n A memory reference to an unused page traps into
the OS (protection violation)
n Each PTE also contains:
u a present bit
F If set: 20 bits are used for the frame address of
the selected page.
F Else these bits are used to locate the selected
page in a paging file (on disk)
u some bits identify the paging file used
u a dirty bit  (ie: a modified bit)
u some protection bits (ex: read-only, or read-write)