Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!ctrsol!srcsip!manyjars!mnkonar From: mnkonar@manyjars.SRC.Honeywell.COM (Murat N. Konar) Newsgroups: comp.sys.mac Subject: Re: PMMU... Message-ID: <32679@srcsip.UUCP> Date: 26 Sep 89 22:18:42 GMT References: <822@rodan.acs.syr.edu> <13822@well.UUCP> Reply-To: mnkonar@src.honeywell.com (Murat N. Konar) Distribution: usa Organization: ipd Lines: 40 In article <13822@well.UUCP> nagle@well.UUCP (John Nagle) writes: [discussion of 680x0 and PMMUs deleted] > The MC68020 does not have a memory management unit either, but >like the 68010, can be provided with an external one. This is the >"PMMU" (Paged Memory Management Unit) used in the Mac II. It's not >standard on a Mac II, but is an expensive extra-cost option. On >standard Mac IIs, the MMU socket is filled with a chip which is more >of a dummy plug than a component; it just passes the addresses through >unchanged. The PMMU costs about $350 dollars. Whether you consider this expensive in view of what it will get is up to you. > 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. The 030 MMU implements a subset of the 68851 PMMU (the one you can use with the 68020). So software written to take advantage of the 030's MMU should work ok on a 020/851 combo. > Unfortunately, none of this excellent memory management hardware >does you any good unless you run A/UX. The regular Mac operating system >just turns any memory management unit off and runs in "real mode". Even >release 7, much to the disappointment of people who don't like system >crashes, doesn't use the MMU. On a machine with memory management and >a solid operating system, an application can't crash the system no matter >what it does. Essentially all workstation-class machines (Suns, Apollos, [etc] Sorry dude, you almost got it but not quite. System 7 implements virtual memory which REQUIRES a PMMU (either the 030's own or the 851 for the 020). What System 7 does not implement is protected memory where the MMU is used to prevent code in one address space from writing directly to another address space. ____________________________________________________________________ Have a day. :^| Murat N. Konar Honeywell Systems & Research Center, Camden, MN mnkonar@SRC.honeywell.com (internet) {umn-cs,ems,bthpyd}!srcsip!mnkonar(UUCP)