Proposed Electronic Calculator

Written at the National Physical Laboratory in late 1945 and put to the executive committee in March 1946. Where von Neumann's First Draft sketched an architecture in abstract terms, Turing supplied circuit diagrams, a complete instruction set, sample programs and a price: roughly £11,200. The machine was to be called ACE, the Automatic Computing Engine — engine being a deliberate nod to Babbage.

His instinct ran against the grain of the era: minimise the hardware, push the complexity into the programs. There was no multiplier unit, because multiplication could be written as a subroutine. Memory was mercury delay lines, and since a delay line hands back its contents strictly in turn, every instruction carried the address of the next one — the programmer was expected to place each instruction exactly where the echo would happen to be when it was wanted. This was called optimum coding, and it was as much fun as it sounds.

NPL never built it. Turing lost patience with the delays and left for Manchester in 1948; a cut-down Pilot ACE finally ran in May 1950. The report circulated internally and was not printed in full until the 1970s, which is part of why the architecture ended up carrying von Neumann's name rather than his.