Page fault
A page fault occurs when a page referenced by the CPU is not found in the main memory. The required page has to be brought from the secondary memory into the main memory. A page that is mapped in the RAM/physical memory/main memory has to be replaced if all the frames of main memory are already occupied.
Page Replacement
Page replacement is a process of swapping out an existing page from the frame of a main memory and replacing it with the required page. Page replacement algorithms help to decide which page must be swapped out from the main memory to create a room for the incoming page.
- page fault occurred i.e the referenced page by the CPU is not found in ram.
- Find a free page frame
- If there is a free page frame, use it (go to step no 7)
- Otherwise, Select a page to be replaced using page replacement algorithm
- Write the selected page to the disk if modified bit of that page is 1
- wanted free page frame found
- Fetch disk block into page frame
- When disk read finishes, add VM mapping for the page
- change page table entries