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From: jackb@MDI.COM (Jack Brindle)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.misc,comp.sys.mac.system
Subject: Re: System 7.0 and RAM
Message-ID: <1991Apr11.211546.27442@MDI.COM>
Date: 11 Apr 91 21:15:46 GMT
References: <1991Apr8.010338.14202@murdoch.acc.Virginia.EDU> <1991Apr9.220140.18228@murdoch.acc.Virginia.EDU>
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In article <1991Apr8.010338.14202@murdoch.acc.Virginia.EDU> mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes:
>
>Given the virtual memory capacities of system 7.0, will there be any
>need to purchase additional RAM?  If not, will it even be advantageous
>to have more RAM rather than less?  

I see several reasons to have large amounts of RAM in your Mac with sys 7. 
The first is that the sys 7 system heap is quite large (at least for B4).
Mine currently runs about 1.5 Megs. I'm not sure why it is so large, but
I do have sharing turned on (but not VM). Sharing seems to account for
only about 180K or so, though. The finder grabs another 340K. (Side
question - has anyone figured out why the system heap is so large?) This
leaves a small amount of memory available for applications in a 2 Meg
machine (Mine has 5 MB - it still feels a bit cramped). 

The second reason is that VM eats up disk space. If you have 14 Megs of VM,
then you will use up 14 Megs of disk space. That leave only 26 megs of storage
on a 40 MB hard disk. This is the biggest reason I don't run VM - I need
the extra storage.

Lastly, you should expect some applications to break under VM. If the
applications use a Vertical timing interval task or any other task that
runs at interrupt time, there could be a quick debugger exit (AKA BOMB).
The reason is that if the task is swapped to disk when an interrupt that
triggers the task occurs, the machine will vector to where it thought the
task was (but no longer is). System 7 has functions to lock down the tasks
so they won't be swapped. Of course, pre Sys-7 applications don't know
about the functions, so they don't use them. I sure hope the folks in Redmond
have their (now very good) tech support staff prepped for this one :-).

My question actually becomes one of whether VM is all that useful to run.
It's quite possible the Macintosh Plus/Classic folks will have nothing
to miss with this feature!

- Jack B.
amateur radio: wa4fib/7