Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.arch,comp.unix.wizards
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <6488@brl-smoke.ARPA>
Date: Mon, 28-Sep-87 15:50:00 EDT
Article-I.D.: brl-smok.6488
Posted: Mon Sep 28 15:50:00 1987
Date-Received: Tue, 29-Sep-87 06:32:07 EDT
References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 25
Xref: mnetor comp.arch:2399 comp.unix.wizards:4557

In article <819@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes:
>probably because it didn't in fact have the capability of supporting VM.

Greg Noel was right -- the PDP-11 did support virtual memory.
Basically, three things are necessary for full support of demand-paged
virtual memory:  mappable per-process virtual address space pages,
generation of a trap when a reference is made to an unmapped page,
and ability to restart the faulted instruction after changing the map.
All these facilities existed on high-end PDP-11 models with KT11 MMUs.
(The low-end models did not have such a memory management unit.)

As others have pointed out, there were experimental virtual memory
implementations of PDP-11 UNIX.

>Why do you think DEC developed the Virtual Address Extension (VAX) in the
>first place?

To get a larger per-process address space!

>You can run way more users on and get way better real-time response
>from a PDP 11/70 than any VAX you care to name.

Both 22-bit PDP-11s and VAXes have to perform two levels of address
translation.  Some VAXes provide poorer peripheral I/O paths, but
apart from that there is little difference in their real-time abilities.