Refers to the main means to achieve memory expansion, he takes external memory as a direct extension of memory, so that the limited actual memory (referred to as real memory) and large-capacity external memory unified organization into a virtual memory (referred to as virtual memory), the user feels that the main memory space is infinite.
When a program runs, all of its information is loaded into virtual memory, but in fact the part of the program that is involved in execution can be stored in memory, and the other parts are stored in external memory.