Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!tnc!m0154
From: m0154@tnc.UUCP (GUY GARNETT)
Newsgroups: comp.sys.amiga.introduction
Subject: Amiga Multitasking
Message-ID: <663@tnc.UUCP>
Date: 11 Jan 91 15:22:31 GMT
Reply-To: m0154@tnc.UUCP (GUY GARNETT)
Organization: The Next Challenge, Fairfax, Va.
Lines: 46


In comp.sys.amiga, there was a question about Amiga multitasking; I
thought I would answer it here ...

Multitasking operating systems can be broken down into two broad
categories, "preemptive" and "cooperative".  The Amiga, as well as
OS/2, Unix, and most mainframes use preemptive multitasking. 
Multifinder for the Mac, and some other computers, use cooperative
multitasking.

Preemptive multitasking is particularly well suited for doing several
things at once (which is why it is used so often); some have been
known to call it "true" multitasking.  Cooperative multitasking is
also valid multitasking, but it is less generally usefull (anything
which can be done on a cooperative multitasking system can also be
done with a preemptive system, in theory, but the reverse is not
true).

Programs on the Amiga have a time limit, or "slice"; when the time
expires, Exec (a part of the Amiga operating system) takes control
away from the running program, and gives it to the next eligible
program.  The time slice is so short that it appears that many
programs are running at once.  Programs can also give up control
volunatrily; usually this happens when the program is waiting for
something, like disk drives, user input, or other "real world" events.
The program in effect says "I have noting to do until someone moves
the mouse (or whatever)", and so Exec starts up another program.  This
helps make efficient use of the computer.

Unlike many preemptive multitasking operating systems, the Amiga has
"real-time response".  This means that external events can be detected
and responded to quickly ("quickly" is a relative term; the Amiga can
respond much faster than most other multitasking systems, although
there are several which are designed to respone even faster still). 
For example, the system can be set up so that a certian program runs
between every frame of video (this is how some programs manage color
cycling).

This post was designed to be a general overview ... more detailed
technical questions can go to comp.sys.amiga.programmer (I have
omitted or glossed over a huge number of things) and discussions of
which is better, Mac, Amiga, Unix, OS/9, OS/2, or whatever can go to
comp.sys.amiga.advocacy.  If anyone has general questions, I'll be
glad to continue the discussion here.

Wildstar