Path: utzoo!attcan!uunet!lll-winken!ames!ncar!boulder!sunybcs!rutgers!apple!keith
From: keith@Apple.COM (Keith Rollin)
Newsgroups: comp.sys.mac
Subject: Re: 68030 MMU vs 68551 {was: Virtual Memory for the Mac II}
Message-ID: <24392@apple.Apple.COM>
Date: 20 Jan 89 06:29:02 GMT
References: <1250@cascade.Stanford.EDU> <35421@think.UUCP> <79271@felix.UUCP> <7136@polyslo.CalPoly.EDU>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 51

In article <7136@polyslo.CalPoly.EDU> dorourke@polyslo.CalPoly.EDU (David M. O'Rourke) writes:
>In article <79271@felix.UUCP> kehr@felix.UUCP (Shirley Kehr) writes:
>> different from that for the PMMU they are supplying for Mac II. She
>> said that Apple only implemented a subset of the instruction set on
>> the PMMU that comes with the 030 machines. (I barely know what I'm
>> talking about; please forgive the vagueness)
>
>  Not trying to flame you!!  The PMMU that's built into the 68030 is
>just a subset of the 68551.  It's not Apple's fault, it's motorola's.
>
>  Perhaps someone in the know would be nice enough to outline some of
>the differences, and the ramifications of those differences, between
>the 68030 the 68551.  I've always been curious about what the

I don't know about the ramifications, as I'm no virtual memory programmer, but
here are the differences:

68851 features not in 68030:
- Access levels
- Breakpoint registers
- Root table pointer
- Aliases for tasks
- Lockable entries in the ATC
- ATC entries defined as shared globally

68030 features that differ from 68851:
- 22-entry ATC
- Reduced instruction set
- Only control-alterable addressing moded supported for MMU instructions

The following instructions must be avoided or emulated on 68030:
- PVALID
- PFLUSHR
- PFLUSHS
- PBcc
- PDBcc
- PScc
- PTRAPcc
- PSAVE
- PRESTORE
- PMOVE for the unsupported registers (CAL, VAL, SCC, BAD, BACx, DRP, & AC)

Finally, there are some effective address modes in the 68851 not in 68030.



Keith Rollin  ---  Apple Computer, Inc.  ---  Developer Technical Support
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