Virtual Memory.

Virtual memory is the simulation of a storage space so large that programmers do not need to reprogram or recompile their works when the capacity of a local memory or the configuration of a network changes. The name, borrowed from optics, recalls the virtual images formed by mirrors and lenses--images that are not there but behave as if they are. The designers of the Atlas Computer at the University of Manchester invented virtual memory in the 1950s to eliminate a looming programming problem: planning and scheduling data transfers between main and secondary memory and recompiling programs for each change of size of main memory. Virtual memory is even more useful in the computers of the 1990s, which have more things to hide-on-chip caches, separate RAM chips, local disk storage, network file servers (q.v.), large numbers of separately compiled program modules, other computers on the local bus or local network, or the Internet. The story of virtual memory from then to now is a story about machines helping programmers solve problems in storage allocation, protection of information, sharing and reuse of objects, and linking of program components. Virtual memory, common in all computers and operating systems from the smallest microprocessor to the largest supercomputer, is now invading the Internet.