Nature, Vol 164, Oct 22,1949, p684.

By Dr. T. KILBURN

Reprinted with permission from Nature 164, 684 Copyright (1949) Macmillan Magazines Limited.

THE electronic computing machine described here has been developed in the Electrical Engineering Laboratories at the University of Manchester under the general direction of Prof F.C. Williams, and with the active assistance of the Telecommunications Research Establishment, Malvern.

The use of electronics in machines of this type follows from the
requirement for high-speed operation. A further consequence of this
requirement is the desire to eliminate, so far as possible, the
necessarily slow process of human intervention, which is essential at every
stage of the solution if conventional methods of computing are used. The
minimum extent to which this intervention could be conveniently reduced would be
to present to the machine an oral or written statement of the problem, in much
the same way as
problems are presented to a human computer. Research in electrical engineering
and mathematics has produced computing machines at Manchester, Cambridge and in
the United States, of a type which approximate to this ideal. However, a simple
statement of the problem is insufficient. The problem must be programmed, that
is, it must be split up into a series of simple operations which the machine can
perform. Each operation is represented by an instruction, and these
instructions, and the data which they control, are coded in a form which the
machine can interpret. During the actual solution of the problem by the machine,
however, the need for human intervention is entirely eliminated, for all the
data - instructions and numbers - are loaded into the machine initially. The
solution then occurs at a speed determined only by the the electronic circuits
used. To permit this initial loading, the machine must have a 'memory' or
storage facility. One storage system used in the Manchester machine has been
fully described elsewhere^{1}. It is clear that as the capacity of the storage is
increased, the programming of problems becomes easier; for mathematical
operations common to many problems may be programmed, and introduced permanently
into the storage of the machine. It will then be necessary to construct only
'master programmes', which call upon these 'sub-programmes' appropriately, and
a nearer approximation to the ideal is achieved without further engineering
being necessary.

The important engineering requirements resulting from the above discussion are as follows:

- representation and interpretation of data;
- storage, and input and output devices; and
- computing circuits.

These items are now discussed with reference to the machine at Manchester.

The binary system of numbers^{1} is used throughout the machine, because of the
simplicity and economy with which it can be represented electrically. In this
system of numbers, it is only necessary to represent the digits 0 and 1. Thus,
in a machine operated in the 'series' mode, the binary number 10101 (decimal 21)
is conveniently represented by a train of negative pulses occurring sequentially
on a single wire, as shown in Fig. 1.

Now, if the stored items of data are considered to exist in numbered
'addresses', *s*,
in the main storage of the machine, and if, further, the operations or
functions, *f*, which the machine can perform are numbered, then it follows that
an instruction, in its simplest form, consists of two numbers (*s*,*f*), and may be
represented in the same way as a number. In fact, instructions and numbers
differ only in the way in which they are used by the machine. Instructions are
converted from the 'dynamic' form shown in Fig. 1 into a 'static' form by a set
of trigger circuits called a 'staticisor', and are held
in this condition for sufficient time to enable, say, a stored number to be
'read' from a selected address, *s*, and transferred to a selected
computing circuit *f*.

It will be seen that instructions specify addresses, and not the contents of those addresses. Thus, a programme is universal in the sense that, once devised, it can be used to operate on different sets of data without modification.

The main storage of the machine, which initially holds all the data relevant to a problem, is now discussed.

An important feature of any storage system is its accessibility ratio

wherealpha=t_{1}/t_{2},

Clearly, it is necessary that the part of the storage to which access is
frequently made should have a value of *alpha* approaching the optimum value of
unity, if the time taken in solving a problem is not to be extended by barren
periods of waiting. In the electronic storage system^{1} used, *alpha* = 1. However, it
is unnecessary to have *alpha* = 1 for all the storage, and, in fact, a magnetic
storage system, for which *alpha* = 1/133, is used to supplement the electronic system.
This is made practicable by transferring blocks (tracks) of information between
the magnetic and electronic systems; and by having sufficient electronic storage
to ensure that, on the average, access to the magnetic storage is not a sufficiently
frequent occurrence to affect appreciably the time of solution. With this
proviso, magnetic storage is a useful, if not indispensable, adjunct in view of
its compactness, permanence of record, and reliability.

