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 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 -:

	to Dik Leatherdale’s Atlas emulator web page where you may download a rather out of date version of the emulator. The emulator runs satisfactorily under Windows XP/Vista but under later versions of Windows the built-in help file doesn’t work (though you will find one which works on the same web page). 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. It is hoped to release a newer version of this emulator which will support more of the Supervisor functionality.
	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.