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From: john@anasazi.UUCP (John Moore)
Newsgroups: net.micro.amiga
Subject: Re: Memory Management w/o MMU"
Message-ID: <382@anasazi.UUCP>
Date: Fri, 29-Nov-85 11:19:45 EST
Article-I.D.: anasazi.382
Posted: Fri Nov 29 11:19:45 1985
Date-Received: Thu, 5-Dec-85 06:33:31 EST
References: <370@caip.RUTGERS.EDU> <371@sdcc13.UUCP>
Reply-To: john@anasazi.UUCP (John Moore)
Organization: Anasazi, Phoenix Az.
Lines: 41

In article <371@sdcc13.UUCP> ec150fcy@sdcc13.UUCP (Joseph McCaughey{|stu) writes:
>You know, I am getting sick of all these 'how do you do
>multi-tasking without an MMU' questions.
>
>GET THIS THRU YOUR HEADS OR TAKE AN OPERATING SYSTEMS CLASS:
>
>THE BARE MINIMUM NEEDED FOR MULTITASKING ARE PROCESSOR
>INTERRUPTS OCCURING AT INTERVALS, AND AN INTERRUPT HANDLER.
>
>
>Larry J. McCaughey
Woah... not so fast there. The bare minimum needed for multitasking
is a computer. Period. I have written may real-time systems where
the multitasking was performed by simply calling the tasks round
robin style. What you are talking about is the bare minimum for
a TIME-SLICED multitasker.

However, the point about MMU's is not that you can't do multitasking
without one, but that for general multipurpose systems (such as UNIX),
it is bad news to do it without MMU's or some form of memory protection.

The system here is an IBM PC/AT running UNIX in non-protected mode. The
result sometimes is nasty system crashes during applications development
in C. All it takes is one blown pointer and you are in potential trouble.
On the PC, you have much better memory protection on a 68000 (assuming
small or medium model C). A pointer of zero stomps the start of your
data area rather than the start of the Operating System. Since those
pointers are 16 bit, they can only mash the 64K of memory in the
data segment (which may unfortunately also contain other tasks and
code spaces). This offers a little protection. The idea of doing
this in a 68000 makes my skin crawl. However, I'll probably shortly
end up with an Amiga or a 520-St pretty soon, and just have to live
with it - grumpily.

-- 
John Moore (NJ7E/XE1HDO)
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