Unwinding performance and power on Colossus, an unconventional computer

Abstract

In 1944 the computing machine known as Colossus became operational in support of British cryptanalysis and decryption of German High Command wireless traffic. This first electronic digital and very unconventional computer was not a stored-program general purpose computer in today’s terms, despite printed claims to the contrary. At least one of these asserts Colossus was a Turing machine. While an appropriate Turing machine can simulate the operation of Colossus, that is not an argument for generality of computation. Nor does the behavior of Colossus resemble that of a Turing machine, much less a universal Turing machine (UTM). Nonetheless, we shall see that a UTM could have been implemented on a clustering of the ten Colossus machines installed at Bletchley Park, England, by the end of WWII in 1945. Improvements require even fewer machines. Several advances in input, output, speed, processing, and applications—within the hardware capability of the time and respectful of the specification of Colossus—are also offered.