Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!njin!princeton!phoenix!mbkennel
From: mbkennel@phoenix.Princeton.EDU (Matthew B. Kennel)
Newsgroups: comp.sys.mac
Subject: Re: When will MacOS get virtual memory?
Message-ID: <3841@phoenix.Princeton.EDU>
Date: 5 Oct 88 00:19:12 GMT
References: <5624@zodiac.UUCP> <76000290@p.cs.uiuc.edu> <39513@yale-celray.yale.UUCP>
Reply-To: mbkennel@phoenix.Princeton.EDU (Matthew B. Kennel)
Distribution: na
Organization: Princeton University, NJ
Lines: 49

In article <39513@yale-celray.yale.UUCP> jellinghaus-robert@yale.UUCP writes:
>In article <76000290@p.cs.uiuc.edu> gillies@p.cs.uiuc.edu writes:
>>The sad thing is, the vast majority of Macintoshes in this world
>>cannot support virtual memory.  This is because the 68000 chip has a
>>design flaw that doesn't allow virtual memory to be implemented.
>
>Someone really should tell Sun or Apollo.  They've produced many workstations
>with 68000's running UNIX, which DOES support virtual memory and page swapping.
>Gosh, wonder how they do it?
>
With 68000s?  No, I don't think so.

I'm pretty sure they used to use 68010 & custom memory management chips
(sets) before the 68851 came out, and for the most part, they now use 68020
& 68851s.  I believe the 68000 has a problem with restarting instructions
after a trap, such as would occur on a page fault.

>>I guess if Apple really intended to support virtual memory soon in all
>>macintoshes, then it would have put a 68020 or at least a 68010 in the
>>Mac SE.  But since most machines (SE's, Plus's) have the 68000, these
>>machines will not have virtual memory any time soon.
>
>Virtual memory doesn't REQUIRE hardware memory management.  It's a lot easier
>if you have the PMMU, but it's not impossible to do on a 68000.  Apple, though,
>probably won't bother to write its new OS for anything less than a 68020/68851
>combo... gotta keep pushing into the future, ya know!
>

Short of interpreting every instruction, just how do _you_ propose to
do virtual memory without hardware memory management on a 68000?

Suppose I want a nice big chunk of memory to do some casual geophysical
modeling on my Mac 128k:  a = NewHandle(10000000L);

What do you do?


I guess, that in some sense, purgable resources are somewhat like "virtual
memory" (stuff stays on disk until needed), but there, you have to give an
explicit command to load it in before you use anything, and then an explicit
command to tell it that you're done with it.

>>Don Gillies, Dept. of Computer Science, University of Illinois
>(Sanity check... this guy's in the CS dept.  He may be a professor.  And I'm
>just a lowly undergrad... did I just shoot my mouth off?)
>
>Rob Jellinghaus                | "SINGAPORE?!?  You're supposed to be a

possibly.  But I'm just a lowly undergrad too.