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From: rcd@opus.UUCP (Dick Dunn)
Newsgroups: net.arch
Subject: Re: Why Virtual Memory
Message-ID: <178@opus.UUCP>
Date: Wed, 30-Oct-85 02:08:42 EST
Article-I.D.: opus.178
Posted: Wed Oct 30 02:08:42 1985
Date-Received: Fri, 1-Nov-85 03:10:27 EST
References: <480@seismo.CSS.GOV> <384@unc.unc.UUCP>
Organization: NBI,Inc, Boulder CO
Lines: 22

> It is interesting to note that 10 years ago or so, all large systems
> had virtual memory whereas small systems did not.
> 
> Now the largest systems (e.g., Cray 2) do not have virtual memory,
> whereas it is more and more common for small systems ("microprocessors", 
> and I use the term in quotes) to have virtual memory.

If you mean by "virtual memory" something like paging capability--or in
particular, hardware support for a logical address space larger than the
physical address space, it is NOT true that all large systems had virtual
memory 10 years ago.

The Cray 2 is not markedly different with respect to memory address mapping
than the Cray 1, the CDC 7600, or the CDC 6600--each the fastest commercial
machine of its day.  The 6600 takes us back about 20 years.

Quite simply, in machines as large as these, you cannot pretend that you
have memory that isn't really there, for the class of problem they tend to
be used to solve.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...At last it's the real thing...or close enough to pretend.