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)