96 Part 2 ÷ The instruction-set processor: main-line computers

Section 1 ÷ Processors with one address per instruction

than electronic devices. It must be fast enough so that, even when it has to be used frequently in a problem, a large percentage of the total solution time is not spent in getting data into and out of this medium and achieving the desired positioning on it. If this condition is not reasonably well met, the advantages of the high electronic speeds of the machine will be largely lost.

Both light- or electron-sensitive film and magnetic wires or tapes, whose motions are controlled by servo-mechanisms integrated with the control, would seem to fulfil our needs reasonably well. We have tentatively decided to use magnetic wires since we have achieved reliable performance with them at pulse rates of the order of 25,000/sec and beyond.

4.6. Lastly our memory hierarchy requires a vast quantity of dead storage, i.e. storage not integrated with the machine. This storage requirement may be satisfied by a library of wires that can be introduced into the machine when desired and at that time become automatically controlled. Thus our dead storage is really nothing but an extension of our secondary storage medium. It differs from the latter only in its availability to the machine.

47. We impose one additional requirement on our secondary memory. It must be possible for a human to put words on to the wire or other substance used and to read the words put on by the machine. In this manner the human can control the machine's functions. It is now clear that the secondary storage medium is really nothing other than a part of our input-output system, cf. 6.8.4 for a description of a mechanism for achieving this.

4.8. There is another highly important part of the input-output which we merely mention at this time, namely, some mechanism for viewing graphically the results of a given computation. This can, of course, be achieved by a Selectron-like tube which causes its screen to fluoresce when data are put on it by an electron beam.

4.9. For definiteness in the subsequent discussions we assume that associated with the output of each Selectron is a flip-flop. This assemblage of 40 flip-flops we term the Selectron Register.

5.1. In this section we discuss the features we now consider desirable for the arithmetic part of our machine. We give our tentative conclusions as to which of the arithmetic operations should be built into the machine and which should be programmed. Finally, a schematic of the arithmetic unit is described.

5.2. In a discussion of the arithmetical organs of a computing machine one is naturally led to a consideration of the number system to be adopted. In spite of the longstanding tradition of building digital machines in the decimal system, we feel strongly in favor of the binary system for our device. Our fundamental unit of memory is naturally adapted to the binary system since we do not attempt to measure gradations of charge at a particular point in the Selectron but are content to distinguish two states. The flip-flop again is truly a binary device. On magnetic wires or tapes and in acoustic delay line memories one is also content to recognize the presence or absence of a pulse or (if a carrier frequency is used) of a pulse train, or of the sign of a pulse. (We will not discuss here the ternary possibilities of a positive-or-negative-or-no-pulse system and their relationship to questions of reliability and checking, nor the very interesting possibilities of carrier frequency modulation.) Hence if one contemplates using a decimal system with either the iconoscope or delay-line memory one is forced into a binary coding of the decimal system-each decimal digit being represented by at least a tetrad of binary digits. Thus an accuracy of ten decimal digits requires at least 40 binary digits. In a true binary representation of numbers, however, about 33 digits suffice to achieve a precision of 10¹⁰. The use of the binary system is therefore somewhat more economical of equipment than is the decimal.

The main virtue of the binary system as against the decimal is, however, the greater simplicity and speed with which the elementary operations can be performed. To illustrate, consider multiplication by repeated addition. In binary multiplication the product of a particular digit of the multiplier by the multiplicand is either the multiplicand or null according as the multiplier digit is 1 or 0. In the decimal system, however, this product has ten possible values between null and nine times the multiplicand, inclusive. Of course, a decimal number has only 1og₁₀2 ~ 0.3 times as many digits as a binary number of the same accuracy, but even so multiplication in the decimal system is considerably longer than in the binary system. One can accelerate decimal multiplication by complicating the circuits, but this fact is irrelevant to the point just made since binary multiplication can likewise be accelerated by adding to the equipment. Similar remarks may be made about the other operations.

An additional point that deserves emphasis is this: An important part of the machine is not arithmetical, but logical in nature. Now logics, being a yes-no system, is fundamentally binary. Therefore a binary arrangement of the arithmetical organs contributes very significantly towards producing a more homogeneous machine, which can be better integrated and is more efficient.

The one disadvantage of the binary system from the human point of view is the conversion problem. Since, however, it is completely known how to convert numbers from one base to