Why we need Virtual memory?

@Mahesh Dahale • 16 Nov, 2009 Windows XP Professional uses a process called demand paging to exchange data between random access memory (RAM) and paging files on the hard disk. When you install Windows XP Professional, Setup creates a virtual-memory paging file, PAGEFILE.SYS, on the partition in which you installed
Windows XP Professional. The default or recommended paging file size for Windows XP Professional is equal to 1.5 times the total amount of RAM. For best results, never set the value of the paging file size to less than the recommended amount. Typically, you can leave the size of the paging file set to the default value and let Windows XP Professional manage the file size. In some circumstances, such as when you run a large number of applications simultaneously, you might find it advantageous to use a larger paging file or multiple paging files.

by this you will get an idea

@Sahithi Pallavi • 16 Nov, 2009 Thanks for your information mahesh...!

@faizaan • 17 Nov, 2009 virtual memory is one of the most important concept which must be properly understood in order to know how actually operating system allows those large programs (programs larger than size of main memory) to run without any problem.

As you know that program is divided into number of pages of some fixed size .In olden days to run a program all pages of that program need to be present in main memory but as time passed and multi programming came into picture & program sizes increased drastically so it was not possible to keep all pages of all programs in main memory but yes each program can have all its pages on secondary storage (disk).So virtual memory allows us to keep only those pages currently needed by our program & keep all other pages on secondary storage so when ever there is need of page it can be brought from disk.

Thus we can say that virtual memory allows to keep copy of whole address space of process in secondary storage.

We need to do extra work of using logical address & mapping from pages in disk to pages in main memory in order to run program that is much larger than our actual physical main memory.

Please correct me if i am wrong as i am also student studying o.s in current semister.

@Sahithi Pallavi • 18 Nov, 2009 Thank you fiazaan...!