paging

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paging

[′pāj·iŋ]
(computer science)
The scheme used to locate pages, to move them between main storage and auxiliary storage, or to exchange them with pages of the same or other computer programs; used in computers with virtual memories.

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 hard 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 physical memory, whereas swapping simply allows multiple processes to run concurrently, so long as each process on its own fits within physical memory.

paging

(1) In a virtual memory computer, it is the transfer of program segments (pages) into and out of memory. Although paging is the primary mechanism for virtual memory, excessive paging is not desirable (see thrashing).

(2) A communications service that began in the U.S. in the 1980s. It evolved from a one-way beeper to one-way numeric text to one-way alphanumeric text to a two-way text service.
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