Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!umich!yale!think!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.arch
Subject: Re: Personal OS
Message-ID: <36849@think.Think.COM>
Date: 27 May 90 04:53:24 GMT
References: <402@newave.UUCP> <3300131@m.cs.uiuc.edu> <9437@pt.cs.cmu.edu>
Sender: news@Think.COM
Reply-To: barmar@nugodot.think.com (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 36

In article <9437@pt.cs.cmu.edu> lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:
>In summary: if my workstation had a single address space, I'd sell it
>and buy something adequate.  (Sorry, Ed.)

Actually, one of the best reasons for a single address space is to simplify
sharing of data between applications, which promotes integrated software.
MIT-descended Lisp Machines are a good example of this.  The debugger can
display source code from the editor, the editor has access to the compiler
warning database, etc.  It's true that integrated software can be written
when the applications have walls between them, but it's harder and rarely
very extensible.  A protocol has to be provided, and you are usually
restricted to the forms of communication that are preconceived by the
designers; if all the applications are designed together this may be good
enough, but if they have differing heritages you may be out of luck.

Many Macintosh utilities take advantage of the single address space.  For
example, there's an auto-save utility, which periodically causes the
application to invoke its "Save" menu item; it has to look around in the
application's address space to find the identity of the menu entry.

>>   7.  overlay loader (because early macs (& Alto) had no VM)
>I'm old enough to remember when overlays were the way you built
>applications on minicomputers.  They were a god-awful source of bugs
>and grief, and a horrendous waste of effort.  In summary: bletch.

Remember, the article was written in the early 80's, when VM on personal
computers was almost unthinkable (indeed, hard disks on personal computers
were considered a luxury -- I think the Winchester disk is probably one of
the most important computer technologies of the 80's).  If overlays are
automated, as is done on the Mac, then bugs are reduced.  It's a reasonable
compromise, but it's certainly not as nice as VM with demand paging.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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