virtual memory

(apparent) increase in the main memory of a computer by using part of the hard disk as additional memory

This article is about the computer term. For the TBN game show, see Virtual Memory (game show).  Virtual memory is an abstraction implemented in a computer that gives an application program the impression it has contiguous working memory, while in fact it is physically fragmented, and may even overflow onto disk storage. While virtual memory is a revolutionary approach for programmers, the benefits for an end-user are only indirect, including quickly developed, more flexible, and simpler programs. All modern general purpose computer operating systems provide virtual memory for ordinary applications, like word processors, spreadsheets, multimedia players, etc. Older operating systems, such as DOS of the 1980s, or mainframe operating systems of the 1960s, had no virtual memory functionality.
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Noun
1. (computer science) memory created by using the hard disk to simulate additional random-access memory; the addressable storage space available to the user of a computer system in which virtual addresses are mapped into real addresses
(synonym) virtual storage
(hypernym) memory, computer memory, storage, computer storage, store, memory board
(classification) computer science, computing

In computer systems, the memory as it appears to, i.e., as it is available to, the operating programs running in the central processing unit (CPU). Note: The virtual memory may be smaller, equal to, or larger than the real memory present in the system.

<memory management> A system allowing a computer program to behave as though the computer's memory was larger than the actual physical RAM. The excess is stored on hard disk and copied to RAM as required.
Virtual memory is usually much larger than physical memory, making it possible to run programs for which the total code plus data size is greater than the amount of RAM available. This is known as "demand paged virtual memory". A page is copied from disk to RAM ("paged in") when an attempt is made to access it and it is not already present. This paging is performed automatically by collaboration between the CPU, the memory management unit (MMU), and the operating system kernel. The program is unaware of virtual memory, it just sees a large address space, only part of which corresponds to physical memory at any instant.
The virtual address space is divided into pages. Each virtual address output by the CPU is split into a (virtual) page number (the most significant bits) and an offset within the page (the N least significant bits). Each page thus contains 2^N bytes (or whatever the unit of addressing is). The offset is left unchanged and the memory management unit (MMU) maps the virtual page number to a physical page number. This is recombined with the offset to give a physical address - a location in physical memory (RAM).
The performance of a program will depend dramatically on how its memory access pattern interacts with the paging scheme. If accesses exhibit a lot of locality of reference, i.e. each access tends to be close to previous accesses, the performance will be better than if accesses are randomly distributed over the program's address space thus requiring more paging.
In a multitasking system, physical memory may contain pages belonging to several programs. Without demand paging, an OS would need to allocate physical memory for the whole of every active program and its data. Such a system might still use an MMU so that each program could be located at the same virtual address and not require run-time relocation. Thus virtual addressing does not necessarily imply the existence of virtual memory. Similarly, a multitasking system might load the whole program and its data into physical memory when it is to be executed and copy it all out to disk when its timeslice expired. Such "swapping" does not imply virtual memory and is less efficient than paging.
Some application programs implement virtual memory wholly in software, by translating every virtual memory access into a file access, but efficient virtual memory requires hardware and operating system support.
(2002-11-26)

<operating system> (VMS) DEC's proprietary operating system originally produced for its VAX minicomputer.
VMS V1 was released in August 1978. VMS was renamed "OpenVMS" around version 5.5. The first version of VMS on DEC Alpha was known as OpenVMS for AXP V1.0, and the correct way to refer to the operating system now is OpenVMS for VAX or OpenVMS for Alpha. The renaming also signified the fact that the X/Open consortium had certified OpenVMS as having a high support for POSIX standards.
VMS is one of the most secure operating systems on the market (making it popular in financial institutions). It currently (October 1997) has the best clustering capability (both number and distance) and is very scalable with binaries portable from small desktop workstations up to huge mainframes.
Many Unix fans generously concede that VMS would probably be the hacker's favourite commercial OS if Unix didn't exist; though true, this makes VMS fans furious.
FAQ.
Usenet newsgroup: news:comp.os.vms.
[How does its performance compare with other OSes?]
(1999-06-03)