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From: papa@pollux.usc.edu (Marco Papa)
Newsgroups: comp.sys.amiga
Subject: Re: Multi-tasking and OS books
Message-ID: <17207@usc.edu>
Date: 13 May 89 22:28:41 GMT
References: <17148@usc.edu> <24279@agate.BERKELEY.EDU> <18268@cup.portal.com> <17183@usc.edu> <11336@polyslo.CalPoly.EDU> <17195@usc.edu> <11373@polyslo.CalPoly.EDU>
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Reply-To: papa@pollux.usc.edu (Marco Papa)
Organization: Felsina Software, Los Angeles, CA
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In article <11373@polyslo.CalPoly.EDU> dorourke@polyslo.CalPoly.EDU (David M. O'Rourke) writes:
>  I believe there's a misunderstanding, M-Finder switchs on several calls and
>"Get Next Event & SystemTask", both GNE & SysTask are called several times
>by an "Macintosh" program.  There are calls specific to MF that do a better
>job of Task switching, but they don't "require" you to use them for
>multi-tasking to occur.  It's very hard to write a mac program that doesn't
>make a OS call.

But that is the whole point about program switching: on the Mac you HAVE to do 
a Dos call. Take any compute-bound program like a ray-tracer.  While it is
computing the rays there are NO system calls made.  On the Mac this program is
uninterruptible, unless you decide to SLOW down the system by injecting bogus
system calls within the computation.  Another problem happens with 
communication programs at high speeds.  When using multiple programs with
MultiFinder switching to another program can be deadly for a download. Not so
on a "real" multi-tasking OS like AmigaDOS or OS/2.

>  BTW:  I've done some checking on OS/2, are you 100% sure it doesn't task
>switch on OS call.  It's suppose to run on older 80286's which I'm sure don't
>have the hardware to do it pre-emptive.  Perhaps a calm, cool and collected
>usenet person could set the record straight.

You must be kidding. What do you think XENIX runs on? 80286s! And don't tell me
that UNIX is not preemptive.  Notice that UNIX was also implemented on 8088
machines: PC/IX run on an IBM XT.  You seem to confuse memory protection and
supervisor mode. You don't need memory protection to implement a pre-emptive 
OS; you need only supervisor mode. If you are confused about OS/2, go to your 
local Waldenbook or Dalton store: they have a miriad of OS/2 books that will
show you that OS/2 is a "preemptive OS" that supports tasks, processes and
threads (in fact from a black box point of view the multi-tasking 
capabilities of the Amiga are very similar to the ones offered by OS/2: not
a surprise since both OSs use concepts very well understood in the late 70s;
something Apple sems to have not had the time to grasp :-).

-- Marco Papa 'Doc'
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