Missing Used Field DEC VAX lacks a Used field. Simulates the use of one through a mirror table and the present bit. Reset present bit rather when it would want to reset the non-existent use bit. So far, we have considered single process systems where there is only one page table. Multiple processes can be accommodated by giving each process a quota of frames and applying a policy individually to each process.
Local policies resolve competition for frames local to a process.
Global policies have processes competing against each other for frames. Usually, the frame table is not directly supported by the hardware, so the OS maintains one in software. Sharing Pages Can think of allowing sharing of pages between processes. Useful, but causes problems. Backing Store Have an explicit swap area on disk.