History

In the 1940s and 1950s, before the development of a virtual memory, all larger programs had to contain logic for managing two-level storage (primary and secondary, today's analogies being RAM and hard disk), such as overlaying techniques. Programs were responsible for moving overlays back and forth from secondary storage to primary.

The main reason for introducing virtual memory was therefore not simply to extend primary memory, but to make such an extension as easy to use for programmers as possible.

Many systems already had the ability to divide the memory between multiple programs (required for multiprogramming and multiprocessing), provided for example by "base and bounds registers" on early models of the PDP-10, without providing virtual memory. That gave each application a private address space starting at an address of 0, with an address in the private address space being checked against a bounds register to make sure it's within the section of memory allocated to the application and, if it is, having the contents of the corresponding base register being added to it to give an address in main memory. This is a simple form of segmentation without virtual memory.

Virtual memory was developed in approximately 1959–1962, at the University of Manchester for the Atlas Computer, completed in 1962.^[3] However, Fritz-Rudolf Güntsch, one of Germany's pioneering computer scientists and later the developer of the Telefunken TR 440 mainframe, claims to have invented the concept in 1957 in his doctoral dissertation Logischer Entwurf eines digitalen Rechengerätes mit mehreren asynchron laufenden Trommeln und automatischem Schnellspeicherbetrieb (Logic Concept of a Digital Computing Device with Multiple Asynchronous Drum Storage and Automatic Fast Memory Mode).

In 1961, Burroughs released the B5000, the first commercial computer with virtual memory. It used segmentation rather than paging.

Like many technologies in the history of computing, virtual memory was not accepted without challenge. Before it could be implemented in mainstream operating systems, many models, experiments, and theories had to be developed to overcome the numerous problems. Dynamic address translation required a specialized, expensive, and hard to build hardware, moreover initially it slightly slowed down the access to memory.^[2] There were also worries that new system-wide algorithms of utilizing secondary storage would be far less effective than previously used application-specific ones.

By 1969 the debate over virtual memory for commercial computers was over.^[2] An IBM research team led by David Sayre showed that the virtual memory overlay system consistently worked better than the best manually controlled systems.

Possibly the first minicomputer to introduce virtual memory was the Norwegian NORD-1. During the 1970s, other minicomputers implemented virtual memory, notably VAX models running VMS.

Virtual memory was introduced to the x86 architecture with the protected mode of the Intel 80286 processor. At first it was done with segment swapping, which became inefficient with larger segments. The Intel 80386 introduced support for paging underneath the existing segmentation layer. The page fault exception could be chained with other exceptions without causing a double fault.