Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: MMR and MMU questions
Message-ID: <21983@cbmvax.commodore.com>
Date: 28 May 91 20:54:10 GMT
References: <1991May23.194235.5616@mnemosyne.cs.du.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Distribution: usa
Organization: Commodore, West Chester, PA
Lines: 44

In article <1991May23.194235.5616@mnemosyne.cs.du.edu> echadez@isis.cs.du.edu (Edward Vincent Chadez) writes:

>I have some rudimentary questions regarding the Mega-Midget Racer (from
>CSA).  Even more basic are my questions on the MMU.  First of all,
>what function does the MMU accomplish?  

An MMU provides a programmable mapping function between a CPU and main memory.
This is used to set up translations between a processor's memory address (the
virtual address) and the hardware's memory address (the physical address).  
Along with a translation, various protections can be applied.  This is most
often used to support virtual and protected memory systems.

>I realize that to use Dave's SetCPU program, an MMU is required.  

To use all the SetCPU function, you need an MMU.  SetCPU will still do CPU IDs
and other things without it, but it won't let you run alternate KickStarts.

>Which brings me to my second question, what __exactly__ does SetCPU do??  I 
>know only of the feature which allows the operating system to be moved to 
>32-bit ram.  

SetCPU sets up the MMU, if available, to translate a chunk of memory to ROM
space.  This lets it load either a copy of your ROM, or a disk image, into
RAM and use it as ROM.  This lets the user run with a faster ROM on full 32 bit
machines and also run different version of the ROMed OS.

>Finally, is static ram required to perform the latter function??

No.

>And finally, what risks would I be taking trying to run a 16Mhz MC68881
>as a coprocessor to a 20Mhz or 33Mhz MC68030??  

As long as you have a 16MHz clock to go with it, it should work.  If you're
clocking a 16MHz 68881 with a faster clock, don't expect it to work.  If the
FPU interface is done right, it should work with a CPU going as much as twice
as fast as the FPU clock, according to what I recall from Motorola's notes on
the subject.


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
      "That's me in the corner, that's me in the spotlight" -R.E.M.