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page replacement

_ Limited number of pages available in memory

_ Need to optimize swapping (placement and replacement)

_ Increase in multiprogramming through replacement optimization

_ Increase in degree of multiprogramming ) over allocation of memory

_ Assume no free frames available

     - Option to terminate the process

          _ Against the philosophy behind virtual memory

          _ User should not be aware of the underlying memory management

     - Option to swap out a process

          _ No guarantee that the process will get the CPU back pretty fast

     - Option to replace a page in memory

_Modified page-fault service routine

     - Find the location of the desired page on the disk

    - Find a free frame

         _ If there is a free frame, use it

         _ Otherwise, use a page replacement algorithm to find a victim frame

         _ Write the victim page to the disk; change the page and frame tables accordingly

    - Read the desired page into the (newly) free frame; change the page and frame tables

    - Restart the user process