Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/13

 note. How is the burying and disinterring of the note to be done? There are of course many ways. One is to keep a list of these notes in one or more standard size delay lines (1024), with the most recent last. The position of the most recent of these will be kept in a fixed TS, and this reference will be modified every time a subsidiary is started or finished. The burying and disinterring processes are fairly elaborate, but there is fortunately no need to repeat the instructions involved each time, the burying being done through a standard instruction table BURY, and the disinterring by the table UNBURY.

 7. External Organs.

(i) General.— It might appear that it would be difficult to put information into the calculator and to take it out, on account of the high speeds associated with the calculator, and the slow speeds associated with mechanical devices; but this difficulty is not a real one. Let us consider for instance the output organ. We will allow the mechanical part of the output organ to work at whatever pace suits it, to take its own time in fact. However we will require it to give out signals stating when it is ready to accept information. This signal provides a gate for the feeding of the information out to the output organ, and also signifies to the calculator that it may note that information as recorded and proceed to feed out some more. The preparation for feeding (p.384) the information out consists merely in transferring it from dynamic storages onto trigger circuits.

In the case of the output arrangements we have the full power of the calculator behind us, i.e. we can do the conversion of the information into the required form as an ITO. In the case of the input organ we must go more warily. If we are putting the instruction tables into delay lines, then when the power has been turned off all memory will have been effaced, including the instruction tables. We cannot use instruction tables to get the information back, because the instruction tables are not there. We are able to get over this difficulty as will be seen below.

(ii) Output Organ.— The output will go on to 32 columns of some Hollerith cards. All the 12 rows may be used. On the receipt of a signal from the calculator a card will begin to pass through a punch or ‘reproducer’. Shortly before each row comes into position for punching a signal is sent back to the calculator and trigger circuits controlling the punches are set up. After the punching another signal is sent to the calculator and the trigger circuits are cleared. The reproducer punch also gives a signal on the final exit of the card. The circuit is shown in connection with CA (Fig. 26).

(iii) Input Organ.— Let us first describe the action of this without worrying about the difficulty concerning absence of instruction tables. It is very similar to the output organ in many ways. The input is from 32 columns and 12 rows of a Hollerith card. When the calculator is ready a card release signal goes out to the card reader and a card begins to pass through. As each row comes into position for reading a signal is sent back to the calculator, which then prepares to accept the output from the reader at the moment appropriate for sending it to its destination in the delay line. It is assumed that this destination is already decided by the calculator. A signal is sent back to the calculator on the final exit of the card.

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