In order to make the account of the machine complete it is necessary to describe the means by which the behaviour of the machine can be controlled manually. In practice these are best learnt in connection with the machine itself, but an attempt to describe them completely here will nevertheless be made.
In principle two controls are sufficient. We have already mentioned the input mechanism: this becomes a method of controlling the machine when a means of changing tapes if provided. If to this is added a key which clears everything (i.e. sets all the electronic stores to zero, including accumulator etc.) we can do all that we require, at any rate if the input programme has been stored in an appropriate part of the magnetic store. [The first instruction executed will then be ////, which will cause the magnetic instruction set up in H to be obeyed; this can then load a bootstrap routine into page 0, as discussed below in detail.] In principle also it is sufficient for the machine to signal to the operator with the output mechanism, or the hooter. We regard these facilities as standard, but there are a number of others which we regard as providing variants of the standard machine.
We have mentioned the dummy stops. For each of the instructions /L and /G there is a switch deciding whether that instruction shall be a `waste time' or a stop. When the machine is in the standard condition these are both time wasters. We have also mentioned the hand switches H. Besides these there are a number of other switches and keys; (`switches' are stable in either position, whereas `keys' must be kept pressed if they are to remain in the `active' position). We list them below together with those which have been mentioned before.
|Switch for the dummy stop /L|
|Switch for the dummy stop /G|
|Hand switches H|
|MAN/AUTO switch||Selection of|
|Manual instruction switches MI0, MI1, ..., MI19||instructions|
|Completion signals on-off switch CS||Control of|
|Key for single completion signals KCS||completion|
|Slow completion signals key||signals|
|Key accumulator clear KAC|
|Key B clear KBC|
|Key control clear KCC||Clearing keys|
|Key multiplicand clear KMC|
|Key everything clear KEC|
|Digit keys P0,...,P39||Manual writing|
|Key line clear [KLC]|
|Input controlling switch||Input and output|
|Printer on-off switch||control|
|Switches for connecting input to printer|
|Write-power switch||Writing suppression|
|Other writing suppression methods|
The purpose of the clearing keys, other than KLC, are self explanatory. When the MAN-AUTO switch is in the AUTO position the instructions are selected and obeyed in the manner already explained in detail. When this switch is in the MAN position a different arrangement applies. In this case the actual instruction is the combination set up in the manual instruction switches. The presumptive instruction is only of interest when the function symbol is TO, TB or TG. For this purpose it may be taken that the presumptive instruction was ££££. Thus it is not possible by manual instructions to make any other use of the B tube than transfers and additions involving B7. [One gathers this applies even to `B-exceptional' operations such as TT for which the presumptive and actual instructions are normally identical.] The effect of obeying an instruction in the MAN condition is determined by the equations on p.E [in the appendices], with one modification, viz. that everywhere where appears C should be read instead. This applies to the `normally understood' equation , and permits the interpolation of manual instructions amongst automatic ones. It also applies to the relative transfers /Q, /O, /M and results in the transfer, if applicable, being delivered to the line immediately previous to that which would have been reached on the automatic system. The user is recommended to avoid such manual instructions.
The switches and keys for the control of completion signals in effect decide when the instruction, determined by the rules of the AUTO or MAN operation, whichever is applicable, shall be obeyed. If the completion signals switch is in the on position the completion signals occur at the times described on p.E [in the appendices], i.e. the machine works `full speed ahead'. If a dummy stop which has been switched on is encountered the completion signals cease. Completion signals may also be given with the single completion signals key. These completion signals are not interrupted by stops. They may be given when the completion signals switch is on. It is therefore possible to restart after a stop by giving a single completion signal. The chief application of the single completion signals key is in the verification of the correctness of routines by the comparison of check sheets with the actual behaviour of the machine. It is also in this connection that the dummy stops find their use. They make it possible to hurry through parts which are known to be correct. Another aid to this checking process is the `slow completion signals key', which provides completion signals at a rate of 50 per second.
