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94 Part 2 The instruction-set processor: main-line computers

Section 1 Processors with one address per instruction

To summarize, transfers into the memory will be of two sorts:

Total substitutions, whereby the quantity previously stored is cleared out and replaced by a new number. Partial substitutions in which that part of an order containing a memory location-number-we assume the various positions in the memory are enumerated serially by memory location-numbers-is replaced by a new memory location-number.

3.4. It is clear that one must be able to get numbers from any part of the memory at any time. The treatment in the case of orders can, however, b more methodical since one can at least partially arrange the control instructions in a linear sequence. Consequently the control will be so constructed that it will normally proceed from place n in the memory to place (n + 1) for its next instruction.

3.5. The utility of an automatic computer lies in the possibility of using a given sequence of instructions repeatedly, the number of times it is iterated being either preassigned or dependent upon the results of the computation. When the iteration is completed a different sequence of orders is to be followed, so we must, in most cases, give two parallel trains of orders preceded by an instruction as to which routine is to be followed. This choice can be made to depend upon the sign of a number (zero being reckoned as plus for machine purposes). Consequently, we introduce an order (the conditional transfer order) which will, depending on the sign of a given number, cause the proper one of two routines to be executed.

Frequently two parallel trains of orders terminate in a common routine. It is desirable, therefore, to order the control in either case to proceed to the beginning point of the common routine. This unconditional transfer can be achieved either by the artificial use of a conditional transfer or by the introduction of an explicit order for such a transfer.

3.6. Finally we need orders which will integrate the input-output devices with the machine. These are discussed briefly in 6.8.

3.7. We proceed now to a more detailed discussion of the machine. Inasmuch as our experience has shown that the moment one chooses a given component as the elementary memory unit, one has also more or less determined upon much of the balance of the machine, we start by a consideration of the memory organ. In attempting an exposition of a highly integrated device like a computing machine we do not find it possible, however, to give an exhaustive discussion of each organ before completing its description. It is only in the final block diagrams that anything approaching a complete unit can be achieved.

The time units to be used in what follows will be:

1 m sec = 1 microsecond = 10- 6 seconds

1 msec = 1 millisecond = 10- 3 seconds

4. The memory organ

4.1. Ideally one would desire an indefinitely large memory capacity such that any particular aggregate of 40 binary digits, or word (cf. 2.3), would be immediately available-i.e. in a time which is somewhat or considerably shorter than the operation time of a fast electronic multiplier. This may be assumed to be practical at the level of about 100 m sec. Hence the availability time for a word in the memory should be 5 to 50 m sec. It is equally desirable that words may be replaced with new words at about the same rate. It does not seem possible physically to achieve such a capacity. We are therefore forced to recognize the possibility of constructing a hierarchy of memories, each of which has greater capacity than the preceding but which is less quickly accessible.

The most common forms of storage in electrical circuits are the flip-flop or trigger circuit, the gas tube, and the electromechanical relay. To achieve a memory of n words would, of course, require about 40n such elements, exclusive of the switching elements. We saw earlier (cf. 2.2) that a fast memory of several thousand words is not at all unreasonable for an all-purpose instrument. Hence, about 105 flip-flops or analogous elements would be required! This would, of course, be entirely impractical.

We must therefore seek out some more fundamental method of storing electrical information than has been suggested above. One criterion for such a storage medium is that the individual storage organs, which accommodate only one binary digit each, should not be macroscopic components, but rather microscopic elements of some suitable organ. They would then, of course, not be identified and switched to by the usual macroscopic wire connections, but by some functional procedure in manipulating that organ.

One device which displays this property to a marked degree is the iconoscope tube. In its conventional form it possesses a linear resolution of about one part in 500. This would correspond to a (two-dimensional) memory capacity of 500 x 500 = 2.5 x 105. One is accordingly led to consider the possibility of storing electrical charges on a dielectric plate inside a cathode-ray tube. Effectively such a tube is nothing more than a myriad of electrical capacitors which can be connected into the circuit by means of an electron beam.

Actually the above mentioned high resolution and concomitant memory capacity are only realistic under the conditions of television-image storage, which are much less exigent in respect to

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