Starting the machine

Our explanations and examples have up to now assumed the machine somehow to have reached certain particular states, and the reader has been expected to restrain temporarily his curiosity as to how these states were reached. In this section we attempt to satisfy this curiosity. This does not involve making any new statements about the construction of the machine; it is an application of the properties of the machine already (and almost completely) described.

The essential process to describe is that of `manually writing given information into a track'. This is done in two parts, of which the first is the writing of the information into a page or two partnered pages of the electronic store and the second consists in transferring this information to the track in question. The writing into the electronic store is done a line at a time. The procedure is as follows

• CS off
• Set function part of MI e.g. to T£ or T/.
• MAN-AUTO switch to MAN
• Operate KEC
• CS on
• Set store line part of MI to the address of the first line to be written in, e.g. if the content required is E//T operate P0, P19.
• Set the store line part of MI to the address of the second line-pair to be written in, e.g. to E/.
• Operate the keys for the content of this second line.
• Repeat with the remaining lines required.

This procedure can be varied when errors are made. The most satisfactory arrangement in the opinion of the writer is to operate KLC whenever an error is made and then rewrite the line. Alternatively the `erase' key may be used.

To copy from the electronic store to the track the procedure is

• C.S. off
• MI to ////
• H to a magnetic instruction describing the transfer required.
• Operate KCS

It will be appreciated that there are a number of variations of procedure possible, and that the procedure above makes no allowance for checking, although checking should in fact be done. An important variation is the possibility of using H in place of P0,...P19.

The procedure outlined above should be non-recurrent. It is intended that as soon as the input programme, or even the bare bones of the input programme, have been put into the magnetic store, this method will no longer be used. There will be an interim period during which a variety of different methods may be used, because the programmes for the final arrangement have not yet been put in, but these are of no interest. The final arrangements will also not be described, for they have not yet been decided. [But see the discussion of, e.g., the WRITE routine on p. .]

It must not of course be imagined that the `initial conditions' of the various routines which we have given as examples would normally be achieved by such a `writing-in' process. They will normally be achieved during the action of some other routine which uses the given routine as subroutine. Moreover when making up these (sub-) routines one does not normally have any particular means in mind by which the initial conditions are to be achieved, apart from the general conventions governing the change of routine. They are simply made up in the (normally justified) belief that these initial conditions will arise in some useful connection.

In connection with starting procedures the use of KEC should also be mentioned. We will suppose that the machine is out of control, i.e. that we do not know what it is doing, either because the machine itself has made some unknown error, or because the operator has forgotten what he has done, or has done something whose consequences he does not choose to consider. We assume the switches to be in the positions for normal operation, i.e.

• C.S. on
• Dummy stops off
• Auto condition

We will also assume H set to ////. Now consider the effect of operating KEC. As mentioned in the description of the console this clears the store, and the special stores B and C. It has other effects which do not concern us so much here. As the key is allowed to return to its normal condition the connections causing the clearing effect will be broken in some unknown order. There may possibly be bounce on the contacts causing the clearing effect to return temporarily. We shall show that this complicated behaviour of the key need not be known in any detail, and that the machine as a whole `comes under control' very shortly after the key has returned to its normal position. We make only the following assumptions.

i)

The period of bounce does not exceed 156 ms.

ii)

During the period of bounce the effect of altering a line of store or control or B-tube is the alteration as described for normal operation combined with the possible replacement of some digits 1 by 0.

For this purpose the processes

• Adding 1 to control (I.N.)
• Selecting the instruction from the store and copying onto control tube (P.I.)
• Obeying the instruction

must be treated as separate.

iii)

Digits 10 to 19 of lines // to U/ of track 0L [i.e., 0 (left)] contain //.

It will be seen that if at any prepulse-time ${\tt\bf C}_+ \le 6$ and iii) still holds, then ${\tt\bf C}_+ \le {\tt\bf C}+1$ and iii) holds at the next prepulse. It follows that ${\tt\bf C}_+ \ge 6$ cannot hold until the fifth prepulse after the beginning of the bounce period. It will be seen that all the intervening instructions are magnetic transfers occupying more than 30 ms and that therefore when ${\tt\bf C}_+ = 6$ for the first time, the bounce period is over, and iii) still holds. When the next instruction has been obeyed the machine will be in the same condition as if it had proceeded in the normal manner from the condition with control and store clear. There is no externally distinguishable behaviour of the machine in the two cases, for in neither case is there any hooting, printing or punching before the point at which the common behaviour starts.

It will be seen that the KEC method of starting is valid also if some other magnetic transfer is set up on H, provided that the routine brought down to [electronic] p. 0 satisfies iii). Such routines may be termed `self-starter' routines. They include routines brought down to p. 1 and starting at /@, though there is a delay of about two seconds before the routine is entered in this case.

[At least on the basis of Turing's descriptions of console functions, it would appear that if KEC were only operated with completion signals off, then we could dispense with the redundant initial magnetic transfers at the start of `self-starter' routines, and for that matter a lot of this argumentation ...]