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2 definitions found
 for virtual memory
From WordNet (r) 3.0 (2006) :

  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]

From The Free On-line Dictionary of Computing (18 March 2015) :

  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)
  

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