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OSX: Understanding RAM and System Memory

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This is an issue that cropped up a few times on the MacCast forums, and I realized that it is something that most people are only half aware of. The majority of Mac users understand that OS X is relatively RAM dependent, and that more is better; but there is more to it than that. I am not claiming to “know it all”, but I certainly have a good background in this and hope that through collaboration with other MacCast forum members I can help some people gain a better understanding of their computers.


Random Access Memory

Firstly then, we need to understand the concept of RAM. RAM (or Random Access Memory) is simply a circuit board that is used for storage of data that is assumed (by the Operating System usually) to be required regularly or soon. This speeds up the computer in effect because data transfer rates are much higher than those of most other forms of computer data storage (such as Hard Disk Drives).


With that in mind, Mac OS X has the ability to report a certain amount of information over the current activity of your RAM at any point in time. Knowing this information can often help in diagnosis of current hardware and software issues, future upgrade decisions and many other applications. To access this information you would need to open Activity Monitor, stored in a folder named Utilities which is located in the Applications folder.




Using this application you can change the tab to System Memory, which refers mainly to RAM activity. Here you will find a pie chart, color coded to show 4 categories of usage. Here is a quick and general (not entirely technically accurate, but more a sense) idea of what these categories mean.


Usage Categories

Wired Memory - In Mac OS X “Wired” system memory refers to data that cannot be cached to another disk (or storage/memory module), meaning they must remain in the RAM.


Active Memory - This is simply data that is currently being stored in your RAM and currently in use by OS X and applications running on OS X.


Inactive Memory - Data in this case is no longer being used by OSX or applications, but is determined to be best left in the RAM in case it is called upon later. Should the RAM space be required by another application or OS X itself, this data will be over-written. This is used to your advantage but can be thought of as free memory in that it isn’t required or locked down in any way.


Free Memory - This is the memory that is not currently storing any data and is ready to use at any time.


When Should I Worry?

Obviously, the more Free Memory you have the better, but a lack of it should not concern you. As you use applications the amount of Free Memory will drop, and it will evolve into Active and eventually Inactive Memory. The only time you need to start thinking about upgrading your RAM is when the combination of Free and Inactive Memory is very low. These statistics are best checked when the computer is under full load and slow downs or other issues are occurring, or occasionally when under what you define as “normal” use for that computer (as it is in your hands) to estimate your computers “normal” usage and in turn strain.

I hope this helps and I would appreciate any corrections or alteration suggestions.


[Further Ref: Apple OS X: Reading system memory usage in Activity Monitor (107918)]

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