The address space
available to a process
running in a system with a memory management unit
). The virtual address space
is divided into page
s. Each physical 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 bit
s). Each page
thus contains 2n byte
s (or whatever the unit of addressing is). The offset
is left unchanged and the virtual page
number which is mapped by the MMU
to a physical page
number. This is recombined with the offset to give a physical address
- a location in physical memory
Virtual memory is usually much larger than physical memory. Paging allows the excess to be stored on hard disk and copied to RAM as required. This makes it possible to run programs for which the total code plus data size is greater than the amount of available RAM. This is known as "demand paged virtual memory". A page is copied from disk to RAM 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 MMU and the operating system kernel, and the program is unaware of it.
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 runtime 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 expires. 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.