Lockup in Read-Modify-Write with a PMMU

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2 total messages Started by Jim Wall Wed, 13 Aug 1986 20:59

#1007

Author: Jim Wall
Date: Wed, 13 Aug 1986 20:59

20 lines
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Is Motorola still on the net?

Probably, if I'm still at Fortune, they're still on the net,
somethings never change.

When you do a read-modify-write instruction on a systen that
uses a PMMU or the infamous MMB from Motorola the odds are taht
a table walk will be required, unfortunately, the bus request from
the PMMU will be ignored by the 68020 because the RMC is an 
indivisable cycle and so bus requests are ignored, the upshot is
that the system hangs. 

This is ture for the MMB, nothing I see will fix this for the PMMU
chip in its final form. Any one care to comment? I'm assuming this 
to be the case, and since I have no time to pursue it, software 
will work around it. THis is partially for peoples edification, and
partially to see if I'm understanding the situation correctly.

						-Jim Wall
						Fortune Systems Corp.

Re: Lockup in Read-Modify-Write with a PMMU

#1009

Author: Michael Cruess
Date: Fri, 15 Aug 1986 21:17

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Old versions of the MMB (manufactured more than about 2  years  ago)  would
lock  up.  Newer versions of the MMB do not.  The PMMU (68851) does not and
never has.  Please see *MC68851 Paged Memory Management Unit User's Manual*
Section 6.3.1.7 READ-MODIFY-WRITE (RMW) CYCLE.

Michael Cruess                                              oakhill!cruess
Motorola, Inc.
Microprocessor Products Group

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