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From: wagner@utcs.uucp
Newsgroups: net.micro.amiga
Subject: Re: How to prevent system death from errors in user code?
Message-ID: <1986Jun6.10:29:48.20091@utcs.uucp>
Date: Fri, 6-Jun-86 10:29:48 EDT
Article-I.D.: utcs.1986Jun6.10:29:48.20091
Posted: Fri Jun  6 10:29:48 1986
Date-Received: Fri, 6-Jun-86 10:43:39 EDT
References: <8605310909.AA00952@pavepaws> <1265@utcs.uucp> <281@chronon.chronon.UUCP>
Reply-To: wagner@utcs.UUCP (Michael Wagner)
Organization: University of Toronto Computing Services, general purpose UNIX
Lines: 53

Both this and a previous article are reposts, due to some software
problems.  Sorry for those who see both sets before the cancel messages
propegate.

In article <281@chronon.chronon.UUCP> eric@chronon.UUCP (Eric Black) writes:
>In article <1265@utcs.uucp> wagner@utcs.UUCP (Michael Wagner) writes:
   [ I wrote that MMUs aren't the only way to deal with memory 
	protection.  I listed other memory protection schemes that
	did not require memory mapping ]
>
>Explain to me how these techniques are not "Memory Management".  Reminder:
>MMU stands for "Memory Management Unit", NOT "Memory Mapping Unit".
>You are confused by the fact that the current spate of single-chip
>solutions to some memory management problems all seem to have logical
>address mapping (whether virtual or not).  Ignoring the advantages
>that address mapping will give (e.g. easier implementation of shared
>code!), the protection capability of an MMU does not stem primarily
>from allowing or not allowing addressability; rather, it is the
>ability to permit or deny particular types of access to specific ranges
>of addresses (whether those addresses get mapped before going to the
>RAM chips or not).
>
>Sound like what you were talking about?  Do you think that mainframes
>which implemented memory keys and the like did it "on-chip"?
>-- 
>Eric Black   "Garbage In, Gospel Out"
>UUCP:        {sun,pyramid,hplabs,amdcad}!chronon!eric


I think we are basically agreed, but are having a wording problem.
It is true that MMU stands for Memory Management Unit.  In a similar
vein, an air conditioner is anything that conditions the air (including,
presumably, my humidifier, and the heater currently sitting disconnected
in the closet), but common convention takes it to mean a cooling device.
I fell into the sloppyness of refering to one of the 'current spate'
of memory mapping units.

My point was that, if they thought a memory mapper was too expensive,
less expensive solutions (that offer less functionality) exist.  
I listed a few.  I think Eric and I agree there.

Incidentally, much of the support in mainframes for memory keys was
in the CPU ('on-chip' is a litle difficult to parse in that context,
since the CPU was typically a whole board of chips).  Every memory fetch
also sent a key to the CPU (so the data bus was wider).  The CPU compared
the data key with the current running key.  In the case of the Amiga, one
could consider the 68000 having two keys, Supervisor and Problem Program,
and external logic for checking.  There are, of course, many other ways
of doing this too.  After all this discussion, it's still sad that
Amiga didn't implement any of the many possibilities.

Michael
(wagner@utcs)