What is meant by demand paging? Explain.

Demand paging is a technique to implement virtual memory. In demand paging simply when a page is required it is brought into memory. As an alternative of swapping the whole process into memory only the required pages are brought into the memory. Therefore it will not read pages which are not used and hence results in reduced swap time and memory needed.

For distinguishing the pages in memory and those on disk a valid invalid bit is used. If the bit is valid the page is in memory moreover the page is either not in memory or is not valid. The page table entry for a page that isn't currently in the memory is marked invalid.