Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!cbmvax!daveh From: daveh@cbmvax.UUCP (Dave Haynie) Newsgroups: comp.sys.mac Subject: Re: PMMU... Message-ID: <8028@cbmvax.UUCP> Date: 28 Sep 89 18:08:41 GMT References: <13822@well.UUCP> Distribution: usa Organization: Commodore Technology, West Chester, PA Lines: 28 in article <13822@well.UUCP>, nagle@well.UUCP (John Nagle) says: > In article <822@rodan.acs.syr.edu> jstewart@rodan.acs.syr.edu (Ace Stewart [Jonathan III]) writes: > The MC68030 has a built-in MMU comparable to (but not > compatible with) the PMMU. So all '030-based machines are in theory > capable of running a virtual memory operating system with protected > memory. Actually, the 68030 Internal PMMU is very similar to the external 68851 PMMU that can be used with the 68020 in machines like the Mac II. There are a few additional addressing modes for some instructions in the '851, there's no support for CALLM/RTM in the '030's MMU (or the '030 for that matter), and the '030 adds two "transparent translation" registers, which support simple mapping of I/O regions in a virtual system. The 68851 also has a larger Address Translation Cache (64 vs. 22 entries). So, while there are slight differences, it's quite simple to write code that works on both MMUs just fine. If you're looking for a major change in MMU architecture, the 68040 will be along someday soon... > John Nagle -- Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Too much of everything is just enough