utzoo.7110
Posted: Thu Sep 11 13:32:38 1986
Date-Received: Thu, 11-Sep-86 13:32:38 EDT
References: <8494@duke.duke.UUCP> <147@eneevax.UUCP>
Organization: U of Toronto Zoology
Lines: 25

> >"Virtual memory", strictly speaking, refers to memory that acts like it's
> >always there but sometimes isn't.  That is, demand paging or some analogous
> >scheme ... Address translation, by itself, is not virtual memory.
> 
> Hmmmmm.....  I think I want to take issue with this.  Although what Henry
> says is valid, his point is too narrow.  Two immediate counter-examples are
> the SDS-940 (later the XDS-940) and the PDP-11, both of which are virtual-
> memory machines, and yet essentially just only do address translation...

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.  (Well, you can sort of do virtual memory on the 11, but none of the
popular operating systems bother.)  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.

> Perhaps more to the point would be to say that the memory image seen by the
> program is independent of the real memory provided by the machine...

This would be a reasonable meaning for the term "virtual memory", but it is
not the standard one.  Check any good operating-systems textbook.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,decvax,pyramid}!utzoo!henry