by Barry Dysert
(last updated October 19, 2015)
Virtual memory is the name given to the contiguous address range used by a computer program. For a program to run, its instructions must reside in physical memory. However, when there are several programs running simultaneously or when there's not enough physical memory to encompass the amount of memory needed by a program, the computer uses virtual memory to fulfill the program's needs.
For example, if two programs are running at the same time, each program treats memory as if it's one large contiguous block beginning at address zero and extending as far as necessary. Since each program can't access the memory at physical address zero simultaneously, the computer creates blocks (address ranges) of virtual memory that make it seem to each program that it has all of the computer's memory to itself.
Virtual memory is also used when programs need more memory than physically exists in the computer. The computer uses a process called paging to allow programs to access more memory than physically exists. Through paging, the computer keeps track of what blocks of physical memory are currently available, writes the contents of those blocks to disk, and then reads in from disk the previously written data that the currently executing program needs. By using the disk as a temporary storage medium, many different programs can be running at the same time and accessing large portions of memory without ever having to be concerned with whether the data is physically in memory at the time or needs to be paged in from disk.
This tip (12623) applies to Windows 7 and 8.
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