In a operating systems that use paging for memory management, page replacement algorithm are needed to decide which page needed to be replaced when new page comes in. Whenever a new page is referred and not present in memory, page fault occurs and Operating System replaces one of the existing pages with newly needed page. Different page replacement algorithms suggest different ways to decide which page to replace. The target for all algorithms is to reduce number of page faults.
Page Fault – A page fault is a type of interrupt, raised by the hardware when a running program accesses a memory page that is mapped into the virtual address space, but not loaded in physical memory. In this algorithm page will be replaced which is least recently used. Let say the page reference string 7 0 1 2 0 3 0 4 2 3 0 3 2 . Initially we have 4 page slots empty. Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —> 4 Page faults 0 is already their so —> 0 Page fault. when 3 came it will take the place of 7 because it is least recently used —>1 Page fault 0 is already in memory so —> 0 Page fault. 4 will takes place of 1 —> 1 Page Fault Now for the further page reference string —> 0 Page fault because they are already available in the memory.
Example-2, Let’s have a reference string: a, b, c, d, c, a, d, b, e, b, a, b, c, d and the size of the frame be 4. lru There are 7 page faults using LRU algorithm.