Demand paging
consider how an executable programs might be loaded from disk into memory. One option is to load the entire program in physical memory at program execution time. However a problem with this approach is that we may not initially need the entire program in memory. Suppose a program start with a list of available options from which the user is it to select. Loading the entire program into memory results in loading the executable code for all option, regardless of whether an option is ultimately selected by the user or not . An alternative strategy is to load page only as they are needed. This technique is known as demand paging and is commonly used in virtual memory system.
when we want to execute a process, we swap it into memory. Rather then swapping the entire process into memory, however we use a lazy swapper. A lazy swapper never Swap a page into memory unless that page will be needed. Since we are now viewing a process as a sequence of pages, rather than as one large contiguous address space, use of the term swapper is technique incorrect. A swapper manipulate entire process where a page is concerned with the individual page of a process. So we use a pager rather then swapper, in connection with demand paging.
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