What is virtual memory?

How does it differ from primary memory and secondary memory?

The technique which allows a program to use main memory than what a computer really has is known as virtual memory. It gives an illusion that they have main memory available more than what is physically provided in the computer. The entire program and its data are not place in main memory. Only the instructions and data which are to be currently executed are brought from secondary memory into main memory. The remaining part of programs resides in secondary memory. The addresses which are used in programs are called logical addresses. The addresses indicate the position of instructions and data in the program. The logical addresses are translated into physical addresses by memory management unit (MMU).The CPU generates logical address while executing programs.

Differences of virtual memory from primary memory and secondary memory:

1) Virtual memory enables the execution process on system whose main memory size is less than the total memory required by the process. Whereas main memory is the memory of the computer which is used to run the program. And, secondary memory is the memory of the computer which is used to store the program.

2) Virtual memory is not fixed memory of computer i.e. illusion even it help to run the program on the system. But main memory is a type of memory of the system which is used to store program. And, secondary memory is also type of memory of system which is used to store program data & instruction.

3) Virtual memory has no type but primary memory has two type i.e. RAM & ROM. And, secondary memories are hard disk, floppy disk, CD ROM etc.

4) Virtual memory is not used as backup or permanent storage whereas primary memory is also not used as backup but secondary memory is used as backup. So data and program can be used as present & future reference.

5) Virtual memory is not required if the capacity of program is less than primary memory. Secondary memory is not in used.

6) Virtual memory gets its memory from secondary memory but not primary memory.

8) Virtual memory is used as on-line memory where main memory is also used as on-line memory but secondary is used as online & offline memory.

9) Capacity or size of virtual memory can be increased by increasing of its size in secondary memory with the help of OS. But capacity of primary memory & secondary memory can be increased by adding primary & secondary storage devices.

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