Turing´s Pilot ACE: Why Not Important?

author: Frederick P. Brooks, Jr., University of North Carolina at Chapel Hill
published: Sept. 10, 2012,   recorded: June 2012,   views: 4274


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Alan Turing’s contribution to the theory of computing is unmatched. Here we look beyond that, at the architecture and implementation of his proposed computer, the National Physical Laboratory’s Pilot ACE.

Architecturally, the ACE is the very epitome of an “unclean” machine. This is understandable, given the stated objectives of Turing and the team: to build a running computer with a little equipment as possible. Hence the architecture follows directly from the implementation and the technologies chosen.

The architecture represents a wide deviation from that proposed in Burks, Goldstine, and von Neumann’s “Preliminary Discussion of the Logical Design of an Electronic Computing Instrument” (1946). It introduces several important new concepts. The most important is the use of chained instructions rather than sequential ones, to enable optimum placement of instructions and data in the serial main memory. Each instruction contains the (highly abbreviated) address of the next instruction. This technique became familiar via the popular IBM 650 (1954) A second important innovation is the one-instruction transfer of a variable-sized block of words to/from the backing store. The I/O provisions are exceptional for the date and substantially enhanced the usefulness of the machine.

Some architectural decisions seem quite ill-considered, such as the delegation of multiplication to a subroutine. Wilkes’ deep experience with scientific computing on desk calculators gave him a better instinctive grasp of operation relative frequencies, so his EDSAC didn’t embody this mistake. The ACE also embodies the (universal) mistake of radically underestimating the need for main memory and the provision of adequate addressing for sizes quite beyond the initially affordable. NPL had various troubles realizing the machine, so it didn’t operate until October, 1950, and couldn’t be harnessed for use until May, 1951, almost two years after Kilburn’s Manchester Baby (1948) and Wilkes’s Cambridge EDSAC (1949). It served as a well-used scientific computer for some years, and spawned descendents: the English Electric ACE, the EE Deuce, the Bendix (later CDC) G-15 and G-20.

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