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Demand Paging In Operating System

   
   ⇰   DEMAND PAGING :-
  A demand paging system is basically advanced version of  paging system with swapping where processes reside in secondary memory and pages are loaded into memory only on demand, not in advance.  When we want to execute a process, we swap it into memory. Rather than swapping the entire process into memory.

   Let us consider an executable program loaded from disk into memory. One option is to load the entire program in physical memory at program execution time. It may be possible that we may not initially need the entire program in memory. Suppose a program starts with a list of available options from which the user is to select. Loading the entire program into memory results in loading the executable code for all options, regardless of whether or not an option is ultimately selected by the user. An alternative strategy is to load pages only as they are needed. This technique is known as demand paging. 

   Demand paging is commonly used in virtual memory system. With demand paging, virtual memory pages are loaded only when they are demanded during program  execution. Pages that are never accessed are thus never loaded into physical memory. We use a lazy swapper for this. A lazy swapper never swaps a page into memory unless that page will be needed. 


  According to the concept of Virtual Memory, when some process is executing, only a part of the process needs to be present in the main memory which means that only a few pages will only be present in the main memory at any time. However, to decide which pages are need to kept in the main memory and which need to be kept in the secondary memory is difficult because we cannot say in advance that a process will require a particular page at particular time. Therefore, to overcome this problem, there is a concept called demand paging is introduced. 

 While executing a program, if program reference a page which is not available in the main memory because it was swapped out a few times ago, the processor treats this invalid memory reference as a page fault and transfers control from the program to the operating system to demand the page back into the memory.


  ⇰  ADVANTAGES :-
   i> Demand paging provides large virtual memory. 
  
  ii> There is more efficient use of memory after applying this. 

 iii> It provides unconstrained multiprogramming. 

 iv> There is no limit on degree of multiprogramming. 


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