Virtual memory

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Virtual memory is, basically, a chunk of hard drive space that holds memory not in use. This is a tradeoff: RAM space is expensive, but fast; disk space is cheap, but slow.

'Swapping' is moving a hunk of memory from RAM to disk or back; if you have too little memory for your applications, it can take a long time to swap everything, slowing the OS to a crawl. The minimum amount of RAM for day-to-day use of OS X should be 512MB, with 1GB and up recommended.

New in Tiger is the option to encrypt the data as it is swapped to disk, called secure virtual memory.

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