Free memory
As one might expect, this is the RAM that is sitting there with nothing to do. It is completely free for the system to use when needed.
Wired memory
Critical information stored in RAM by the system, its kernel, and some key application components. This stuff is basically frozen it allocates its space and never moves to the hard drive or gets replaced with user-level data when RAM becomes scarce. An interesting thing to note about wired memory is that it scales based on how much total system memory is installed. For example, a Mac with 1GB of RAM may show 400MB of wired memory, while a Mac with 4GB of RAM may use 700MB. The more memory you have, the more your Mac wants to use it!
Active memory
Information currently in use or very recently used. If youve got Safari, GarageBand, iTunes, and Photoshop all going at the same time, your active memory is likely fairly high. Quit one or two of these applications and active memory will shrink. But not all of it will go straight back to free memory
some goes to the next category.
Inactive memory
One of the great things about Mac OS Xs memory management system is that it never stops working for you. Inactive memory is basically a handy storage space for convenient access to your most used tools. Lets say you were working in iPhoto and decide to quit it. Some of the information that was stored in active memory by this application will be moved to inactive memory. This way, when you open iPhoto next time it will load up faster than before. Why? Because the computer is not reading everything from its slow hard drive again. I just ran a quick test myself to illustrate this point: After a system restart, I opened iPhoto and it took 3 Dock icon bounces to launch. I quit iPhoto, did some other stuff, and came back to it a few minutes later. This time it launched in 1 bounce.
How do I know if I should upgrade my Macs RAM?
Now that we all better understand how our RAM is being used, its much easier to determine whether or not more system memory is needed. Most people look at the number next to free memory and think a low amount means they need more RAM. This isnt always the case. As previously mentioned, active memory can get released to inactive memory when its no longer in use. And this is almost always a good thing, since it can increase speed and responsiveness. So low free memory is something to be happy about! Your RAM isnt sitting there being wasted Mac OS X is utilizing it for your benefit.
Instead, you should be concerned when free AND inactive memory are both very low on a regular basis. This situation means most of your RAM is going to wired & active memory because your running applications are demanding all of it right now. Wired & active memory hold precedence, therefore they can grab as much RAM as they need and can force free & inactive memory to give up their space.
