Atlas (computer)

Atlas

The Atlas was the machine that invented your operating system. Built by Tom Kilburn's team at Manchester with Ferranti and Plessey — commissioned on 7 December 1962 by John Cockcroft — it was, for a while, credibly the most powerful computer on Earth; the saying went that when Atlas went offline, half of Britain's computing capacity disappeared. Only three were built (plus three Atlas 2s), but what ran on them redefined what a computer is.^[1]

The machine began as MUSE — the microsecond engine, named for the Atomic Energy Authority's spec that it execute one instruction per microsecond (1 MIPS, in modern units). To keep that rate while feeding a menagerie of peripherals, Atlas's designers decoupled the program's view of memory from the hardware's: the programmer saw a single flat "one-level store," while the machine silently shuffled 512-word pages between fast core and slow drum — the invention of paged virtual memory, now so ubiquitous it's hard to name a machine without it.^[2]

The Supervisor, and the first kernel

Running it all was the Supervisor — 35,000 lines of assembler delivered by David Howarth's six-person team in what contemporaries called a Herculean effort — the first true operating-system kernel: it multiplexed the machine among programs (multiprogramming), arbitrated peripherals, and exposed hardware services through extracodes — software-implemented pseudo-instructions, the ancestor of the system call. Atlas shipped in 1962 initially running only an Autocode compiler; the full Supervisor, with ALGOL 60 and Fortran, arrived January 1964. The University ran it twenty hours a day, up to a thousand programs at a stretch, renting spare cycles to Ferranti's customers at £500 an hour.^[3]

Where it sits

Atlas's offspring are everywhere and invisible: virtual memory, the supervisor/kernel split, multiprogramming, the system-call interface — and the Cambridge Titan (Atlas 2) that carried CPL's only real implementation. Three machines, a decade of service, and the permanent invention of how computers lie to their programs.

Check your understanding

• What did MUSE stand for, and what performance spec drove it?
• What is the "one-level store" — what we now call what?
• What was the Supervisor, and what mechanism is the ancestor of the system call?
• What was Atlas's original only-language at commissioning?