Demand paging

In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process's working set of pages are located in physical memory. This is an example of a lazy loading technique. Demand paging follows that pages should only be brought into memory if the executing process demands them. This is often referred to as lazy loading as only those pages demanded by the process are swapped from secondary storage to main memory. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory during the process startup. Commonly, to achieve this process a memory management unit is used. The memory management unit maps logical memory to physical memory. Entries in the memory management unit include a bit that indicates whether a page is valid or invalid. A valid page is one that currently resides in main memory. An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed: Attempt to access page. If page is valid (in memory) then continue processing instruction as normal. If page is invalid then a page-fault trap occurs. Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated (illegal memory access). Otherwise, we have to page in the required page. Schedule disk operation to read the desired page into main memory. Restart the instruction that was interrupted by the operating system trap. Demand paging, as opposed to loading all pages immediately: Only loads pages that are demanded by the executing process.