In a memory hierarchy system, data and programs are first stored in secondary or auxiliary memory. Program and its related data are brought in main memory for execution. What if the size of Memory needed for Program is more than the size of memory? Virtual memory is a idea used in some large computer systems which allow user to construct programs as though a large memory space were available, equal to total of secondary memory. Every address produced by CPU goes through an address mapping from so-called virtual address to a physical address in main memory. Virtual memory is employed to give programmers the illusion that they have a very large memory at their disposal although computer in fact has a relatively small main memory. A Virtual memory system offers a mechanism for translating program-generated addresses in correct main memory locations. This is done dynamically whereas programs are being executed in CPU. The translation or mapping is handled automatically by hardware through a mapping table.