Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!amgraf!huver
From: huver@amgraf.UUCP (Huver)
Newsgroups: comp.sys.amiga.hardware
Subject: RE: 68030 in a Lucas and MMU stuff
Message-ID: <365@amgraf.UUCP>
Date: 22 Dec 90 05:58:52 GMT
Organization: Amgraf Inc.,  Kansas City
Lines: 42


Putting a 68030 into an adaptor socket, onto LUCAS '020 socket IMMEDIATELY
gives you a free MMU (albeit a scaled-down, read on).  Other than this,
there is nothing to be gained in any form (you must disable 030 data
caching, unless of cource you'll add your own cache memory design).

The 68851 MMU has built-in chip-select logic, so it is much more straight-
forward a hookup than 68881/2.  However, as Dave Haynie pointed out, with
'851 in place, the system looks like:

	CPU <-+-> MMU <---> outside world
	      |
	     FPU

Even with MMU disabled (become a pass-through), law of physics says you will
see some timing difference; it is much worse if MMU is enabled, for address
translation obviously takes some time.

The MMU in 68030 is a subset of '851.  So if you're into MMU hacking and
value the capacity (AmigaDOS doesn't), you might want to fool with '851.
Make an adaptor board that holds both CPU and MMU, then wire MMU outputs
back to LUCAS CPU socket.  But prepare to witness malfunctioning due to
signal timing skew.

If you use FRANCES, you have no need of an MMU.  FRANCES provides Kickstart
in its space (via copying from WCS) which is exactly what SetCPU does anyway.
And with heavy real-time nature of Amiga tasks, I suspect you won't see a
virtual memory paging AmigaDOS for quite some time to come.  By then you'll
probably have saved up for a new 68040 Amiga?

>Which leads me to ANOTHER question: Can the 68851 be hooked up to a 68010 
>(my current processor) with ease, and if so would something like SetCpu be 
>able to detect and make use of it, despite the fact that an '020 or '030 is 
>not present in the system?

As said, '851 was designed for 68020.  For 68010, Motorola 68451 could be
used (don't know if they still make it).  But it's a whole different ball
game there, you'll have to commit your own heavy hacking to make it work;
certainly not worth thinking about if you value your hair.


-huver   ...!uunet!amgraf!huver