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Experience has shown that the sections of an electronic digital computer which are easiest to maintain are those which have a simple logical structure. Not only can this structure be readily borne in mind by a maintenance engineer when looking for a fault, but it makes it possible to use fault-locating programmes and to test the equipment without the use of elaborate test gear. It is in the control section of electronic computers that the greatest degree of complexity generally arises. This is particularly so if the machine has a comprehensive order code designed to make it simple and fast in operation. In general, for each different order in the code some special equipment must be provided, and the more complicated the function of the order the more complex this equipment. In the past, fear of complicating unduly the control circuits of the machines has prevented the designers of electronic machines from providing such facilities as orders for floating-point operations, although experience with relay machines and with interpretive subroutines has shown how valuable such orders are. This paper describes a method of designing the control circuits of a machine which is wholly logical and which enables alterations or additions to the order code to be made without ad hoc alterations to the circuits. An outline of this method was given by one of us [Wilkes, 1951] at the Conference on Automatic Calculating Machines at the University of Manchester in July 1951.

The operation called for by a single machine order can be broken down into a sequence of more elementary operations; for example, shifting a number in the accumulator one place to the right may involve, first, a transfer of the number to an auxiliary shifting register, and secondly, the transfer of the number back to the accumulator along an oblique path. These elementary operations will be referred to as micro-operations. Basic machine operations, such as addition, subtraction, multiplication, etc., are thought of as being made up of a micro-programme of micro- operations, each micro-operation being called for by a micro- order. The process of writing a micro-programme for a machine order is very similar to that of writing a programme for the whole calculation in terms of machine orders.

For the method to be applicable it is necessary that the machine should contain a suitable permanent rapid-access storage device in which the micro-programme can be held-a diode matrix is proposed in the case of the machine discussed as an example below-and that means should be provided for executing the micro-orders one after the other. It is also necessary that provision should be made for conditional micro-orders which play a role in micro-programming similar to that played by conditional orders in ordinary programming.

Since the only feature of the machine which has to be designed specially for any particular set of machine orders is the configuration of diodes in the matrix, or the corresponding configuration in whatever equivalent device is used, there is no difficulty in making changes to the order code of the machine if experience shows them to be desirable; in fact, the design of the machine in the first place can be carried out completely without a firm decision on the details of the order code being taken, as long as care is taken to provide accommodation for the greatest number of micro-orders that are likely to be required. It would even be possible to have a number of interchangeable matrices providing for different order codes, so that the user could choose the one most suited to his particular requirements.

2. Description of the Proposed System

The system will be described in relation to a parallel machine having an arithmetical unit designed along conventional lines. This will contain a set of registers and an adder together with a switching system which enables the micro-operations in the various machine orders to be performed. Some of the micro- operations will bq~4mple transfers of a number from one register to another with or without shifting of the number one place to the left or the right, while others will also involve the use of the adder. Any particular micro-operation can be performed by applying pulses simultaneously to the appropriate gates of the switching system. In certain cases it may be possible for two or more micro-operations to take place at the same time.

It will be convenient to regard the control system as consisting of two pads. A register is needed to hold the address of the next order due to be executed, and another to hold the current order while it is being executed, or at any rate during part of that time. Some means of counting the number of steps in a shifting operation or a multiplication must also be provided. One method of meeting these requirements is to provide a group of registers and an adder together with a switching system which enables transfers of numbers, with or without addition, to be made. This part of the control system will be called the control register unit In any case the operations which need to be performed on the numbers standing in the control register unit during the execution of an order are, like the operations performed in the arithmetical

1Proc. Cambridge Phil. Soc., pt. 2, vol. 49, April 1953, pp. 230-238.

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