[2] It is notable for being the first machine with virtual memory (at that time referred to as "one-level store"[3]) using paging techniques; this approach quickly spread, and is now ubiquitous.

Called the Titan, or Atlas 2,[4] it had a different memory organisation and ran a time-sharing operating system developed by Cambridge University Computer Laboratory.

Two further Atlas 2s were delivered: one to the CAD Centre in Cambridge (later called CADCentre, then AVEVA), and the other to the Atomic Weapons Research Establishment (AWRE), Aldermaston.

[8] Through this period, Tom Kilburn's team at the University of Manchester had been experimenting with transistor-based systems, building two small machines to test various techniques.

This was clearly the way forward, and in the autumn of 1956, Kilburn began canvassing possible customers on what features they would want in a new transistor-based machine.

[12] This led to David Howarth, newly hired at Ferranti, expanding the operating system team from two to six programmers.

led by the tireless and energetic Howarth (who completed his Ph.D. in physics at age 22), the team eventually delivered a Supervisor consisting of 35,000 lines of assembler language which had support for multiprogramming to solve the problem of peripheral handling.

Atlas met this goal, and was officially commissioned on 7 December by John Cockcroft, director of the AEA.

[15] Ferranti sold two other Atlas installations, one to a joint consortium of University of London and BP in 1963, and another to the Atomic Energy Research Establishment (Harwell) in December 1964.

The AEA machine was later moved to the Atlas Computer Laboratory at Chilton, a few yards outside the boundary fence of Harwell, which placed it on civilian lands and thus made it much easier to access.

Nevertheless, the head of Ferranti's Software Division, Hugh Devonald, said in 1962: "Atlas is in fact claimed to be the world's most powerful computing system.

Among its admirers was C. Gordon Bell of Digital Equipment Corporation, who later praised it: In architecture, the Manchester Atlas was exemplary, not because it was a large machine that we would build, but because it illustrated a number of good design principles.

Atlas was multiprogrammed with a well defined interface between the user and operating system, had a very large address space, and introduced the notion of extra codes to extend the functionality of its instruction set.

[21] The machine had many innovative features, but the key operating parameters were as follows (the store size relates to the Manchester installation; the others were larger): Atlas did not use a synchronous clocking mechanism — it was an asynchronous processor — so performance measurements were not easy, but as an example: One feature of the Atlas was "Extracode", a technique that allowed complex instructions to be implemented in software.

They were used to call mathematical procedures which would have been too inefficient to implement in hardware, for example sine, logarithm, and square root.

Typical examples would be "Print the specified character on the specified stream" or "Read a block of 512 words from logical tape N".

Other UK machines of the era, such as the Ferranti Orion, had similar mechanisms for calling on the services of their operating systems.

Being a university computer it was patronised by a large number of the student population, who had access to a protected machine code development environment.