Simulating more memory than actually exists, allowing the computer to run larger programs and keeping more of them open concurrently. A common function in all modern computer operating systems and hardware platforms, virtual memory uses the hard disk to temporarily hold what was in real memory.
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 or 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. Modern CPUs have memory management units (MMUs) that provide full support for virtual memory. They translate between the programs "virtual addresses" and the "real addresses" in memory and disk, which are changing all the 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 a hardware and operating system implementation that works with all applications. See Windows swap file.
Memory Is Extended to Disk
Virtual memory allows more programs to be opened simultaneously by using the hard disk as temporary storage of memory pages.
Page Out, Page In
When memory is full and a running program needs instructions from a page on the disk, pages are swapped to make room.
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 or 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. Modern CPUs have memory management units (MMUs) that provide full support for virtual memory. They translate between the programs "virtual addresses" and the "real addresses" in memory and disk, which are changing all the 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 a hardware and operating system implementation that works with all applications. See Windows swap file.
Memory Is Extended to Disk
Virtual memory allows more programs to be opened simultaneously by using the hard disk as temporary storage of memory pages.
Page Out, Page In
When memory is full and a running program needs instructions from a page on the disk, pages are swapped to make room.
 | Reproduced with permission from Computer Desktop Encyclopedia. Copyright (c) 1981-2008 The Computer Language Company Inc. All rights reserved. |
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