what is virtual memory?

 Virtual memory: It doesn't exist . It is just an illusion. If our program size is more than main memory . It divides the program in the main memory size. Then turn by turn execute the program. 

• Virtual memory gives an illusion to the program that programs of larger size than actual physical memory can be executed.
• It is the feature or concept of OS . It happens when program size is more than the size of main memory.
• It is the feature of OS where large programs can store themselves in form of pages while their execution only the required pages or portion of processes are loaded into the main memory .
• Virtual memory can be implemented by demand paging and demand segmentation.(paging-equal size of pages. segmentation- size of pages are not equal) 
• Virtual memory doesn't really exists , but the part of secondary memory are made as virtual memory.

  FOR EXAMPLE : If the size of program is 60kb but the size of main memory is 20kb. It divides the program into 3 parts. 20kb ,20kb , 20kb . One by One it will execute but for user it happens all at once. For that reason we say it is an illusion .