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From: dhp@ihlpa.UUCP (Douglas H. Price)
Newsgroups: net.micro.atari16,net.micro.amiga,net.micro.68k
Subject: Re: 68000 Memory Managment
Message-ID: <1722@ihlpa.UUCP>
Date: Fri, 22-Aug-86 18:55:05 EDT
Article-I.D.: ihlpa.1722
Posted: Fri Aug 22 18:55:05 1986
Date-Received: Sat, 23-Aug-86 10:24:05 EDT
References: <508@elmgate.UUCP> <64@mit-prep.ARPA> <510@elmgate.UUCP> <4374@gatech.CSNET>
Organization: AT&T Bell Laboratories
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Xref: mnetor net.micro.atari16:1690 net.micro.amiga:4372 net.micro.68k:1155

> Just because you are implementing "paging" does not mean that you must
> implement "demand paging". The PDP-11 had memory mapping registers but
> did not allow demand paging. Before a process ran you had to ensure that
> all the memory they were allowed to access was resident.
>
> Jeff Lee
> CSNet:	Jeff @ GATech		ARPA:	Jeff%GATech.CSNet @ CSNet-Relay.ARPA
> uucp:	...!{akgua,allegra,hplabs,ihnp4,linus,seismo,ulysses}!gatech!jeff

Wrongo, Oh Bit Breath!  PDP-11s from an 11/44 on up can demand-page quite well,
thank you, though rather inefficiently.  I used to work for a firm called
Datalogics, in Chicago, IL, which had a demand paged operating system called
DURESS which had originally been written in about 1977 as a derivative of RT-11
and RSX-11D.  

This operating system used all three address spaces (USER, SUPERVISOR and KERNEL)
and was fully demand paged.  In fact, you will find there is a service order
available on the PDP-11/44 designed to correct a bug in the memory managment
unit that we turned up when we first tried to port DURESS to it.  

Another comment; DURESS could run proverbial rings around the available UNIX
versions of that era.  It made a PDP-11 look like a real machine to the user
in terms of performance.  On the other hand, it had a really grungy programmer
interface, and a semi-flat file system (yuck!)

PDPs less than an 11/44 (11/23 through the 11/34) had a simplified memory manager
which did away with SUPERVISOR space, and split I and D, which in my opinion
severely crippled them.

-- 
						Douglas H. Price
						Analysts International Corp.
						@ AT&T Bell Laboratories
						..!ihnp4!ihlpa!dhp