Q. What is a virtual address?

Von Neumann had suggested that execution of a program is possible only if program and data are residing in memory. In such a condition program length along with data and other space required for execution can't exceed total memory. Though it was found that at time of execution the complete part of instruction and data is not required as most of the time just few areas of program are being referenced. Keeping this in mind a new concept was putforward where only a required part is kept in memory whereas the rest of program and data reside in secondary storage. Program or data part that are stored on secondary storage are brought to memory whenever needed and part of memory that is not required is returned to the secondary storage. So a program size bigger than actual physical memory can be executed on that machine. This is known as virtual memory.