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From: rang@cs.wisc.edu (Anton Rang)
Newsgroups: comp.sys.mac.system
Subject: Changes for preemptive multitasking (was Re: When will mac do...)
Message-ID: <RANG.91May7015648@nexus.cs.wisc.edu>
Date: 7 May 91 05:56:48 GMT
References: <1991May3.035637.4446@vax5.cit.cornell.edu> <811@newave.UUCP>
	<15931@helios.TAMU.EDU>
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Organization: UW-Madison CS department
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In-Reply-To: jeff@archone.tamu.edu's message of 7 May 91 04:11:12 GMT

In article <15931@helios.TAMU.EDU> jeff@archone.tamu.edu (Jeff Raymond) writes:
>Windows 3.0 can do it, the Amiga can do it, so why not the Mac?  I'm not
>an Apple engineer nor do I claim to know how they do what they do, but if
>this functionality can be grafted on top of DOS' aching bones (across
>multiple platforms), certainly it can be done on the Mac as well.

  It can be done, though without a 68020 and PMMU it's may not be
worth the effort (protected memory and separate address spaces really
help here).  The big problem, at least as I see it, is that the
current libraries aren't re-entrant.  Some of these things would be
easy to change; others wouldn't.

  For instance, take QuickDraw.  Suppose that process A is busily
doing a DrawPicture.  What happens when process B is context switched
in and tries to update its window which was moved earlier?  I suspect
there are parts of QuickDraw which expect various global variables and
data structures to be in consistent states, and that you'd have a
problem.  (What if you're in the process of creating a new window and
it's not fully linked into the window list when a new process tries to
create one, too?)

  I don't think the file manager currently processes more than one
request either, though that isn't as serious a problem (especially
when asynchronous I/O can be used) since it at least queues multiple
requests for service.

  Some parts of the system which have been introduced for MultiFinder
will help quite a bit when preemptive multitasking finally comes (and
I do believe it will, though I'll make no predictions on dates).  The
notification manager, for instance, is a great tool even with MF.

  Programs which depend on global variables may have to be changed,
though a lot of the functionality could be emulated by using the MMU
to trap accesses to global variable addresses and emulate them (at
some cost in speed).  Programs which use the managers should work
fine, since the *internals* can be changed to allow for pre-emptive
multitasking without changing the interface.  (At least, I haven't
come up with any obvious cases where it can't be done yet.)

  It's possible to do.  It'll be expensive and take a long time.  I'd
hope that Apple's already working on this stuff, and that their newer
code (e.g. System 7) is written with it in mind....

	Anton
   
+---------------------------+------------------+-------------+----------------+
| Anton Rang (grad student) | rang@cs.wisc.edu | UW--Madison | "VMS Forever!" |
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