Valid-Invalid Bit:-

• With each page table entry a valid–invalid bit is associated.
(1 => in-memory, 0 => not-in-memory)

• Initially valid–invalid but is set to 0 on all entries.

• During address translation, if valid–invalid bit in page table entry is 0 => page fault.

Page Fault:-

• If there is ever a reference to a page, first reference will trap to OS => page fault.

• OS looks at another table to decide:

Invalid reference => abort.

Just not in memory.

• Get empty frame.

• Swap page into frame.

• Reset tables, validation bit = 1.

• Restart instruction:

block move.

auto increment/decrement location.



What will occur if there is no free frame:-

• Page replacement will occur – find some pages in memory, but not really in use, swap it out.

• Performance will decrease – want an algorithm which will result in minimum number of page fault.

• Sevaral times same page may be brought into memory