from
The Free On-line Dictionary of Computing (8 July 2008)
virtual memory
<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)