Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!mintaka!wookumz.ai.mit.edu!rjc
From: rjc@wookumz.ai.mit.edu (Ray Cromwell)
Newsgroups: comp.sys.amiga.introduction
Subject: Re: Amiga Multitasking
Message-ID: <1991Jan12.040249.1510@mintaka.lcs.mit.edu>
Date: 12 Jan 91 04:02:49 GMT
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Sender: daemon@mintaka.lcs.mit.edu (Lucifer Maleficius)
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  Lets also add to this:

   The Amiga, since it was designed to multitask from the beginning allows
tasks to communicate between one another using Signals, MsgPorts and
Semaphores.  At the lowest level are Signals, which allow tasks to
signal each other and wait for certain signals. (and while waiting,
other tasks are allowed to run making effective use of CPU time.) When a 
task receives a signal it 'Wakes up' and starts running. Using signals,
the Amiga supports Message Ports. These are like signals in that, tasks
may wait for them. Messages offer more however. Messages can contain data, so
tasks can 'share' information instead of holding multiple copies of data, or
reading it from disk. Semaphores are like signals and message ports. They
are primarily used to share public data effectively. Semaphores can
be like auctions. Tasks can 'bid' for the ability to own some data
for a certain period of time and guarantee that another task won't be
trying to access that data while your task is using it. 

 The Amiga's DOS system has another kind of communication called Packets.
Packets are like Message ports, and are mainly concerned with asking
'handlers' to do something, like open a file for you, and read some data.

 I am not an expert on the Mac, so I don't know how the Mac
shares things like Interupts, Devices, Memory, etc.  The main problem
I see for the Mac is, most of the software not written explicitly
for Multifinder won't cooperate enough to make it usuable as a multitasking
system. And even if it does, the MacOS is still at the core, and
Multifinder is just a front-end. (Just like Windows runs on top of MS-DOS)

  To me, true multitasking means multitasking from the beginning.
OS/2 multitasks from the beginning, so does Unix, and fortunately
so does AmigaOS. 

 The Amiga faces a challenge of supporting new video devices in a friendly
manner. (Retargetable Graphics) Since the Amiga didn't have this from the
beginning, it will be hard to make existing software work on newer 
graphic devices. The Mac ironicly got this part right with QuickDraw
(I'm willing to bet, QuickDraw wasn't written to handle multiple tasks
updating the display however.).
 My opinion of course, is I think multitasking is more important than
supporting different display devices.

One last note about multitasking. Systems which support multiple users
usually implement something called 'Aged Multitasking.' This prevents one
user from being stingy and steal lots of processor time from other users. 
Imagine a low priority task like printing the current date periodically, and
a high priority task like a Ray-Tracer. On the Amiga (a single user system)
the Ray Tracer can steal lots of CPU time from a low priority task, even
though the low task doesn't want that much time (since it only runs
periodically.) On 'aged' systems, the OS keeps track of how long
each task runs. If a task gets starved too much, it gives a little extra
CPU time to it. This has a disadvantage in that it can prevent real time
response, and has even more overhead involved in task switching.