Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!keith
From: keith@Apple.COM (Keith Rollin)
Newsgroups: comp.sys.mac.system
Subject: Re: VM rule of thumb (sic)
Message-ID: <53567@apple.Apple.COM>
Date: 2 Jun 91 23:07:40 GMT
References: <1991May30.132020.23146@xn.ll.mit.edu> <12384@scolex.sco.COM>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 44

>
>delaney@xn.ll.mit.edu (John R. Delaney) writes:
>
>>And by the way, what we have on the Mac's right now strikes me as
>>demand-paged memory, not virtual memory. But please, let's not start a
>>"true virtual memory" thread to go along with the "true multi-tasking"
>>thread.

I don't want to start a war; I just want to ask a question: why
don't you consider "demand-paged memory" to be "virtual memory".
Isn't the former just a spcific kind of the latter? That was
my impression. Is it not the case?


In article <12384@scolex.sco.COM> erics@sco.COM (eric smith) writes:
>
>From what I understand of VM on the Mac (I'm not running 7.0 yet)
>it definitely is virtual memory and not demand paging. In demand paging,
>an application's pages would not be read in from their original disk
>location until they were accessed. Thus if a page was never used it would
>never be read in at all. What I gather about the Mac's VM is that an
>application is read in as before and also written to the special disk area
>that contains the entire VM contents. Applications may then be swapped in
>and out from there as needed, but I believe that the entire application
>has to be in physical memory in order to run (correct me if I'm wrong).
>In any case, the application must completely fit into the virtual space.
>This is not demand paging.

System 7.0's VM _is_ demand paging. Pages are read in as needed. Also,
not all pages belonging to an application need to be in memory for
the application to run. However, because a running application is
likely to need all its various parts at some time, the pages holding
the applicaton code will eventually be brought into RAM.

And don't forget that this is all on top of the segmenting scheme
that Macs have always had. CODE segments aren't even read out of
their file unless they are needed.

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