General principles of the design of all-purpose computing machines

Professor Hartree in his paper has recalled that all the essential ideas of the general-purpose calculating machines now being made are to be found in Babbage’s plans for his analytical engine. In modern times the idea of a universal calculating machine was independently introduced by Turing (1938) in connexion with a logical problem, which there is unfortunately no time to mention, and the construction of actual machines was begun independently in America, towards the end of the late war. A ‘universal’ machine is one which, when given suitable instructions, will carry out automatically any well-defined series of computations of certain specified kinds, say additions, subtractions, multiplications and divisions of integers or finite decimals. This is a rather doubtful definition, since it depends on what is meant by a ‘welldefined’ series of computations; and undoubtedly the best definition of this is ‘one that can be done by a machine ’. However, this description is not quite so circular as it may seem; for most people have a fairly clear idea of w hat processes can be done by machines specially constructed for each separate purpose. There are, for example, machines for solving sets of linear algebraic equations, for finding the prim e factors of large integers, for solving ordinary differential equations of certain types, and so on. A universal machine is a single machine which, when provided with suitable instructions, will perform any calculation that could be done by a specially constructed machine. No real machine can be truly universal because its size is limited—for example, no machine will work out π to lO¹⁰⁰⁰ places of decimals, because there is no room in the world for the working or the answer; but subject to this limitation of size, the machines now being made in America and in this country will be ‘ universal ’ —if they work a t all; that is, they will do every kind of job that can be done by special machines.