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From: ejbjr@ihlpg.ATT.COM (Branagan)
Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <3919@ihlpg.ATT.COM>
Date: Mon, 28-Sep-87 12:08:47 EDT
Article-I.D.: ihlpg.3919
Posted: Mon Sep 28 12:08:47 1987
Date-Received: Tue, 29-Sep-87 06:44:57 EDT
References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP>
Organization: AT&T Bell Laboratories - Naperville, Illinois
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Xref: mnetor comp.arch:2400 comp.unix.wizards:4558 comp.os.minix:1739

> > >....  The PDP11 didn't have virtual memory either,
> > >if my memory serves me.  ....
> > 
> > At the risk of re-opening an old debate, the PDP-11 \does/ have virtual
> > memory.  It's just that, for various technical reasons, the original Unix
> > implementation for it chose to use swapping instead of paging as its virtual
> > memory technique.
> 
> And neither did any other operating system for the PDP-11 (RSX, RSTS, RT-11),
> probably because it didn't in fact have the capability of supporting VM.
> Why do you think DEC developed the Virtual Address Extension (VAX) in the
> first place?

Virtual memory != Support for demand paging

The PDP-11 (later models only) does have virtual memory (i.e. supports
a translation from logical to physical address).  This was necessary to
support more than 64K of physical memory on a machine only capable of
addressing 64K - sort of backwards from the typical situation of using
virtual memory to support processes which use more memory than physically
available on the machine (or to the process).

The PDP-11 does not have support for paging (i.e. does not generate
convienient hardware interupts allowing a page to be brought into memory).
Actually the problem here is that it is difficult to figure out how much
of the instruction that faulted has executed already, and completing
execution of the instruction which generated the fault (I've seen this
done by interpreting the instruction in software, but it is very slow
and gross).  It looks like the designers made an attempt to add paging
capabilities, but it was very difficult without major design and microcode
changes (which lead to the VAX 11/780).

SUMMARY:

	The PDP-11 (many models) does support virtual memory, though
	not for the same purposes as most machines.  The PDP-11 has very
	poor support for paging, but it is theoretically (not practically)
	possible.
-- 
-----------------
Ed Branagan
ihnp4!ihlpg!ejbjr
(312) 369-7408 (work)