A page fault occurs when an access to a page that has not been brought into main memory takes place. The operating system verifies the memory access, aborting the program if it is invalid. If it is valid, a free frame is located and I/O is requested to read the needed page into the free frame. Upon completion of I/O, the process table and page table are updated and the instruction is restarted.
when a process is executed with only few pages in memory,& when an instruction is encountered which refers to any instruction or data in some other page,which is not present in the main memory,a page fault occurs.