paging

operating system A technique for increasing the memory space

available by moving infrequently-used parts of a program's

working memory from RAM to a secondary storage medium,

usually disk. The unit of transfer is called a page.

A memory management unit (MMU) monitors accesses to memory

and splits each address into a page number (the most

significant bits) and an offset within that page (the lower

bits). It then looks up the page number in its page table.

The page may be marked as paged in or paged out. If it is

paged in then the memory access can proceed after translating

the virtual address to a physical address. If the

requested page is paged out then space must be made for it by

paging out some other page, i.e. copying it to disk. The

requested page is then located on the area of the disk

allocated for "swap space" and is read back into RAM. The

page table is updated to indicate that the page is paged in

and its physical address recorded.

The MMU also records whether a page has been modified since it

was last paged in. If it has not been modified then there is

no need to copy it back to disk and the space can be reused

immediately.

Paging allows the total memory requirements of all running

tasks (possibly just one) to exceed the amount of physicalmemory, whereas swapping simply allows multiple processes

to run concurrently, so long as each process on its own fits

within physical memory.

(1996-11-22)