Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!edcastle!lfcs!nick
From: nick@lfcs.ed.ac.uk (Nick Rothwell)
Newsgroups: comp.sys.mac.system
Subject: Re: Memory (de)fragmentation
Message-ID: <4372@castle.ed.ac.uk>
Date: 31 May 90 10:08:46 GMT
References: <16797@phoenix.Princeton.EDU> <JUH.90May28095602@hutcs.hut.fi> <JUH.90May29144638@hutcs.hut.fi> <1990May29.145842.18701@kth.se> <JUH.90May30163659@hutcs.hut.fi>
Reply-To: nick@lfcs.ed.ac.uk (Nick Rothwell)
Organization: Jenny Agutter Appreciation Society of Edinburgh
Lines: 32
In-reply-to: juh@cs.hut.fi (Juha Hyv|nen)

In article <JUH.90May30163659@hutcs.hut.fi>, juh@cs (Juha Hyv|nen) writes:
>Why can't (application) heaps move? Is it the same in all (operating)
>systems or is it just the way Apple choose to do?

Because the applications will have pointers to things in the heap, so
their addresses mustn't change. The whole point behind handles is that
stuff can be moved transparently under the application under certain
well-defined circumstances (when the application is required not to
hold onto dereferenced handles). Even so, there's a call to create a
*pointer* to a heap object, and handles can be locked, so the problem
is still there. Moving an entire application around is, I would
surmise, much more complicated.

I don't know of any systems which can move application data areas
in physical memory without using handles or somesuch. Moving code
is sometimes achievable since code can usually be made
position-independent and relocatable.

Virtual memory solves these problems, of course, since then the
fragmentation of the application's memory is virtual (sic), and
shouldn't cause physical fragmentation; things can be moved in
physical memory (or to/from disk) without the virtual addresses
changing.

This implies that VM makes the whole handles business obselete (yes?)

		Nick.
--
Nick Rothwell,	Laboratory for Foundations of Computer Science, Edinburgh.
		nick@lfcs.ed.ac.uk    <Atlantic Ocean>!mcsun!ukc!lfcs!nick
~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~
		   Ich weiss jetzt was kein Engel weiss