virtual memory
    n 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
         [syn: virtual memory, virtual storage]

virtual memory

    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)