demand paging

memory management A kind of virtual memory where a page

of memory will be paged in if an attempt is made to access

it and it is not already present in main memory. This

normally involves a memory management unit which looks up

the virtual address in a page map to see if it is paged

in. If it is not then the operating system will page it in,

update the page map and restart the failed access. This

implies that the processor must be able to recover from and

restart a failed memory access or must be suspended while some

other mechanism is used to perform the paging.

Paging in a page may first require some other page to be moved

from main memory to disk ("paged out") to make room. If this

page has not been modified since it was paged in, it can

simply be reused without writing it back to disk. This is

determined from the "modified" or "dirty" flag bit in the

page map. A replacement algorithm or policy is used to

select the page to be paged out, often this is the leastrecently used (LRU) algorithm.

Prepaging is generally more efficient than demand paging.

(1998-04-24)