<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)

Virtual memory T cells (TVM) are a subtype of T lymphocytes. These are cells that have a memory phenotype but have not been exposed to a foreign antigen.

Virtual memory compression (also referred to as RAM compression and memory compression) is a memory management technique that utilizes data compression to reduce the size or number of paging requests to and from the auxiliary storage. In a virtual memory compression system, pages to be paged out of virtual memory are compressed and stored in physical memory, which is usually random-access memory (RAM), or sent as compressed to auxiliary storage such as a hard disk drive (HDD) or solid-state drive (SSD).

<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 (http://cis.ohio-state.edu/hypertext/faq/bngusenet/comp/os/vms/top.html)}. Usenet newsgroup: news:comp.os.vms. [How does its performance compare with other OSes?] (1999-06-03)

1. <architecture> A memory location accessed by an application program in a system with virtual memory such that intervening hardware and/or software maps the virtual address to real (physical) memory. During the course of execution of an application, the same virtual address may be mapped to many different physical addresses as data and programs are paged out and paged in to other locations. 2. In IBM's VM operating system, {Virtual Device Location}. (2001-01-02)

Virtual hosting is a method for hosting multiple domain names (with separate handling of each name) on a single server (or pool of servers). This allows one server to share its resources, such as memory and processor cycles, without requiring all services provided to use the same host name.

A virtual event is an online event that involves people interacting in a virtual environment on the web, rather than meeting in a physical location. Virtual events are typically multi-session online events that often feature webinars and webcasts.