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From: peter@sugar.UUCP (Peter da Silva)
Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <831@sugar.UUCP>
Date: Sun, 27-Sep-87 22:07:04 EDT
Article-I.D.: sugar.831
Posted: Sun Sep 27 22:07:04 1987
Date-Received: Thu, 1-Oct-87 05:13:55 EDT
References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP> <1755@ncr-sd.SanDiego.NCR.COM>
Organization: Sugar Land UNIX - Houston, TX
Lines: 42
Summary: That's not virtual...
Xref: mnetor comp.arch:2422 comp.unix.wizards:4574 comp.os.minix:1748

In article <1755@ncr-sd.SanDiego.NCR.COM>, greg@ncr-sd.SanDiego.NCR.COM (Greg Noel) writes:
> In article <819@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes:
> >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.
> 
> You don't give the criteria by which you make this rather bald claim, but I'll
> try to respond to it anyway.

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. If you allow the task to manage the "VM", then you're letting
overlays in and anything with secondary memory qualifies. This implies
that it should be able to recover from a page fault.

Now, someone else has claimed that the PDP-11 in some incarnations was able to
do this. Since they seem to know whereof they speak, I'll accept it. But I
stand on my claim that PDP11 does nore imply VM.

What you're talking about...

> 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.

Is mapped memory (at least that's what DEC calls it, and it's a reasonable
description. Especially since we're talking about DEC processors.

> BTW, I'm assuming that you meant "interactive" when you said "real-time,"
> since in an actual hard-real-time environment, I would want my tasks running
> with the memory management turned off and in a soft-real-time environment, I
> would want my tasks locked down.  In either case, the overhead for virtual
> memory would be similar and the faster processor would win.

Yeh, yeh. For hard real-time I wouldn't want anything more complicated in the
way of operating systems than a scheduler. Down here in Houston real-time
generally refers to SCADA, which makes most people's "soft realtime" look
like granite.
-- 
-- Peter da Silva `-_-' ...!hoptoad!academ!uhnix1!sugar!peter
--                 'U`  Have you hugged your wolf today?
-- Disclaimer: These aren't mere opinions... these are *values*.