Simulating more random access memory (RAM) than actually exists, allowing the computer to run larger programs and multiple programs concurrently. A common function in most every OS and hardware platform, virtual memory uses the hard disk to temporarily hold what was in real memory.
Virtual memory allows multiple programs to load in memory at the same time. Each application addresses memory starting at zero, but virtual memory takes control of the memory addressing and lets each application function as if it had unlimited memory.
Note that virtual "memory" and virtual "machine" are not the same. Virtual memory is used all the time, whereas a virtual machine is an optional approach for running applications (see virtual machine).
Virtual Memory Pages
The computer's real memory is broken up into smaller segments, called "pages," typically 4KB in size. When real memory fills up, pages not currently in use by open applications are written to a virtual memory "swap file" on the disk for temporary storage. When any swapped out page is required again, once again a page in real memory is written to the disk to make room, and the disk page is retrieved. Memory is the computer's workspace, and since there is often a hundred times more disk space than memory space, virtual memory dramatically increases the computer's capacity to do work. However, there is a penalty. When a user has too many open programs, there can be excessive amounts of page swapping, causing applications to slow down. In addition, switching between applications is no longer immediate (see thrashing).
Hardware Is Required
Virtual memory can be implemented in software only, but efficient operation requires specialized hardware circuits. All modern, general-purpose CPUs have memory management units (MMUs) that support virtual memory. They provide "page tables" that are used to translate between the program's "virtual" addresses and the "real" addresses in memory and disk, which may change at any time. Although a program may initially load as a contiguous block of code, it can wind up in pages randomly scattered around real memory.
Virtual memory claims are sometimes made for specific applications that bring additional parts of the program in as needed; however, true virtual memory is built into the operating system and hardware and works with all applications. See Windows swap file.
Virtual memory allows more programs to be opened simultaneously by using the hard disk as temporary storage of memory pages.
When memory is full and the current program needs instructions that are not in memory, pages are swapped. In this example, program A needs a page from the disk, and a page from program C is swapped out to make room.
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