A photograph of the face of an electronic storage cathode ray tube, during
the course of the solution of a problem, is shown in Fig. 2. The
binary digits 0 and 1 can be seen in the photograph, where they appear as dots,
and vertical dashes, respectively. A single storage tube has a capacity of 2,560
digits, arranged in two rasters of television type, where each
raster has 32 lines, and each line of a raster holds 40
digits. The lines of the rasters, each of which holds
one 40-digit number or two 20-digit instructions,
correspond with the storage addresses previously
mentioned. It will be seen that the 40 digits of a line
are arranged in 8 groups of 5, which is convenient for
visual reading of data, and corresponds with teleprinter
practice. Input is at present achieved by a keyboard,
arranged in the form of a typewriter, by which the state of
any digit in the store can be changed from 0 to 1, or *vice versa*.
Output is achieved by visual reading from the known answer
lines, in teleprinter code, by inspection of a cathode ray tube
face. These methods of input and output will very
shortly be replaced by teleprinter apparatus. The
conversion from decimal to binary, and from binary
to decimal numbering, at the input and output,
respectively, will be achieved by the machine itself.

The magnetic system of storage^{2} uses a 12-in.
diameter nickel-plated drum, rotating in synchronism
with the scanning of the electronic storage tubes.
Each recording track on the drum holds the same
number of digits as a cathode ray tube, and the
synchronize method of operation ensures one-to-one
correspondence between digit position on track and
tube.

The present machine has an electronic storage capacity of 5,120 digits, and a magnetic storage capacity of 40,960 digits.

In deciding whether or not to include a certain type of computing element in the machine, the frequency of occurrence of that computing operation must be balanced against the time taken for the machine to obey a sub-programme designed to carry out the operation. Thus, special circuits are provided to perform addition, subtraction, multiplication and certain logical operations, but a sub-programme is used to perform division.

The component elements of the computing and
other circuits used are similar to those used in
videopulse circuits in radar^{3}. As an example, it will
therefore be sufficient to consider schematically one
type of adding circuit. The rules of binary addition
are such that

x+y+w>= 2 implies a 1 inw'x+y+w< 2 implies a 0 inw'andx+y+w- 2w'=z,

where *x* and *y* are corresponding digits of the numbers
to be added; *w'* is the carry digit; *w* is the carried
digit (that is, *w'* delayed by one digit period); *z* is the
sum digit; and *x*, *y*, *w'*, *w* and *z* are represented by
zero or unit amplitude pulses.

These rules are conveniently obeyed by the circuit
shown schematically in Fig. 3, the important elements
being the two amplitude discriminators, *P* and *Q*.
These are arranged to deliver an output pulse during
a digit period, if the sum of the pulses in the input
pulse trains during that period is equal to or greater
than 2 and greater than 0 respectively. Discriminator *P* determines whether a 1 is to be carried, and discriminator *Q* delivers the output *z*.

A subtracting circuit may be constructed in a similar manner to obey the rule of subtraction.

In multiplication (the circuit used was developed by A. A. Robinson) the 80-digit product is formed by repeated addition to a running total of the multiplicand, which is suitably shifted (delayed in time) for each 1 in the multiplier.

In addition to these arithmetical circuits, it is useful to be able to operate on data digit by digit, carried digits being suppressed. For this purpose circuits which are much simpler than those mentioned above are provided to perform the logical operations 'and', 'or' and 'not equivalent to'.

Associated with the computing circuits is a storage
tube called the accumulator, which holds sufficient
lines (two) to record the partial answers. An elementary
computation is, in general, performed upon a
number, *x*, obtained by reading the accumulator, and
a number, *y*, obtained by reading a selected address
in the main storage. The answer, *z*, is written into
the accumulator over the previous contents, *x*. At
any desired stage of the solution, a partial answer may
be read from the accumulator and written into a
selected line of the main storage, as a result of a
suitable instruction.

The most important remaining feature of the machine is the control of the order in which the instructions contained in a programme are obeyed, and this is now described, with reference to the schematic diagram of the machine, shown in Fig. 4.

