實體記憶體和虛擬記憶體

實體記憶體(physical memory)
(不用闡述了)

虛擬記憶體(virtual memory)
virtual memory uses a disk as an extension of RAM so that the effective size of usable memory grows correspondingly. The kernel will write the contents of a currently unused block of memory to the hard disk so that the memory can be used for another purpose. When the original contents are needed again, they are read back into memory.This is all made completely transparent to the user; programs only see the larger amount of memory available and don't notice that parts of them reside on the disk
from time to time. Of course, reading and writing the hard disk is slower (on the order of a thousand times slower) than using real memory, so the programs don't run as fast. The part of the hard disk that is used as virtual memory is called the swap

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