CS 3733 Operating Systems

Reading: Tanenbaum Section 3.4

Virtual Memory

• Each page table entry has a bit indicating whether the page actually occupies a page frame.
• Initially when the process is allocated resources, the page table is set up with all (or most) of the entries marked invalid.
• When a particular page is referenced, the hardware checks to see if the page operation is permitted (e.g. writable) and if not, the program aborts.
• When a particular page is referenced, the hardware checks to see if the page table entry is valid. If it is, everything is as before.
• If an invalid page is referenced, a page fault trap will occur and the following must be done:
  • Remove the process from the CPU. When restarted, the instruction which caused the page fault will be reexecuted.
    (Be careful to undo operations such as MOV -(R2), (R3)+.)
  • Find a free frame from the free frame list and update this list.
  • Schedule a disk operation to read page into frame.
  • Update the page table.
• If the process never attempts to access a particular page of memory, it is never brought in.
• PSECTS in VAX assembly language are defined on page boundaries. You can put restrictions such as noexc or nowrt on pages and keep the protection information in the page table.
• The major components of page fault service are:
  • Service the page fault interrupt.
  • Swap in the page.
  • Restart the process.

Page Replacement
When a new page needs to be brought in, there will not always be an empty frame. Which frame should be replaced?

• FIFO - first-in-first-out
  
  • This is very simple to implement - just keep a FIFO queue and add an entry to the back when a page is placed in a frame.
  • Unfortunately the most heavily used pages are often the ones brought in first.
  • Note that the FIFO queue gets updated only when there is a page fault, not on each reference. This implies that it is possible to generate reference strings which do worse when there are more frames.
  • Example: Calculate the number of page faults obtained for the following reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
    • When the memory has 3 frames
    • When the memory has 4 frames

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  Revision Date: 10/6/97

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