Programming

Of the 20 bits allocated for each program instruction, 10 were used to hold the instruction code, which allowed for 1,024 (210) different instructions. The machine had 26 initially,[10] increasing to 30 when the function codes to programmatically control the data transfer between the magnetic drum and the cathode ray tube (CRT) main store were added. On the Intermediary Version programs were input by key switches, and the output was displayed as a series of dots and dashes on a cathode ray tube known as the output device, just as on the SSEM from which the Mark 1 had been developed. However, the Final Specification in October 1949, benefitted from the addition of a teleprinter with a 5-hole paper-tape reader and punch.[13]
Mathematician Alan Turing, who had been appointed to the nominal post of Deputy Director of the Computing Machine Laboratory at the University of Manchester in September 1948,[10] devised a base 32 encoding scheme based on the standard ITA2 5-bit teleprinter code, which allowed programs and data to be written to and read from paper tape.[19] The ITA2 system maps each of the possible 32 binary values that can be represented in 5 bits (25) to a single character. Thus "10010" represents "D", "10001" represents "Z", and so forth. Turing changed only a few of the standard encodings; for instance, 00000 and 01000, which mean "no effect" and "linefeed" in the teleprinter code, were represented by the characters "/" and "@" respectively. Binary zero, represented by the forward slash, was the most common character in programs and data, leading to sequences written as "///////////////". One early user suggested that Turing's choice of a forward slash was a subconscious choice on his part, a representation of rain seen through a dirty window, reflecting Manchester's "famously dismal" weather.[20]


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