The Ferranti Atlas 1 was, for a brief while, reckoned to be the most powerful computer in the world. So powerful in fact that it was said that When Atlas goes down, the computing capacity of the entire country is halved.

Fifty years on it is chiefly remembered for being the machine for which the now, almost universal system of Virtual Memory (paging) was invented. But it was more than that. It was a very complex machine with no less than 100 hardware instructions and a repertoire of 262 additional pseudo-instructions (Extracodes).

The Atlas Supervisor (operating system) was also innovative introducing spooling as we now understand it as well giving support for multi-programming at an early date.

Finally the Brooker-Morris Compiler Compiler was a programming language which allowed compiler writers to specify a formal description of a programming languge, match users’ source code against it and generate compiled code much more easily than hitherto.

There are two known Atlas emulators -:

	Dik Leatherdale’s Atlas emulator. The emulator runs Atlas programs with the user being able to stop execution at a given point and execute instructions in one-shot mode in a manner resembling the debugging mode of Microsoft Visual Studio. The emulator has been developed under various version of Windows from XP onwards but most recently under Windows 10. Some original software is available viz. ABL – Atlas Basic Language – the standard assembler. ABL is built in to the emulator, rather than being an Atlas program. Compiler Compiler (CC) – a high(ish) level language devised by Tony Brooker and Derrick Morris which features the ability to automatically parse other high-level languages and thus ease the task of compiler writers OCTAL – a crude loader SPECIAL – A means of invoking compilers not resident on the main software tape or disc. SERVICE – a collection of useful utilities not yet available but “coming soon”. In addition, there is some inauthentic software available FABL – a version of ABL for “compiling” the Atlas Fixed Store. In the real Atlas the Fixed store was in read-only memory and was programmed physically, but in the emulator it is more convenient to compile it like any other program. ABL is built in to the emulator, rather than being an Atlas program. MAGUTILS – like SERVICE a collection of useful utilities this time aimed at manipulating the contents of tapes and discs. MAGUTILS was written to gain experience in using the Compiler Compiler and also includes a lerge amount of ABL code. RESTART – An enhanced version of OCTAL able to restart programs which have been checkpointed by the emulator user during execution. The help file can be found here
	to Prof. Roland Ibbett’s web pages where may be found a rather different Atlas Simulator which runs in the HASE simulation environment. HASE is available for Windows Vista onwards, Linux or OSX. This simulator animates the internal workings of the Atlas Computer showing the information flows around the processor.

Both the emulators described here were demonstarated during the celebrations for the 50^th anniversary of Atlas in 2012.