Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!rutgers!cbmvax!valentin
From: valentin@cbmvax.commodore.com (Valentin Pepelea)
Newsgroups: comp.sys.mac.system
Subject: Re: PPC, IAC, and True Multitasking (tm)
Message-ID: <13888@cbmvax.commodore.com>
Date: 20 Aug 90 07:02:17 GMT
References: <1990Aug3.040513.14844@d.cs.okstate.edu> <2760@bridge2.ESD.3Com.COM>
Reply-To: valentin@cbmvax (Valentin Pepelea)
Organization: Commodore, West Chester, PA
Lines: 53

In article <2760@bridge2.ESD.3Com.COM> ngg@bridge2.ESD.3Com.COM (Norman
Goodger) writes:
>
>	Despite the -:)'s, I say there is no such thing as "True 
>	MultiTasking", you either have "pre-emptive" or "cooperative"
>	MultiTasking. Take your pick...

Actually, you either have pre-emptive multitasking, or you have nothing. Taking
it your way, my Apple II has cooperative multitasking since it allows me to
load two specially written executables at two different locations, which then
branch into each other when they have completed part of their task.

Of course the Mac has much more built-in support for such a cooperative effort,
but multitasking is not a feature which can be quantitized. Either you have it,
or you don't. An operating system has a pre-emptive scheduler, or it doesn't
have one at all. There is no such thing as a cooperative scheduler. The term
cooperative is used when there is an absence of a scheduler.

> Pre_emptive Multitasking I suspect will slow down performance
> of current Mac's to much to be acceptable.

You misunderstand the concepts behind (pre-emptive) multitasking. The idea is
to have the kernel interrupt the task in progress if another task of a higher
priority is ready to run or if the quantum of the current one is over. If there
is no other other task ready to run, task switching never occurrs.

Therefore, whether each task is responsible for lanching cooperatively other
tasks, or whether the operating system does that, has nothing to do with the
speed at which these tasks will run. Factors which could affect the speed are

1. the size of the time slice (quantum)
2. overhead of the dispatcher
3. the memory model (independent or common address spaces, memory protection,
   virtual memory, etc.)

If anything, a preemptive operating system will be faster, because it will
perform task switching only when necessary. The preemptive operating system
will also allow the user to set task priorities. Thus if the user wants task B
to run only when task A is waiting for an event to complete, all it has to do
is lower its priority below that of task A.

Such is the nature and purpose of preemtive schedulers.

> Lets stamp out the True-MultiTasking rumor...there is no such thing.

Thank you for your support.

Valentin
-- 
The Goddess of democracy? "The tyrants     Name:    Valentin Pepelea
may distroy a statue,  but they cannot     Phone:   (215) 431-9327
kill a god."                               UseNet:  cbmvax!valentin@uunet.uu.net
             - Ancient Chinese Proverb     Claimer: I not Commodore spokesman be