**Chapter 4**

Preliminary discussion of the logical design of an electronic computing instrument^{1}

*Arthur W. Burks */ *Herman H. Goldstine */ *John von Neumann*

PART I

1. Principal components of the machine

1.1. Inasmuch as the completed device will be a general-purpose computing machine it should contain certain main organs relating to arithmetic, memory-storage, control and connection with the human operator. It is intended that the machine be fully automatic in character, i.e. independent of the human operator after the computation starts. A fuller discussion of the implications of this remark will be given in Sec. 3 below.

1.2. It is evident that the machine must be capable of storing in some manner not only the digital information needed in a given computation such as boundary values, tables of functions (such as the equation of state of a fluid) and also the intermediate results of the computation (which may be wanted for varying lengths of time), but also the instructions which govern the actual routine to be performed on the numerical data. In a special-purpose machine these instructions are an integral part of the device and constitute a part of its design structure. For an all-purpose machine it must be possible to instruct the device to carry out any computation that can be formulated in numerical terms. Hence there must be some organ capable of storing these program orders. There must, moreover, be a unit which can understand these instructions and order their execution.

1.3. Conceptually we have discussed above two different forms of memory: storage of numbers and storage of orders. If, however, the orders to the machine are reduced to a numerical code and if the machine can in some fashion distinguish a number from an order, the memory organ can be used to store both numbers and orders. The coding of orders into numeric form is discussed in 6.3 below.

1.4. If the memory for orders is merely a storage organ there must exist an organ which can automatically execute the orders stored in the memory. We shall call this organ the *Control.*

1.5*. *Inasmuch as the device is to be a computing machine there must be an arithmetic organ in it which can perform certain of the elementary arithmetic operations. There will be, therefore, a unit capable of adding, subtracting, multiplying and dividing. It will be seen in 6.6 below that it can also perform additional operations that occur quite frequently.

The operations that the machine will view as elementary are clearly those which are wired into the machine. To illustrate, the operation of multiplication could be eliminated from the device as an elementary process if one were willing to view it as a properly ordered series of additions. Similar remarks apply to division. In general, the inner economy of the arithmetic unit is determined by a compromise between the desire for speed of operation-a non-elementary operation will generally take a long time to perform since it is constituted of a series of orders given by the control-and the desire for simplicity, or cheapness, of the machine.

1.6. Lastly there must exist devices, the input and output organ, whereby the human operator and the machine can communicate with each other. This organ will be seen below in 4.5, where it is discussed, to constitute a secondary form of automatic memory.

2. First remarks on the memory

2.1. It is clear that the size of the memory is a critical consideration in the design of a satisfactory general-purpose computing