On the left of the figure is 'Control' *C*, an electronic storage tube,
holding two lines, which operates in conjunction with an adding circuit. When a
new instruction is required from the instructions stored in one of the main
electronic storage tubes, a 'prepulse' initiates a standard sequence of events.
Initially, unity is, in general, added to the control instruction line of *C* (*CI*), and the new content of *CI* controls the 'staticisor', *S*. The new, or
'present', instruction (*PI*) is then selected from the main electronic storage,
and written on the *PI* line of *C*, as a result of the condition of *S*. During this
initial phase of the operation, the content of *CI* has operated as an instruction
to extract the next present instruction from the storage. During the final phase
of the operation *PI*, not *CI*, controls *S*. Thus in general *PI* selects a line of
the storage, and causes some function to be performed on the contents of that
line. When the present instruction has been obeyed, a further prepulse is
given automatically, and the standard sequence is
repeated. The time elapsing between adjacent prepulses is, except when
multiplying, 1.8 m.sec.

It will be understood that the effect of adding unity to *CI*, after each
prepulse, is to cause the instructions in a programme to be obeyed sequentially.
Although, in general, this is required, some alternative must be available, if
only to prevent instructions being consumed and made redundant at the rate of
about 500 per sec. In fact, the same instructions are used repeatedly during the
solution of a problem, and this is made possible by having the facility called
'transfer of control', by which the place in the list of instructions, as
'remembered' by *CI*, can be changed when desired. Such a transfer may be
unconditional or conditional. In the unconditional case, use is made of one of
two instructions which cause a chosen number to be read from the main electronic
storage, and either written over or added to the content of *CI*. In the
conditional case, the programme is arranged so that the sign of the content of
the accumulator, *A*, as determined as a 'test' instruction, is the criterion.
(A negative number -*q* is represented by 2^{40} - *q*.) If the content of *A*, which is
indicative of the present state of solution, is positive or zero, unity is added
to *CI* after the next prepulse, in the normal manner. If, however, the content is
negative, 2 is added to *CI* at this time, and one instruction in the list is
omitted. If this instruction, which may or may not be omitted, is one of the
type which modifies *CI*, a branching point has been created in the list of
instructions, and the branch taken is conditioned by the present state of the
solution.

On the right of Fig. 4 the accumulator *A* and its associated computing
circuits, any of which may be switched into play by appropriate instructions,
are indicated. Multiplication involves the use of the tube *M*, the multiplicand
being stated on the *D* line, and the multiplier on the *R* line. Products are added
into the accumulator automatically.

Storage tube *B* is a valuable auxiliary to the machine and is used for
modifying instructions. Instructions can, of course, be modified by the normal
processes (*A* and *M*, etc.) in the same way as numbers, but this is often
inconvenient and wasteful of time and storage space. Therefore each instruction
(*s*,*f*) is preceded by a single digit called the *b* digit. If *b* = 0, the content of
line *B0* of *B* (normally zero) is added into the present instruction as this
instruction is written into the *PI* line of *C*, that is, before this instruction
is used. If *b* = 1, the content of line *B1* of *B* is used in the same manner. Numbers
are written into *B* from the main electronic storage, exactly as they are written
into *M*, say.

Loading between the magnetic and electronic stores, and *vice versa*, is at
present achieved manually.

Immediate future development of the machine will
include the provision of more lines on the *B* tube, further extension of the
capacity of electronic and magnetic store as required, improved input and output
facilities, and automatic interchange of information between electronic and
magnetic stores.

I wish to acknowledge my indebtedness to Prof. M. H. A. Newman, and Mr. A. M. Turing for much helpful discussion of the mathematical requirements of digital computing machines; and to Mr G. C. Tootill, who has collaborated in the design of the machine.

*References*

- Williams, F.C., and Kilburn, T.,
*J. Inst. Elect. Eng.*,**96**, Pt. III, No. 40 (March 1949). - Unpublished work by J.C. West and G.E. Thomas. For a
description of a similar system, see Booth, A.D.,
*Electronic Eng.*(July 1949). - Williams, F.C.,
*J. Inst. Elect. Eng.*,**93**, Pt. IIIA, 303 (1946).