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From: dyer@atari.UUCP
Newsgroups: comp.sys.atari.st
Subject: Re: ST hard
Message-ID: <858@atari.UUCP>
Date: Fri, 2-Oct-87 12:16:57 EDT
Article-I.D.: atari.858
Posted: Fri Oct  2 12:16:57 1987
Date-Received: Sun, 4-Oct-87 02:32:46 EDT
References: <8710011017.AA11188@ucbvax.Berkeley.EDU>
Distribution: world
Organization: Atari Corp., Sunnyvale CA
Lines: 28


> 4. The problem with the 68851 (?) MMU chip is the 50ns or so delay.

It is possible to do reasonable MMUs with *no* delays.  But you
have to be clever, and you have to do it right on the chip.  It's
hard to build chips in your garage....


> 6. What is called MMU in the ST is not what the rest of the computer world
>    call a MMU, whose functionnalities are needed to implement safe
>    multitasking (segment protection and translation).

Correct.  Atari US doesn't call it an MMU -- we call it a Memory
Controller Unit (MCU) -- but our people in Japan decided to
change its name on all the schematics.


> 7. It should not be very hard to insert a MMU between the 68000 and the
>    rest of the chips.

It is.  Very hard.  There are NO nanoseconds available -- even
one gate delay would kill memory access.

-- 
-Landon Dyer, Atari Corporation        {sun,amdcad,imagen,hoptoad}!atari!dyer
The views expressed here do not necessarily reflect those
of Atari or the AI software that has taken over my brain.
YOW! I am waiting for my warranty-expired interrupt!