On a system with a very large Virtual Memory Address space,
  The virtual memory table can be very large.

  Assume:
    Page and page frame is 4K (common cluster size) 2^12

    Real memory is 512 MB  2^29

    Virtual memory space is 4 GB 2^32

    Table:
    ( 29 bits page id + 1 bit P/A + 1 bit dirty + additional flags) 
     * 2^20 entries  =  about 4 MB