Virtual memory: It is computer will comprise a limited amount of real memory accessible to it. Programs frequently need more memory than the quantity of real memory. Moreover, in a multiprogramming system, various processes will be competing for similar limited supply of real memory. The operating system overcomes such conflicts by assigning an amount of virtual memory to each procedure that might be bigger than the net amount of real memory. This is possible by storing up unused portions of a process's address space on disk, till such time as it is needed. Whenever required, it is swapped in to part of real memory, whose preceding contents are swapped out to disk.