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From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga,comp.sys.mac
Subject: Re: Mac Multitasking? Hee-hee!
Message-ID: <2235@cbmvax.UUCP>
Date: Wed, 19-Aug-87 13:30:34 EDT
Article-I.D.: cbmvax.2235
Posted: Wed Aug 19 13:30:34 1987
Date-Received: Sat, 22-Aug-87 02:22:08 EDT
References: <20164@ucbvax.BERKELEY.EDU>
Organization: Commodore Technology, West Chester, PA
Lines: 63
Xref: mnetor comp.sys.amiga:7559 comp.sys.mac:5722

in article <20164@ucbvax.BERKELEY.EDU>, oster@dewey.soe.berkeley.edu (David Phillip Oster) says:
> Amiga fans can help not by explaining why multi-tasking is good, but
> by explaining how the Amiga can do multi-tasking without suffering
> from the interactive response problems that plague Unix, VMS, and that
> plagued the Lisa.

That's a fair question.  First of all, we're not talking about a multi-tasking
system that swaps out stuff to disk.  I've personally run into the effect
you're talking about on our Apollo systems.  The Apollo systems here have
typically a megabyte or two of real memory, and they run very large programs.
The Amiga in my office, on the other hand, has seven megabytes of real memory,
and runs small and medium sized programs, all of which can share run-time
libraries.  Real memory at this point is nearly as cheap as mass storage, so
I'm very unlikely to need swaps out to disk.  That takes care of the largest
interactive response problem.

It's also a single user machine, not multiuser like a UN*X machine.  And the
OS does a few things based on this that might not be implemented in an OS
for multiple users like UN*X or VMS.  Task scheduling is based on fixed task
priorities.  Higher priority tasks always get all the CPU time they need.  Whatever
is left gets passed down to lower priority tasks.  So, providing you run your
interactive tasks at a higher priority than your background tasks, 
everything's cool.  Your interactive stuff is always going to run at full
speed while you're interacting with it, and your background tasks get all the
CPU time that's free when you stop to think about what to type next.  If you
run multiple interactive tasks, those that you're not interacting with just
go to sleep waiting for your attention once again; they take up no time while
just waiting for user input.  

How well this works, of course, somewhat dependent on programmer's using the
proper system calls for things.  If I have a task that wants to interact with
the Intuition system, let's say, the proper thing to do is set up intertask
signals, and then Wait() for them, something like Wait(sigregister & SIGBIT).
If I instead wrote while (! (sigregister & SIGBIT)); I would certainly waste
lots of available CPU time.  It sounds like similar clean waiting stuff is
being put into the Mac OS; there's no reason it shouldn't be able to benefit
from similar things.

> The Macintosh has always had event-driven multi-tasking, as opposed to
> time-slice. I have written many programs that run as background tasks.
> It is just that on the Mac, the interactive application must
> explicitly give up control (which it only does at times that are not
> critical to the perception of responsiveness.) All the major Macintosh
> applications do give up control to background tasks. To see this, run
> my "Menu Clock". If the colon between the hour and the minutes display
> blinks, then the application is giving up control to the clock.

I run my background clock just as any other program, only at a lower priority.
Whever it does something, like update it's display or jump to the front of the
window stack, it's doing it while my interactive program is waiting on 
something (usually me or the disk).  The responsiveness of the user interface
is also based on task priorities; the program I'm interacting with is higher
than the clock, and the Intuition and input programs that manage everyone's 
menus, keyboard, or mouse input are higher than the interactive program, so
you don't see the mouse get sluggish or miss keystrokes even when the system
starts getting loaded.

> --- David Phillip Oster            --My Good News: "I'm a perfectionist."
-- 
Dave Haynie     Commodore-Amiga    Usenet: {ihnp4|caip|rutgers}!cbmvax!daveh
"The A2000 Guy"                    PLINK : D-DAVE H             BIX   : hazy
     "I'd rather die while I'm living, than live while I'm dead"
						-Jimmy Buffett