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From: ralphw@ius2.cs.cmu.edu.UUCP
Newsgroups: comp.sys.apple
Subject: Re: Apple //e upgrade to //gs
Message-ID: <1060@ius2.cs.cmu.edu>
Date: Mon, 2-Mar-87 16:57:12 EST
Article-I.D.: ius2.1060
Posted: Mon Mar  2 16:57:12 1987
Date-Received: Wed, 4-Mar-87 20:22:39 EST
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Organization: Carnegie-Mellon University, CS/RI
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In article <8702251007.aa07502@SPARK.BRL.ARPA> hansen@cs.uiowa.edu (Kurt Hansen) writes:
>In regard to multitasking:
>
>I have looked into the problem from the micro and mini computer side of things
>and have come to the conclusion that for true multitasking you absolutely must
>have hardware protection between processes.

Not strictly true, as long as you are wiling to discipline yourself and
use reserve some of the capabilities of the processor or associated hardware.
For example, OS-9 level 1 for the Radio Shack Color Computer (based on the
Motorola 6809) is a multitasking system.

>.. the reason current micros don't have true multitasking is that they don't
>have hardware process protection.  The 68000 doesn't have it, although the 
>68010 and 68020 do.  The 6502 certainly doesn't, and thus the //gs in order
>to be at least reasonably compatible, doesn't either.

The Apple //e architecture supports bank-switching, so it is not normally 
possible to play with somebody else's program if it's in the other bank.
So a naive form of multitasking would involve setting up a timer interrupt
(say, every 60 seconds) and switching between programs in each bank.
[This would be most useful with the larger bank-switched cards like the
Applied Engineering or Checkmate Cards.]

The trick is knowing when to switch banks without leaving the machine in 
an inconsistent state.  If a user program tries to switch this it could
wedge the machine, so Apple recommends that you don't do it.

For the 65816, it will be more useful to use the segment capability to
provide some limited protection.  This is what allows the Minix operating
system (a multitasking operating system developed by Andy Tanenbaum)
to run on an IBM PC with NO hardware process protection.



-- 
					- Ralph W. Hyre, Jr.

Internet: ralphw@ius2.cs.cmu.edu    Phone:(412)268-{2847,3275} CMU-{BUGS,DARK}
Amateur Radio: N3FGW (c/o W3VC, CMU Radio Club, Pittsburgh, PA)
	(was KA3PLY, will probably change calls again if/when I get Advanced.)