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From: pshuang@ATHENA.MIT.EDU
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Unix on 486 Machines
Message-ID: <9106111507.AA29486@valhalla.MIT.EDU>
Date: 11 Jun 91 15:07:25 GMT
References: <9106110456.AA01606@pender.ee.upenn.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
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In e-mail, jer@pender.ee.upenn.edu asked on Tue, 11 Jun 91 00:56:29 EDT:

 > Is Task Switching means that the processor does Time Division
 > Multiplexing on the processes ? if so then how does it differs from
 > multi-tasking on Unix ?  Or does Task switching means that the user
 > can "stack up" TSRs in memory and in reality only one program is
 > running until the user "switches" to another program that was asleep

Strictly speaking all versions of multitasking on single processor
machines must perforce use some form of task switching.  The
clarification is that (unless you're talking to a bunch of system
programmers) usually the term "task switching" is reserved situations
where it is up the the end user to signal to the computer to start
running a different application (via a hotkey or by some other means)
which had been asleep until now, whereas "true multitasking" refers to
the case where the computer uses some periodic signal, usually the
system timer, to switch between applications for you so that they all
have a chance to do something... exactly what you refer to by time
division multiplexing.  It is not very different from how UNIX
multitasks, although the details of the implementation differ.

[There's also a level of multitasking "below" time-slicing usually
labeled "cooperative", where it is up to the application to call a
certain system function every so often to give up the CPU voluntarily.
This is the form of multitasking you find in Apple's Multifinder as well
as between Windows 3.0 programs (but not between Windows 3.0 and DOS
programs).  Its failing is that an application which is not properly
written or which crashes will never yield the CPU back to the system.]

You will find task switching in shareware products such as Back and
Forth and commercial products such as Software Carousel and Switch-It.
Desqview and Windows 3.0 can provide true multitasking given the right
hardware, or can provide task switching on lesser hardware.  OS/2 also
provides both, but in a more robust implementation (protects processes
from trashing each other and provides better interprocess communication
as UNIX does).  You can also get true UNIX for i386/i486 boxes.

Singing off,
UNIX:/etc/ping instantiated (Ping Huang).