Chapter 4 Preliminary discussion of the logical design of an electronic computing instrument 95
the reliability of individual markings than what one can accept in the storage for a computer. In this latter case resolutions of one part in 20 to 100, i.e. memory capacities of 400 to 10,000, would seem to be more reasonable in terms of equipment built essentially along familiar lines.
At the present time the Princeton Laboratories of the Radio Corporation of America are engaged in the development of a storage tube, the Selectron, of the type we have mentioned above. This tube is also planned to have a non-amplitude-sensitive switching system whereby the electron beam can be directed to a given spot on the plate within a quite small fraction of a millisecond. Inasmuch as the storage tube is the key component of the machine envisaged in this report we are extremely fortunate in having secured the cooperation of the RCA group in this as well as in various other developments.
An alternate form of rapid memory organ is the acoustic feedback delay line described in various reports on the EDVAC. (This is an electronic computing machine being developed for the Ordnance Department, U.S. Army, by the University of Pennsylvania, Moore School of Electrical Engineering.) Inasmuch as that device has been so clearly reported in those papers we give no further discussion. There are still other physical and chemical properties of matter in the presence of electrons or photons that might be considered, but since none is yet beyond the early discussion stage we shall not make further mention of them.
4.2. We shall accordingly assume throughout the balance of this report that the Selectron is the modus for storage of words at electronic speeds. As now planned, this tube will have a capacity of 212 = 4,096 » 4,000 binary digits. To achieve a total electronic storage of about 4,000 words we propose to use 40 Selectrons, thereby achieving a memory of 212 words of 40 binary digits each. (Cf. again 2.3.)
4.3. There are two possible means for storing a particular word in the Selectron memory-or, in fact, in either a delay line memory or in a storage tube with amplitude-sensitive deflection. One method is to store the entire word in a given tube and then to get the word out by picking out its respective digits in a serial fashion. The other method is to store in corresponding places in each of the 40 tubes one digit of the word. To get a word from the memory in this scheme requires, then, one switching mechanism to which all 40 tubes are connected in parallel. Such a switching scheme seems to us to be simpler than the technique needed in the serial system and is, of course, 40 times faster. We accordingly adopt the parallel procedure and thus are led to consider a so-called parallel machine, as contrasted with the serial principles being considered for the EDVAC. (In the EDVAC the peculiar characteristics of the acoustic delay line, as well as various other considerations, seem to justify a serial procedure. For more details, cf. the reports referred to in 4.1.) The essential difference between these two systems lies in the method of performing an addition; in a parallel machine all corresponding pairs of digits are added simultaneously, whereas in a serial one these pairs are added serially in time.
4.4. To summarize, we assume that the fast electronic memory consists of 40 Selectrons which are switched in parallel by a common switching arrangement. The inputs of the switch are controlled by the control.
4.5. Inasmuch as a great many highly important classes of problems require a far greater total memory than 212 words, we now consider the next stage in our storage hierarchy. Although the solution of partial differential equations frequently involves the manipulation of many thousands of words, these data are generally required only in blocks which are well within the 212 capacity of the electronic memory. Our second form of storage must therefore be a medium which feeds these blocks of words to the electronic memory. It should be controlled by the control of the computer and is thus an integral part of the system, not requiring human intervention.
There are evidently two distinct problems raised above. One can choose a given medium for storage such as teletype tapes, magnetic wire or tapes, movie film or similar media. There still remains the problem of automatic integration of this storage medium with the machine. This integration is achieved logically by introducing appropriate orders into the code which can instruct the machine to read or write on the medium, or to move it by a given amount or to a place with given characteristics. We discuss this question a little more fully in 6.8.
Let us return now to the question of what properties the secondary storage medium should have. It clearly should be able to store information for periods of time long enough so that only a few per cent of the total computing time is spent in re-registering information that is "fading off." It is certainly desirable, although not imperative, that information can be erased and replaced by new data. The medium should be such that it can be controlled, i.e. moved forward and backward, automatically. This consideration makes certain media, such as punched cards, undesirable. While cards can, of course, be printed or read by appropriate orders from some machine, they are not well adapted to problems in which the output data are fed directly back into the machine, and are required in a sequence which is non-monotone with respect to the order of the cards. The medium should be capable of remembering very large numbers of data at a much smaller price