What's the difference between free and inactive memory?

[Gibson88]

I have noticed that via iStatPro I have been reaching the point where I only have around 50mb of free RAM at times, but I have about 450mb of inactive RAM. I'm just wanting to know what the difference is, I'm thinking about putting 4GB in my iMac but I don't want to unless I need it. So yeah can someone explain the differences?

 

[CubeHacker]

Free memory is RAM that is 100% empty and unused. Inactive memory is RAM that currently has something in it, but whatever that something is is no longer being used by any application. Thus, the OS can overwrite whatever is in inactive memory if an application needs it. For example, Safari likes to stick stuff in "inactive memory" - cache files, browsing history, etc. just incase you decide to click the back button and revisit something you've already been to. But if you run another application that really needs that inactive RAM, that new application will use it up.

Technically, both inactive and free RAM can be treated as free RAM.

 

[XheartcoreboyX]

Awesome info!

What about wired vs active?

 

[CubeHacker]

Awesome info!

What about wired vs active?

Active memory is exactly what it sounds like - RAM that is currently active and being used by an application. If you close the application thats using that RAM, it should be converted to either free or inactive RAM, depending on how the application works.

Wired memory stores critical OS data, and cannot be freed or overwritten by the user or any other application unless the OS frees that ram itself.