Virtual_memory

Virtual memory is a feature of a computer program (typically an Operating System or similar low-level system) that allows Secondary Storage (such as a hard disk) to be used as extra Primary Storage (RAM). For example, a computer with 200MB of RAM may use 200MB of hard disk space to allow users to run software occupying 400MB or more by swapping pieces of the running programs in and out of main memory as needed.

Modern microprocessors (CPUs) have built-in support for this functionality, without which such systems would suffer greatly decreased performance.