The digit keys P0-P19 provide means for altering individual line pairs of the store. They only operate in the MAN condition with completion signals on. The line of the store which can then be affected is that whose address is contained in the first half of the instruction. It follows from what has already been said that under the conditions in question the instruction on the manual instruction switches will be being repeatedly obeyed. It is advisable therefore to choose some innocuous value for the function part of this instruction. (Any value which does not mention in its equations and does not involve a magnetic transfer is satisfactory. If the normal condition of the switches is considered to be //// probably /T is the most convenient). Under these circumstances the effect of operating key is equivalent to an instruction with the equation . For example if we have set up GF/T on the switches and depress P1,P6, and the short line GF contains MICE, we shall afterwards find that GF contains VICE. At the same time the content of control may have been altered by the obeying of GF/T. It is assumed for this purpose that the write-erase key is not depressed, so that it is in the write position. If this key is maintained depressed during the operation of the digit key the effect is given by the equation i.e. instead of altering digits 0 to 1 the reverse applies. Operation of KLC clears the line, whatever the position of the write-erase key. It will be observed that the same effect is obtained by setting the function symbol momentarily e.g. to TA.
Exercise. Assuming the digit keys P0-P19 to be unserviceable devise a procedure to replace them by the use of H and other controls.
Note. Since H consists of switches, whilst P0-P 19 are press-button keys the latter are somewhat more convenient to use.
The changing of the position of a switch should preferably be done at a time when its position is irrelevant to the working of the machine. All switches and associated equipment are designed with the intention of avoiding transient effects, i.e. if it is irrelevant which stationary position the switch is in, it is equally irrelevant whether it is being flicked back and forth. Also if it should happen that a change is made during a time when its position is relevant, the effect produced at each individual instruction during that period will be either what would be expected in the on position or what would be expected in the off position, and not any third alternative. It may be taken that the period between being definitely on and definitely off does not exceed 30 ms. No statement is made concerning what will happen when two or more switches are simultaneously neither fully on nor fully off. Likewise the operation of a key is preferably done at a time during which the intermediate state of that part of the machine immediately affected by the operation is irrelevant to the behaviour of the rest of the machine. For instance the effect of clearing the accumulator with completion signals on is not always very easily predicted. It is best to do such operations with completion signals off. If however it is necessary to do them with completion signals on the following assumptions apply to keys other than KEC. In every individual digit period it may be assumed that the key is either definitely operated or definitely not operated. The transition period between operation and non-operation may be assumed not to exceed 30 ms. This assumption cannot of course be applied without more knowledge of the detailed circuits and timing than has been given here. It may be said however that it is adequate to justify the use of KAC or KBC for signalling to the machine with completion signals on. This is done in a few routines. The key KEC is in a somewhat different category since it effectively operates a number of separate keys. The use of KEC with completion signals on is considered on pp. -.
The effect of undesired magnetic writing transfers can be disastrous. In many problems no such transfers need be made. During the solutions of these problems it is usual to suppress all writing transfers by switching off the `write power'. In most other problems there are certain tracks on which one does not wish to write, e.g. the tracks on which the routines are written. In these cases one wishes to be able to suppress the writing on these. For this purpose one may suppress the writing on a block of sixteen tracks by the removal of a valve [i.e., vacuum tube] from the writing circuits. A `block' consists of a set of tracks whose numbers are of form where and remains fixed throughout the block. One may also suppress writing on a single track by removal of a valve. The removal of valves is normally not admissible, but is licensed in these cases.
The facilities additional to the output and the hooter by which the state of the machine may be observed comprise a number of monitor tubes, which make visible the contents of the various electronic stores and some neons. The contents of the accumulator, B-tube, control and multiplicand are displayed on different tubes and the content of the store is displayed on two further tubes. Which pages are displayed on these tubes is determined by switches. Any pair can be chosen. The neons comprise
A number of special points must be noted. The quantity given on the multiplicand line, interpreted according to the plus convention, is the modulus of the content of the multiplicand. The sign of the content of the multiplicand must be found from the neon. On the control tube one finds not only the quantity C, (which we have also called the instruction number I.N., and which, after adding 1, describes the line in which the next instruction to be obeyed, is to be found), but also the last instruction obeyed. This is the true instruction, not the `presumptive instruction', i.e. the content of the B-tube has been taken into account. This does not of course apply to the case when the MAN-AUTO switch is set at MAN. In this case one may think of the control tube as showing the instruction which would have been obeyed in the AUTO condition. There is no means of monitoring the presumptive instruction, other than by looking at the appropriate line of the store before obeying the instruction.
The monitor tubes can of course be used to observe the general progress of a computation, but this usually proceeds too fast for observation of detail. They are most useful for observing the machine in a stationary condition. This occurs with