Can an 030 need a PMMU?

[Brian H. Powell]

In article <10395 at dartvax.Dartmouth.EDU>, 63848e at eleazar.dartmouth.edu (Jeffrey Buchsbaum) writes:

> I called Dove Computer today to ask about an '030 board ugrade for the II.
> They told me that I would still need to buy a 68851 chip(PMMU) with their
> 030. Is is possible that an 030 can still need a PMMU?

> Their 030 board clips into the 020 sockets, to clarify any confusion,
> and comes with inits and stuff to let the caches work.

     I'm not much of a hardware authority, so use salt grains when reading
this.  It sounds from your description that their board has a socket for a
68851 on it.  If you don't have a PMMU, you usually have a dummy chip sitting
in the slot to pass through the proper logical and physical lines, etc.
     Unfortunately, the signals that the dummy chip needs to handle differ
between a 68020 and a 68030.  If you replace the dummy chip with a real 68851,
the PMMU part of the 68030 works correctly, even though the 68851 is
effectively ignored.  Quoting from a 68030 manual (page 12-3):

     When used in a system originally designed for both an MC68020 and an
MC68851, the MC68851 may be left in the system or removed (and replaced with a
jumpered header).  However, if left in the system, the MC68851 is not
accessible to the programmer with the M68000 coprocessor interface.  All MMU
instructions access the MC68030's on-chip MMU.  This is true even if the
MC68030's MMUDIS signal is asserted.  The benefit in removing the MC68851 is
that the minimum asynchronous bus cycle time to the physical bus is reduced
from four clock cycles to three.
     If the MC68851 is removed and replaced with a jumpered header, the
following MC68851 signals may need special system-specific consideration:
[list omitted]...  During translation table searches the MC68851 asserts the
CLI signal but not RMC, while the MC68030 asserts RMC but not CIOUT.
     [etc...]


Brian H. Powell					National Instruments Corp.
	brian at natinst.uucp			12109 Technology Blvd.
	cs.utexas.edu!natinst!brian		Austin, Texas 78727-6204
	AppleLink:D0351				(512) 250-9119 x832