Path: utzoo!utgpu!watmath!att!ucbvax!dewey.soe.berkeley.edu!oster
From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Subtantiatng my criticism [really: VM on PDP 11/70]
Message-ID: <30466@ucbvax.BERKELEY.EDU>
Date: 8 Aug 89 08:50:25 GMT
References: <13277@polyslo.CalPoly.EDU>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
Lines: 39

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??

Sorry to bother the comp.sys.mac.programmer crowd with this. David, don't
be so quick to apologize, Stephen is absolutely wrong.  The PDP-11/70 only
has a 16-bit program counter, so you'd think it can only address 64k. But,
the memory mapping registers map the program counter into one 64k space,
and the data registers into a second 64k space. This was known as split i
& d space.  Each program gets its own pair of i&d space, and the kernel
gets _its_ own space. So, you had virtual memory, but on a whole system
basis. A single program could get no bigger than 128k bytes. Since the
operating system has direct access to the memory mapping registers, it can
be much larger than 128k by accessing other memory spaces as needed.

Now, the Macintosh connection: that Unix system had a huge latency: if you
typed a key, it might be as long as 5 seconds before your job came around
again, and your keystroke got processed. That is okay,if the PDP-11 had a
serial communications card and a high-priority device driver queueing up
characeters for you.

Lets look at the Macintosh. You are in a Paint program, and begin to make
a stroke with the mouse. There is no way you can afford to wait 5 seconds
before that stroke begins to echo.

Unix is okay, but the pipeline components Stephen puts such store by:
sort, ls, grep, awk, and the rest are essentially non-interactive. You
start them up and wait for them to finish. If you want interactive
performance on a wimpy old slow 68000, you don't want unix.

--- David Phillip Oster            --"Unix Version 7 was an improvement not
Arpa: oster@dewey.soe.berkeley.edu --only over its predeccessors, but also its
Uucp: {uwvax,decvax}!ucbvax!oster%dewey.soe.berkeley.edu --successors."