Tapes

Tapes may be classified in a number of ways.

1. They may be titled or not titled, i.e. they may or may not start with a Q-sequence. Untitled tapes are somewhat to be discouraged, and the convention that untitled tapes may be destroyed is under consideration.

2. They may be `input', `output', or `other purpose' tapes. If the printer is being used there is little point in output tapes except where they are to be used later for input, either as they stand, or after combining with others in a copying process.

3. Input tapes will nearly always be used in connection with the regular input routine INPUT. There may however be a few `irregular' input tapes. These are likely to be chiefly useful where single characters are significant standing by themselves.

4. The most important tapes using the routine INPUT are `writing tapes' and `job-steering tapes'.

Writing tapes

These tapes are the standard form of storage for routines and other large blocks of information. One tape is made for each half track, and takes the form

• Q (title)
• KAK@///¼ (magnetic half cue) or KAK@//E¼ (magnetic half cue)
• KPK (check sum)
• Y
• (punching proper)
• Z

The title does not require much explanation. It must be preceded by the character giving its length. The magnetic half cue is similar to the second half of the cue of a routine. It determines the track in which the material on the tape is to be stored in exactly the same way as the second half of a cue of a routine determines the track from which the routine is to be read. In the case of a routine more is given viz. the whole magnetic instruction required for that reading. In the case of a tape the remainder of the magnetic instruction is given in the earlier part of the `destination sequence'. This is KAK@///¼ for tapes destined for a left half track, KAK@//E¼ for ones destined for right half tracks. In the case that the tape is used for a routine the magnetic half of the cue will normally be identical with the second half of the cue of the routine, though this will not apply when the routine is stored in more than one track.

The punching proper results in transferring certain information to page 4, the `systematic working space'. It will normally be punched with the `J' meaningful sequences.

The check sum is the sum of the long lines of page 4 after the punching proper has passed through.

The writing tapes are intended for use with the routine WRITE. If this routine is entered when the beginning of a writing tape is in the input, the title will appear in the output and the information in the punching proper will be written in the appropriate half-track, after a number of precautions have been taken, including a check on the track-selection mechanism, verification that the correct check sum is formed, and a check that no digits were altered in the writing process. For further information on this routine its official account must be consulted.

Writing tapes should normally be produced automatically from information within the machine. The routine for this purpose will shortly be available, but details are not yet known.

It is intended that a typewritten list be kept of all tapes with their titles, destination sequences and check sums, i.e. essentially all of the tape except the punching proper.

Job-steering tapes

It is convenient to use INPUT, in combination with an appropriate tape, as a master routine for a job. In so far as the master routine is non-repetitive this is no slower than putting a master routine away in the magnetic store and then entering it. The advantages of this method, when applicable, are

1. The state of progress visible to controller through position of tape.
2. No necessity to assign magnetic or electronic storage
3. Subroutines can be entered with less formality by using meaningful sequences Y and Z.
4. Writing operations are avoided.

Even when a certain amount of repetition is involved it may be less trouble to have repetition e.g. by copying on the tape rather than to attempt to produce a routine in which the repetition is achieved by passing through the same instructions again and again. An example of this is described in the appendix (p. ).

Exercise. Suppose that ££ABF/EZ is the cue of a routine whose effect is given by $f([\tp{/C}]) = [\tp{:C}]'$, and does not alter any long lines, except the conventional ones and :C, produce a job steering tape for printing out the values of $f$ for six given arguments. Use OUTPUTA.

Directories

The twenty digit rows that are put into a cue-bearing track to assist the routine changing sequence in turning false cues into true cues may be called `tape addresses'. For every job it is necessary to assign an address to each tape involved. The list of these addresses or directory has to be put into the cue bearing tracks (which should be very few) before the job can begin. This process is certainly something of an imposition. It is hoped that it will be possible to have a small number of standard directories of which one will be suitable for almost any job. Those routines which are special to the job may be given true cues and will not need directory entries.