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From: glenn@gla-aux.uucp (Glenn L. Austin)
Newsgroups: comp.sys.mac.system
Subject: Re: Virtual Memory and Sys 7
Message-ID: <1991May9.042425.598@gla-aux.uucp>
Date: 9 May 91 04:24:25 GMT
References: <1991May8.143042.20137@bigsur.uucp>
Organization: The Pit Lane
Lines: 64

webster@bnr.ca (Brent Webster) writes:
>Just a few comments on Apple's version of virtual memory.
>I have a lowly Mac II with 8Meg of ram and a 40Meg hard-drive 
>running 7.0b4 OS.  I been running a Smalltalk-80 application on
>my SparcStation whose image size is about 6.5 Megs and I wanted to
>try running it on my Mac (speed initially is not a concern).

>I went to reconfigure my memory via the Memory control panel but it
>only allowed me to increase the memory to 11 Megs. When I rebooted my
>Mac, to my astonishment, my harddisk was short 11Megs and not the 
>3Megs plus some overhead which I expected.  I can live with that 
>but I am wishing for a more elegant solution.

This is so that the entire memory map is available.  If only the additional
RAM was allocated, where would you put the current RAM so that you could
swap in the additional RAM?  There would always be some overhead, and by
allocating the full memory size, you remove problems with double-writing,
memory collisions, etc.

>Running "About this Macintosh..." indicated that the System was ONLY
>taking about 1.8Meg so I figured I had 9Megs to play with.  WRONG!!!
>The "Largest Unused Block:" was about 7.2 Meg and that's all the 
>Smalltalk-80 application could see.

If you have any cards in NuBus slots, they leave 1MB "holes" in the
memory map.  If you want to increase the largest block, move all the
cards as close to the power supply as possible (on 6-slot Macs), or
as suggested by Apple in the docs (which I haven't seen yet...)

>My questions are:

>  Does Smalltalk-80 have to be reworked to see the other 2Meg or
>  do all Mac applications only get to use the "Largest Unused
>  Block:"?

I'll bet if you went back into the Finder, you would see that the
newest "Largest Unused Block" was now either 1MB or 2MB...
The memory is partitioned under Multifinder (and System 7), and the
memory (map) must be contiguous within the block.

>  Is the "Largest Unused Block:" limited by the size of the actual
>	 ram on board your Mac?

Yes and no.  The largest unused block is an "artifact" of the partitioning
mentioned above.

>  Will I ever be able to run a 12Meg application on a Mac
>  containing 8Meg of ram?

If you allocate the memory correctly, yes.  If I moved my two video boards
to the highest slots, I'd be able to run an application which used the
contiguous memory allocated from the space used by slots 9-C, adding a total
of 4MB to both my available memory and largest unused block.
 
>  Is Apple's version of virtual memory always going to be
>  HARDDISK hungry?

Almost everybody elses is.  Why should Apple be any different? :-)

-- 
===============================================================================
| Glenn L. Austin - Mac Wizard and Auto Racing Driver                         |
| Usenet:  glenn@gla-aux.uucp                                                 |
|  "Turn too soon, run out of room.  Turn too late, much better fate."        |