Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!husc6!encore!zelig.encore.com
From: jdarcy@zelig.encore.com (Mostly Harmless)
Newsgroups: comp.arch
Subject: Re: Macintosh OS (was: 68000 and Workstations.)
Message-ID: <11964@encore.Encore.COM>
Date: 25 May 90 17:41:07 GMT
References: <37@voa3.UUCP>
Sender: news@Encore.COM
Lines: 55

ck@voa3.UUCP (Chris Kern):
> For those of use who don't have any experience with the Mac OS, could
> someone explain what its deficiencies are with respect to multitasking?
> Is it that the OS doesn't do time-slicing?  Does a context switch have
> to wait for some event at the application level?  How is multitasking
> implemented?

Apple's term for it is "cooperative multitasking".  Most Mac applications
spend the bulk of their time in an event loop, in which they repeatedly
call GetNextEvent() until something happens and then they branch all over
the place according to the event type.  What MultiFinder basically does
is steal control from the running application on the third unsuccessful
return from GetNextEvent.  Thus you are correct: context switching is in
fact done only by the (implied) consent of the current program.  A program
that gets stuck in a tight infinite loop and that never calls GetNextEvent
will hang the system.

Most people are probably aware that Desk Accessories aren't multitasking
either.  Basically they're implemented as drivers and they get events by
an unusual method.  When the foreground application gets an event it has
to check which window the event occurred in; if that window is not one
of its own it explicitly passes the event to a Toolbox routine, which in
turn passes it on to the appropriate desk accessory.  It is trivial for
an application to "steal" desk accessory events by simply not bothering
to pass them on, though this is considered "antisocial".

In fact, there is a mechanism by which a driver (and hence a DA) can
arrange to be run every N ticks (1/60 second), but this also depends on
the application calling the Toolbox periodically, at which point the
countdown timer is checked and drivers are awakened if >N ticks have
elapsed.

There are two other ways in which the Mac OS fails to meet what most
consider minimum requirements for true multitasking.  One is memory
management.  The Mac OS basically doesn't do address translation and
everything runs in the MC68K's Supervisor mode.  This means that an
errant application not only *can* but usually *will* overwrite memory
that doesn't belong to it.  Of course, the toolbox itself is in ROM,
but all system globals are vulnerable, as are other applications.

Lastly, there's the lack of IPC.  This is being addressed by Apple's
Inter-Application Communication facilities (about which I know very
little since they were barely around when I stopped doing Mac stuff),
but I don't see anything comparable to pipes, sockets, mailboxes,
message queues or even signals.  Any communication between entities
under the Mac OS is basically done on a "roll-your-own" basis.

I hope this helps people understand the claims about the Mac OS's status
as a platform for multitasking.  If it seems that I've gone into undue
detail I apologize.



Jeff d'Arcy, Generic Software Engineer - jdarcy@encore.com
      Nothing was ever achieved by accepting reality