Demand Paging

• Consider how an executable program 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.
  • An alternative strategy is to initially load pages only as they are needed. This technique is known as demand paging and is commonly used in virtual memory systems.
• A demand-paging system is similar to a paging system with swapping (see Fig. 4) where processes reside in secondary memory (usually a disk).

  Figure 4: Transfer of a paged memory to contiguous disk space.

  
• When we want to execute a process, we swap it into memory. Rather than swapping the entire process into memory, however, we use a lazy swapper. A lazy swapper never swaps a page into memory unless that page will be needed.
• A swapper manipulates entire processes, whereas a pager is concerned with the individual pages of a process. We thus use pager, rather than swapper, in connection with demand paging.