[1] : WordNet (r) 2.0

virtual memory
     n : 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 storage
see also:
virtual storage 

[2] : The Free On-line Dictionary of Computing (27 SEP 03)

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
     
     
see also:
physical ram hard disk demand paged cpu memory management unit 
operating system kernel address space pages virtual address 
page bytes memory management unit physical address physical memory 
locality of reference multitasking demand paging mmu timeslice 
application programs 

Virtual Memory >> V Words
Virtual Memory, definition of term: Virtual Memory
virtual+memory_pag1.html

Virtual >> V Words
Virtual, definition of term: Virtual
virtual_pag1.html

Memory >> M Words
Memory, definition of term: Memory
memory_pag1.html

Virtual Circuit >> V Words
Virtual Circuit, definition of term: Virtual Circuit
virtual+circuit_pag1.html

Virtual Address >> V Words
Virtual Address, definition of term: Virtual Address
virtual+address_pag1.html

Virtual Device >> V Words
Virtual Device, definition of term: Virtual Device
virtual+device_pag1.html

Virtual Focus >> V Words
Virtual Focus, definition of term: Virtual Focus
virtual+focus_pag1.html

Virtual Reality >> V Words
Virtual Reality, definition of term: Virtual Reality
virtual+reality_pag1.html

Virtual Machine >> V Words
Virtual Machine, definition of term: Virtual Machine
virtual+machine_pag1.html

Virtual Image >> V Words
Virtual Image, definition of term: Virtual Image
virtual+image_pag1.html

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