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From: lsr@apple.UUCP (Larry Rosenstein)
Newsgroups: comp.sys.m68k
Subject: Re: How does one implement VM in a 68000?
Message-ID: <1251@apple.UUCP>
Date: Thu, 2-Jul-87 12:26:11 EDT
Article-I.D.: apple.1251
Posted: Thu Jul  2 12:26:11 1987
Date-Received: Sat, 4-Jul-87 07:03:37 EDT
References: <1029@bloom-beacon.MIT.EDU> <2351@hoptoad.uucp>
Reply-To: lsr@apple.UUCP (Larry Rosenstein)
Organization: Advanced Technology Group, Apple Computer
Lines: 27

In article <2351@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
>
>On the Macintosh, I understand that Apple did something similar for
>procedure calls to non-resident libraries; they figured out what the
>chip would do with an instruction like "jsr a0@" to a bad address, and
>if it happens, they fix it up.

Half true.  This was done on the Lisa, not the Macintosh.  Procedure calls
and returns only involved a limited number of instructions, and the O/S
knew how to restart them.  This gave us the ability to dynamically load and
unload code segments.  Since references to data segments could be done with
a variety of instructions, the Lisa O/S could not dynamically load data
segments.  (The Lisa also used the stack expansion technique described
earlier.) 

The Macintosh dynamically loads segments using a totally different scheme,
and cannot (easily) unload a segment is being used somewhere on the call stack.

-- 
Larry Rosenstein

Object Specialist
Apple Computer

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