How virtual Memory works?

The virtual memory system allows only part of the memory required by the process to be resident in physical memory. Say, if a process requires 500 mb memory, but it may only have 20% memory sitting in physical memory, the other 80% may actually stay on the disk.

Demand Paging

1>When a process requests a block of memory, it check the internal page table for this process to determine whether the reference was a valid or invalid memory access.
2>If the reference is invalid, we terminate the process. If it was valid, but we have not yet brought in that page, we now page it in.
3>We find a free frame by taking one from the free-frame list.
4>We schedule a disk operation to read the desired page into the new allocated frame.
5>When the disk operation is complete, we modify the internal table kept with the process and the page table to indicate that the page is now in memory.
6>We restart the instruction that was interrupted by the illegal address trap.