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From: john@anasazi.UUCP (John Moore)
Newsgroups: net.micro.amiga
Subject: Re: MMUs and program correctness
Message-ID: <372@anasazi.UUCP>
Date: Fri, 15-Nov-85 10:51:29 EST
Article-I.D.: anasazi.372
Posted: Fri Nov 15 10:51:29 1985
Date-Received: Mon, 18-Nov-85 07:19:21 EST
References: <862@nmtvax.UUCP>
Reply-To: john@anasazi.UUCP (John Moore)
Distribution: net
Organization: Anasazi, Phoenix Az.
Lines: 35

In article <862@nmtvax.UUCP> pickle@nmtvax.UUCP writes:
>neil@amiga.UUCP from Commodore-Amiga Inc. writes:
>>Multitasking does NOT require an MMU.  Processes are allocated separate
>>places in memory, and then are timesliced (the same way one would if one
>>did have an mmu...).  There is no protection between different processes.
>>One could write a program that trashes memory, and therefore the system
>>(hence the infamous "GURU MEDITATION...").
>>Correctly written programs run without any problems, however.
>
>    From a development point of view, this scheme is really bad: nothing
>would frost my cookies more than having to reboot the system because
>I looked at a bad pointer and a program I was writing decided to munch
>some chunk of memory.  What makes it worse than other micros?  Imagine

	Let me describe my experience with a very similar situation. I
run Venix (a licensed UNIX) on a PC/AT in 8086 (unprotected) mode. For
the last two days I fought a problem where a complex application involving
multiple processes was tubing the whole system. Makes it hard to find a
problem when the system develops amnesia immediately after the problem
occurs. Even worse, one runs the risk of trashing the disk (happened
once so far). This is on a machine with LIMITTED memory protection -
most wild memory stores will fall within 64k of the start of a valid
segment, rather than into the monitor.
	PLEAZE - next time someone brings out a machine with 32 bit
addressability, PLEASE put in an MMU. Then mere mortals can write
and debug programs under Unix or other multitasking OS's, without
"correctly written programs." Have you ever seen a significant
program correctly written the first time???

-- 
John Moore (NJ7E/XE1HDO)
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