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 memorymanagement 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)