Path: utzoo!attcan!uunet!husc6!bu-cs!mirror!frog!john
From: john@frog.UUCP (John Woods)
Newsgroups: comp.sys.mac
Subject: Re: 68030 MMU vs 68551 {was: Virtual Memory for the Mac II}
Message-ID: <1368@X.UUCP>
Date: 21 Jan 89 06:40:00 GMT
References: <1250@cascade.Stanford.EDU> <35421@think.UUCP> <79271@felix.UUCP> <7136@polyslo.CalPoly.EDU>
Organization: Servants of the Great White Frog
Lines: 39

In article <7136@polyslo.CalPoly.EDU>, dorourke@polyslo.CalPoly.EDU (David M. O'Rourke) writes:
>   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.

Ramifications, for the most part, are up to you, but here are the differences,
from the 68030 User's Manual:

"The following functions of the MC68851 are not available in the MC68030 MMU:

	* Access Levels		[hence, no MULTICS -ed]
	* Breakpoint Registers
	* Root Pointer Table
	* Aliases for Tasks
	* Lockable Entries in the ATC
	* ATC Entries Defined as Shared Globally

In addition, the following features of the MC68030 MMU differ from the
MC68020/MC68851 pair:

	* 22-Entry ATC
	* Reduced Instruction Set
	* Only Control-Alterable Addressing Modes Supported for MMU
	  Instructions

In general, the MC68030 is program compatible with the MC68020/MC68851
combination.  However, in a program for the MC68030, the following
instructions must be avoided or emulated in the exception routine for F-line
unimplemented instructions:  PVALID, PFLUSHR, PFLUSHS, PBcc, PDBcc, PScc,
PTRAPcc, PSAVE, PRESTORE, and PMOVE for unsupported registers (CAL, VAL, SCC,
BAD, BACx, DRP, and AC).  Additionally, the effective addressing modes
supported on the MC68851 that are not emulated by the MC68030 must be
simulated or avoided."
-- 
John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101
...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu

Presumably this means that it is vital to get the wrong answers quickly.
		Kernighan and Plauger, The Elements of Programming Style