Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/26

 It should be noticed that a completely blank instruction has a definite meaning, viz. to pass the material on the card in the reader into DL 0000000000.

In Fig.27 TS 01101 typifies any of the TS as regards output connections shown on other diagrams. It is also typical as regards input connections, except as regards TS 4,5,8,1, which have no input connections except those shown on other diagrams.

In the case of operations of type R we shall calculate all of the expressions involved and select them by means of tree elements, digits 18 to 23 being used. The operations so far are:

Digits 000000 TS 8 = TS 9 & TS 10.

Digits 001000 TS 8 = TS 9 v TS 10.

Digits 010000 TS 8 = -TS 10.

Digits 011000 TS 8 = (TS 9 & TS 10) v (-TS 9 & -TS 10).

Digits 100000 TS 8 = 0.

As we shall have very much to say about type S we shall make a few remarks first about type T. In order to be able to obtain a rather direct access from the instructions to the valves we shall introduce a number of valve elements which can be stimulated to order. We may have 64 of these, say FLEX 000000 to FLEX 111111. The circuit will be simply as shown in Fig. 31. It is intended that the outputs of these valve elements should be connected in various ways into the circuit when it is desired to try out new circuit arrangements. It is thought that they may often provide means for doing things simply which could be done lengthily as an ITO. To an extent this represents a compromise between the new system of ‘control by paper’ and the old plugboard and soldering~iron techniques.

We shall also describe the timing arrangements before passing on to type S. We have already mentioned CATIM which determines the timing but we have still to mention what controls CATIM. CATIM is stimulated as soon as the first P1 appears after the signal A, or, in case Q, the first P17. It is extinguished by a variety of means. In cases K and L it is extinguished by the ending of TIMCA indicating that the required minor cycle has just passed through. In cases M, N, R, T, it is only permitted to last for one minor cycle. In case Q it is also only allowed to last for half a minor cycle. In cases O, P the extinguishing signal is CARDEX, which is given by the card reproducer or reader on the final exit of the card, via a trigger-limiter. In case S the signal comes from FINARITH.

The facilities provided under type S are not easily enumerated, because they do not consist of a number of different operations stimulated by different tree valve elements, as for instance applies in the case of the logical processes. Rather they are to be thought of as one process which can be modified in various ways. The standard process always involves converting the content of TS 4 and TS 5 into ‘series form’, i.e. instead of connecting the outputs of TS 4 and TS 5 to their own inputs they are connected to each other's. When they are so connected their content will be described as the ‘partial sum’. Some quantities are then added to or subtracted from the partial sum. If the quantity is to be added then POS is stimulated, otherwise they are subtracted. We may if we wish cancel the original partial sum before/