Virtual Memory

Virtual memory in a separation of user logical memory from physical memory.In this method , we keep only a part of the process in the memory and other part on the disk (Secondary Storage).
It is only part of the program needs to be in memory for execution and logical address space is much larger than physical address space and need to allow pages to be swapped in and out.

It need to maintain entire image of process in disk storage.

User Point of View:-
Programmers or developers are relieved of trying to fit of program into limited memory.

System Point of View:-
It is reduces external  fragmentation

Demand Paging:-
In this technique , a page is brought into the memory for its execution only when it is demanded.
Combination of paging and swapping.

Lazy Swapper -Swaps the page only when it is needed.

Advantage:-
It is reduces memory require.
It is swap time is also reduced.
It is degree of multi programming.

Disadvantage:-Page Fault(Used in the performance measurement)
                                
E_a=[(p*”page fault time”)+(1-p)³ ma)] 

Effective memory access time- E_{a
 Probability of page fault- p
General memory access time- ma
Probability of no page fault- (1-p)³}