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Chapter 8

The UNIVAC system1

I. Presper Eckert, Jr. / James R. Weiner H. Frazer Welsh / Herbert F. Mitchell

Organization of the UNIVAC system

In March 1951, the first UNIVAC2 system formally passed its acceptance tests and was put promptly into operation by the Bureau of the Census. Since the UNIVAC is the first computer which can handle both alphabetic and numerical data to reach full-scale operation so far, its operating record and a review of the types of problems to which it has been applied provide an interesting milestone in the ever-widening field of electronic digital computers.

The organization of the UNIVAC is such that those functions which do not directly require the main computer are performed by separate auxiliary units each having its own power supply. Thus the keyboard to magnetic tape, punched card to magnetic tape and tape to typewritten copy operations are delegated to auxiliary components.

The main computer assembly includes all of those units which are directly concerned with the main or central computer operations. A block diagram of this arrangement is shown in Fig. 1. All of the elements shown are contained within the central computer casework except the supervisory control desk (SC) and the Uniservos,2 to which the lines in the upper right section of the diagram connect.

The supervisory control, in addition to all the necessary control switches and indicator lights, contains an input keyboard. Also cabled to the supervisory control is a typewriter which is operable by the main computer. By means of these two units, limited amounts of information can be inserted or removed either at the will of the operator or by the programmed instructions.

The input-output circuits operate on all data entering or leaving the computer. The input and output synchronizers properly time the incoming or outgoing data for either the Uniservos (tape devices) or the supervisory control devices. The input and output registers (I and 0) are each 60 word (720 characters) temporary storage registers which are intermediate between the main computer and the input-output devices.

The high-speed bus amplifier is a switching central through which all data must pass during transfer between any arithmetic register and the main memory or between the memory and the input-output registers. The arithmetic registers are shown along the bottom of diagram each connected to the high speed bus system.

The L-, F-, X-, and A-registers are each of one word or 12- character capacity and are directly concerned with the arithmetic operations. The V- and Y-registers are of 2- and 10-word capacity, respectively. They are used solely for multiple word transfers within the main memory. Associated with the arithmetic registers are the algebraic adder (AA), the comparator (CP), and the multiplier-quotient counter (MQC).

Addition-subtraction instructions

The addition-subtraction operations are performed in conjunction with the comparator since all numerical quantities are absolute magnitudes with an algebraic sign attached. Before either an addition or subtraction is performed, the two quantities, one already in the A-register and the other either from the memory or from the X-register, depending upon the particular instruction, are compared for magnitude and sign. The adder inputs can then be switched so as always to produce a noncomplemented result for any operation. The choice of adder input arrangement is therefore under the control of the comparator. The comparator also determines the proper sign for the result according to the usual algebraic rules.

One additional function performed by the comparator for addition and subtraction is to control the complementer. This determination is based upon which operation (+, or -) is indicated, and, whether the signs are like or unlike. For a subtract instruction, the sign of the subtrahend is reversed before entering the comparator. The comparator then compares the signs of the quantities in order to determine whether the two quantities are subtracted or added.

Multiplication instruction

The multiplication process requires the services of the adder, the comparator, the multiplier-quotient counter and the four arithmetic registers. During the first step of multiplication the X-reg-

1AIEE-IRE Conf., 6-16, December, 1951.
2Registered trade mark.


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