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From: philip@pescadero.stanford.edu (Philip Machanick)
Newsgroups: comp.sys.mac.hardware
Subject: Re: System 7.0 and RAM
Message-ID: <1991Apr25.212632.22379@neon.Stanford.EDU>
Date: 25 Apr 91 21:26:32 GMT
Article-I.D.: neon.1991Apr25.212632.22379
References: <ALAND.91Apr24115728@chaos.cs.brandeis.edu> <1991Apr25.190940.15729@cbnewsk.att.com>
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In article <1991Apr25.190940.15729@cbnewsk.att.com> ech@cbnewsk.att.com
(ned.horvath) writes:
>In article <1991Apr22.042204.1291@lynx.CS.ORST.EDU> hasses@prism.cs.orst.edu
(Stephen Haase) writes:
>
>>One VERY annoying thing I noticed with system 7's virtual memory was that x
>>amount of HD space does not equal x amount of extra RAM. I got 7.0fc2 going
and
>>then tried to add 3 megs of RAM to my 5 physical RAM. It worked all right,
but
>>it took 8 megs of HD space to give me those extra 3 megs. Needless to say, I
>>removed it quickly since I only have 9 megs free anyways. 
>
>From article <ALAND.91Apr24115728@chaos.cs.brandeis.edu>, by
aland@chaos.cs.brandeis.edu (Alan D.):
>
>> Very annoying, but quite necessary.  You need to have some place to
>> swap OUT the memory you're replacing with the new memory...  And
>> therefore you need as much free space on the HD as you want in virtual
>> memory, notwithstanding the amount of actual RAM you have.
>
>"Convenient for the Systems Programmer," perhaps.  Necessary, no.  The VM
>implementation has to track which pages are in physical memory in any case.
>But if you add a table of ALL pages, with either a RAM address OR a disk
>address for each page, then you need only allocate disk space to
>accommodate the difference between virtual and real, plus one "free" page
>to swap out to.
[...]
There are other reasons to keep the whole of VM on disk. If you get a total
system crash, it's nice to be able to see a reasonably recent snapshot
of system state. If you want to migrate a process on a distributed
architecture, it's relatively easy.
>I think it's important to notice that this is the first VM implementation
>for the Mac, not the last.  A number of us are upset that VM provides no
>additional protection.  The official answer was that it would break "certain
>programs," i.e. SADE.  I think Apple missed an opportunity to do it "right,"
>but the opportunity is scarcely gone forever.
>
I agree with you on protection, but maybe there were good reasons for
requiring that the entire address space be on disk. Who knows - maybe Apple is
planning ahead for the kind of major changes we'd like to see.

Philip Machanick