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The magnetic wheel

The organisation of the magnetic storage into tracks and pages has already been described. To recapitulate, there are 256 tracks each consisting of a left hand page and a right hand page. Each page consists of 1280 digits which although actually arranged on a circle may be more conveniently thought of as arranged in a similar manner to the 1280 digits of a tube.

In order to be able to make use of the information stored in the wheel, arrangements are made to enable one to transfer either a complete trackful or a complete page from the magnetic to the electronic store. The process by which this is done is described as a `reading transfer'. Likewise in order to store information previously held in the electronic store arrangements are made for transfers in the opposite direction, from the electronic store to a track. These are called `writing transfers'. In addition there are `check transfers' by means of which the content of a part of the magnetic store is compared with a part of the electronic store.

These transfers cannot be carried out by instructions satisfying the essential conventions of the `reduced machine', for it is no longer true that in such an operation at most one line of the electronic store is altered. Instead of referring to such a line it is necessary to specify the track concerned and also the tube or tubes. For these reasons the magnetic transfers are controlled on a different principle from the previously described operations. The transfer is described by a special `magnetic' instruction, which is not an instruction as previously understood. When a magnetic transfer occurs an ordinary instruction is also required. However this instruction merely specifies that a magnetic transfer is to be made and states where the magnetic instruction describing the details of that transfer is to be found. Suppose for example that we encounter the instruction VE/:. Referring to the list of meanings of the function symbols we see that /: means `S as magnetic instruction'. Hence VE/: means `The content of line VE is to be obeyed as a magnetic instruction'. Now suppose further that in the line VE there is F/EC. Now F/EC interpreted as a magnetic instruction (as described in detail below) says `Transfer the left half of track 13 to electronic page 7'. The same would happen if (e.g.) AA// or YE// were obeyed when F/EC was set up on the hand switches (H).

We have still to describe the detailed coding of the magnetic transfers. It is thought to be unnecessary to make arrangements to transfer the two halves of a track to any two tubes. Instead the tubes are partnered, each odd numbered tube having the next lower (even numbered) tube as its partner. The two halves of a track, if both are transferred, can only be transferred to partnered tubes. This partnering of tubes is somewhat similar in effect to the partnering of pages in tracks, and we may speak of an even numbered tube as the left member of its partnership and the odd one as the right member.

The magnetic instruction is best thought of as consisting of four parts:

A part describing a track
A part describing a tube
A part which with the aid of a) and b) describes one or two magnetic pages and relates them to an equal number of tubes. In the case that there are two the pages or tubes are partnered.
A part describing the manner of transfer, i.e. whether reading, writing or checking.
Special functions digit
c), d) and e) taken together may be described as the `function' part of the magnetic instruction. The coding for d) may be simply described
00 Reading
10 Checking (read-like)
01 Writing
11 Checking (write-like)
These four must be explained in greater detail.

Reading simply means transferring from the magnetic to the electronic store. The `sixty-fifth line' of any half-track involved is transferred as well as the main body of sixty-four lines.

Checking means comparing the corresponding parts of the electronic and magnetic stores. The check may be said to `fail' if there is any discrepancy. When it fails ${\tt\bf C}'={\tt\bf C}+1$, but ${\tt\bf C}'={\tt\bf C}+3$ if it succeeds. There is also a neon which changes from bright to dark or vice-versa whenever a check fails. In the case under consideration (read-like checking, 10) the sixty-fifth line is checked as well as the rest. It is in this respect that there is a similarity with reading.

Writing means transferring from the electronics to the magnetics without transferring the sixty-fifth lines.

This write-like checking does not involve the sixty-fifth lines.

With all magnetic instructions except the successful checks we have ${\tt\bf C}'={\tt\bf C}+1$.

[Two digits are involved in coding for c). One digit specifies the number of pages involved;] 0 means that one page is transferred, and 1 that both pages are transferred. The other digit decides whether the tubes and pages are paired in the `natural' or `reversed' order. The natural pairing is to take the left page with the named tube and the right with the partner, and in the reversed pairing the left page goes with the partner and the right with the named tube. When only one page or tube is involved the tube concerned is the named tube. Taking the two digits together the effects are

Left page (half track) with named tube
Right page with named tube
Left page with named tube and right page with its partner
Right page with named tube and left with its partner

The magnetic functions can of course also be used for operations which are not magnetic transfers at all. All the functions of this kind have 1 for the `special functions' digit whereas the functions already described have 0. The special functions are described in the next section under input and output.

Since there are only 16 tubes and 256 tracks one only requires 12 digits for a) and b) and 5 for c), d) and e), i.e. 17 in all. The short line assigned for the magnetic instruction has 20 digits, so that there are 3 spares. It is convenient to place these spares in such a way that the various different parts do not overlap teleprinter characters thus

Take for instance the magnetic instruction NEIC. The combination NE determines the track as NE, i.e. 44, the character C determines the tube as C: this means the tube whose first line is /C, more often known as tube 7. The function character I or 01100 is to be broken into 01 stating that the left page (of track 44) is to be taken with the named tube (i.e. 7) and the right page (of 44) with its partner (i.e. 8), 10 meaning check (read), and 0 stating that it is not a special function. [Note that by the rules previously stated, the partner of tube 7 would be tube 6, not tube 8.]

It is hoped that when the details above have been read through once or twice the diagram Fig. G will then suffice to enable one to remember them.

next up previous
Next: The input and output Up: Alan Turing's Manual for Previous: Miscellaneous special functions
Robert S. Thau 2000-02-13