Explain virtual memory.

The term virtual memory considers as to something that appears to be present but in fact it is not. The virtual memory technique permits users to use more memory for a program than the real memory of a computer. A programmer can write a program that requires additional memory space than the capacity of the main memory. Such a program is executed by virtual memory technique. The program is stored into the secondary memory. The memory management unit (MMU) transfers the currently required part of the program from the secondary memory to the main memory for execution. That part of the program is executed through the processor. After execution that part of the program is sent back to the secondary memory together along with the immediate results. After that, the CPU takes the other part of the program for execution. Therefore the main memory always maintains only the currently required part of the program. That type of 'to and fro' movement instructions and data among the main memory and secondary memory is termed as swapping. Therefore a program requiring more memory space than the ability of the main memory can executed by using a swapping technique. This concept is termed as a virtual memory technique.