What is Virtual memory
The term virtual memory signifies something that appears to be present though actually it is not. Virtual memory technique enables users to use more memory for a program than real memory of a computer. A programmer can write a program that requires more memory space than capacity of the main memory. Such a program is executed by virtual memory technique. Program is stored in the secondary memory. Memory management unit (MMU) transfers currently required part of the program from the secondary memory to main memory for execution. This part of program is executed by the processor. After execution this part of program is sent back to the secondary memory together with immediate results. Afterwards CPU takes another part of the program for execution. So the main memory always keeps only currently needed part of the program. This type of 'to and from' movement instructions and data between main memory and secondary memory is termed as swapping. So a program requiring more memory space than capacity of the main memory can executed employing a swapping technique. This concept is called virtual memory technique.