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demand paging
(foldoc) | demand paging
demand paged
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 {least
recently used} (LRU) algorithm.
Prepaging is generally more efficient than demand paging.
(1998-04-24)
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