Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!amdahl!pacbell!att!chinet!aicchi!dbb
From: dbb@aicchi.UUCP (Ben Burch)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Subtantiatng my criticism [really: VM on PDP 11/70]
Summary: A discussion of PDP-11/70 (and 11/44, 11/73, 11/84)
Message-ID: <127@aicchi.UUCP>
Date: 10 Aug 89 18:20:45 GMT
References: <13277@polyslo.CalPoly.EDU>
Reply-To: dbb@aicchi.UUCP (Ben Burch)
Organization: Analysts International; Chicago Branch
Lines: 43

In article <13277@polyslo.CalPoly.EDU> dorourke@polyslo.CalPoly.EDU (David M. O'Rourke) writes:
>
> [Stephen M. Robinson very gently pointed out that the 11/70 doesn't have
>virtual memory.]
>
>  hmmmmm, well then I guess this makes me look pretty foolish.  Guess I
>should brush up on computer history.  I've never seen a Unix system without
>virtual memory.  And how could you do all of this in 64k??  I would be
>interested in how it was to use such a system.  Has Unix grown that much??
>I couldn't concieve of using a BSD 4.3 style system in 64K of ram.  Any
>comments from people who's used such systems would be appreciated, apparently
>I've grown up in a totally different enviroment, and harbor some incorrect
>views about what a "basic" unix system is. :-)
>
>  Amazed that we've come so far.... and sorry for any confusion my previous
>posting might have caused.
>-- 
>\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\|/////////////////////////////////////////
>David M. O'Rourke____________________|_____________dorourke@polyslo.calpoly.edu
>|  God doesn't know, he would have never designed it like that in the first   |
>|_ place. ____________________________________________________________________|

Well, understand that this is from memory, but:
 
The PDP-11/70, while it did not have "virtual memory", did have a paged memory
management architecture which allowed the remapping of 8kb segments of a
tasks 64kb addressing space into a much larger physical addressing space.
I believe that one could shove 4Mb into an 11/70.  This is additionally
compicated by there being executive, supervisor, and user mode addressing
contexts, and seperable I and D spaces.  The current release of RSX-11M+,\
DECs hot OS for this architecture, makes extensive use of these features.
The only data I have on the original Unix implementations for the PDP-11
indicate that paged overlay to disk (explicit, not virtual) was made use of.
Indeed, this is commonly the method on RSX-11M[+] systems, since memory
resident overlays must be swapped in such large segments.  Note that there
are many wierd control bits in the memory management hardware on these
systems which were never used by DEC software, and which seemed to make
a virtual memory system possible with much software assistance.  I have
heard that a Chicago typesetting firm, "Datalogics" (?) did such a hack
to an old version of DOS-11...
 
-Ben Burch
(aicchi!dbb)