Virtual memory on a MacII wo/ PMMU?

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2 total messages Started by Bake Timmons Sun, 01 Nov 1992 10:06

#22

Author: Bake Timmons
Date: Sun, 01 Nov 1992 10:06

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I've got 2megs RAM and no PMMU.  I've tried getting virtual memory under sys7,
but the option does not even show up in the memory control panel.  Is this
because of the PMMU?  What does it provide exactly -- what memory limit does
it change and by how much?  Thanks.

--
Bake Timmons, III

-- "...there's nothing higher, stronger, more wholesome and more useful in life
than some good memory..." -- Alyosha in Brothers Karamazov (Dostoevsky)

Re: Virtual memory on a MacII wo/ PMMU?

#26

Author: John F. Woods
Date: Mon, 02 Nov 1992 12:21

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In <1992Nov1.235504.16257@mercury.unt.edu> mcmains@unt.edu (Sean McMains) writes:
>In article <timmbake.720612419@mcl> Bake Timmons, timmbake@mcl.ucsb.edu
>writes:
>>I've got 2megs RAM and no PMMU.  I've tried getting virtual memory under
>sys7,
>>but the option does not even show up in the memory control panel.
>There's a program
>called "Virtual," which is (I think) made by Connectrix that can be
>purchased with a PMMU if you're really interested in getting this capability.

Since he already has the necessary software (it's built in to System 7), this
is probably an expensive way to purchase a PMMU, which is all that he needs.

>However, virtual memory is ALWAYS slower than real memory, so
>I'd reccommed investing in some more RAM, as it's at an all time low
>price right now.

While it is "always" slower than real memory, sometimes it is unnecessary to
capitalize ALWAYS.  As I said in my previous note, it does slow each memory
access by a clock cycle (best case), which isn't all that much; if you run
far more virtual memory than physical memory, you'll thrash and get nothing
useful done (and at 2MB physical he clearly needs more RAM even for VM);
however, if you have fairly well-behaved applications (which don't randomly
touch every single page of memory just to keep them warm) and a reasonable
job mix (the Macintosh cooperative scheduling actually helps here, sort of),
then virtual memory need not feel slow:  if an application with a 1MB address
space only *uses* 512K during any given run, there is no need to give it any
more than 512K, and it will be essentially just as fast as if it had all that
extra memory (plus or minus startup page fault times and continuing address
translation delays).  And, of course, once in a while, even if you have a
comfortable amount of memory (say 8MB), you decide you just HAVE to run an
application (say, a mondo ray-trace) which simply MUST have 10MB of memory.
In this instance, the run time with virtual memory will be quite a bit less
than without (infinite, since it won't).

But do buy more RAM, even if it is no longer at an all-time low right now
(assuming you find anyone with it in stock :-( ).

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