Page fault handler
When the requested page is not in the main memory a page fault occurs.
This is an interrupt to the OS.
1. If there is empty page in the main memory , roll in the required logical page, update page table. Return to address translation step #3.
2. Else, apply a replacement policy to choose a main memory page to roll out. Roll out the page, if modified, else overwrite the page with new page. Update page table, return to address translation step #3.