VM problems
  Each memory access requires 2 memory accesses
    1st to read the table

    2nd to access the page frame in physical memory. 

  Another issue is size of page table for large systems.
    4 GB (2^32) Virtual memory space.
   32 MB (2^25) Physical memory.
    2 KB (2^11) page and frame size (equal to small cluster).

    2^32/2^11 = 2^21 2 M. slots. 2^25/2^11 = 2^14 (14 bit pointer)

    Table size:
    2 M. * (14 bit pointer + 1 bit P/A flag + 1 bit dirty flag +) 
    >= 4 MB of storage.

    Too big to be practical.