How Your Mac Uses Memory

This column is for those of you who are wondering: What's all this about memory? What is RAM? How do I use it, and when do I need more of it?

Your Mac has several forms of memory. As a music teacher, I am struck by the similarity with the human brain and our own memory functions. There is a hard drive, which is semi-permanent storage of data: The tune you've already learned is stored here. Then there is short-term memory, which corresponds to your Mac's RAM (Random Access Memory): This is what you're using when you're learning a new tune, phrase by phrase. Everything that goes into long-term memory first has to make it through short-term memory.

Running short of memory, one way or another, is one of the most common problems you may have with your Mac, especially in the beginning. Having more RAM installed sometimes clears up difficulties that aren't as obvious as getting an "Out Of Memory" message. When it comes to RAM, many sources, as well as my own experience, persuade me that more is generally better.

RAM is pretty inexpensive right now, especially for more recent computers, and it's usually not too hard to install. Your user's manual probably has instructions for adding more RAM to your computer, along with a warning that Apple recommends that only certified Apple techs install it, and that any damage you cause while doing it yourself is not covered by the warranty. I have to admire the depth of the double message here: "We recommend you do not follow these detailed instructions." I suppose they figure we will sort ourselves out into those who will heed the warning and those who will do it themselves. If you choose to join me in the do-it-yourself category, this is at your own risk.

How much RAM can your Mac use? Check the Low End Mac listings for your model. In some cases, RAM is being manufactured that makes it possible to install more than Apple officially recommends. However, Apple does not guarantee that this will work.

When do you need more RAM? Well, first of all, how much RAM do you have? You can find this out by going to the Apple menu (in Finder) and choosing "About This Computer." This will bring up a display telling you what OS you're running, how much built-in memory you have, and how much Virtual Memory you're using. (Virtual Memory is not as good as RAM, and several things I've read have said you shouldn't have more than 1 MB more VM than you have actual RAM. If you have a lot of RAM, you can turn VM off; you make that choice via the Memory control panel.)

The "About This Computer" display will show you how much RAM the OS is using and how much each program you have running is using. I see to my surprise today that OS 9.1 is using 96.5 MB of RAM. I'm sure it didn't start out that way; as you launch programs, the OS often increases the amount of memory is uses. Anyway, it's a good thing I upgraded from the 64 MB that came with my iMac.

If my OS uses 96 MB and I have 128 MB of RAM in my Mac, I still might not be doing too well. Depending what programs you're using, how much memory you have allocated to each of them, and how many of them you are running at the same time, it can be easy to run short. The "About This Computer" display will tell you how big the largest unused block of memory is. If that block is smaller than the amount of memory that the program you're trying to launch need, you may have trouble. If you quit other programs to free up RAM, try to do it in the reverse of the order in which you launched the programs; otherwise you may leave a program's RAM allocation parked in the middle of memory and might not have as large enough chunk free.

For a detailed graphic display of your memory use, you might want to try the freeware Memory Mapper. This gives you a multicolor bar display of exactly what your RAM is up to.

If you get more RAM, where does it go? Your Apple System Profiler will tell you how many memory slots you have and how many megabytes of RAM is in each of them. This may make a difference in what RAM will be cost-effective for an upgrade for you; if you have to take out a small amount of RAM, no big deal; but in some cases you will have to take out a larger chip.

For example, my Pismo came with 256 MB of RAM, but it's in two 128 MB chips. If I buy a 256 MB chip, I will have to take out a 128 MB chip and only end up adding 128 MB to my current total. That makes the 256 not as cost-effective an upgrade for me as 512 would be, presuming I may someday have the need for, let's see, 640 MB of RAM. Since the need for RAM appears to keep increasing - I hear OS X works best with at least 256 MB - there's a reason to buy what you think you may need in the future. On the other hand, price fluctuations can eliminate any advantage to buying ahead. (Computers have replaced some earlier technologies, but a crystal ball might still be useful in this regard.)

For more information on how your Mac uses memory and how to adjust the memory usage for specific programs, there are good resources in the Mac Help menu.

I'd just add a note that nothing in that Help section prepared me for the huge amounts of memory I've ended up allocating to certain programs, especially email programs, browsers, graphics programs, and anything that was causing me trouble. First I'd increase the "Minimum Size" to the "Suggested Size." If I still had trouble, I'd add some to the suggested size. This went on until a few programs were running several times the "recommended" amount of RAM - and behaving very well.

I have 384 MB of RAM in my iMac now, so I figure I can spare what it takes to keep these programs happy. The improved performance makes my additional RAM a worthwhile investment. Perhaps it might be for you as well.