What is Virtual Memory ? Explain any three terms related to virtual memory.
Virtual Memory : Virtual memory is an attempt, which makes the execution of the processes possible, which may completely not in the main memory. Some part of the process may be on disk.
(1) Locality of reference: Clustering of page references in a certain time zone is called the principle of locality of reference. It gives some basis to forecast whether a page is likely to be referenced in the near future based on its passed behavior and so you can swap out the page.
(2) Page Fault: During the process execution, only few pages are in the memory. At that time an instruction is encountered which refers to any instruction or data in that page which is not in memory. So the page can not be referred and it is called as the page fault.
(3) Working Set: The set of pages in the physical memory actively referred to at any moment is called working set.
(4) Page replacement policy: As no. of processes and no. of pages in main memory for each process increase, all the page frames become occupied at some point of time. For the new page, the operating system has to overwrite some existing page in memory. The page to be replaced is selected by page replacement policy.
(5) Dirty Page: Before overwriting a page, the operating system has to check whether the page has to modified or not. If the page is modified, it becomes a dirty page. The dirty bit is used for checking whether the page is dirty or not.
(6) Demand Paging: A page is swapped in when it is demanded. This policy is known as demand paging.