Virtual memory is a model—one of many possible models—for managing the resource of physical memory, or main memory. Such management is necessary because a microprocessor, the heart of a computer, has direct access only to main memory, while all programs and data are stored on permanent media such as hard disks.

Reading or writing main memory is as simple as executing a single computer instruction. In contrast, any access to hard disks, digital versatile discs (DVDs), compact discs-read only memory (CD-ROMs), or floppy disks is indirect and requires relatively complex communication protocols involving dozens to thousands of computer instructions. Therefore, accessing a file or running a program requires that the data on disk first be moved into main memory. Virtual memory is one method for handling this management of data. To illustrate what it is and how it differs from other possibilities...