The input and output mechanisms

Information can be fed into the Mark II machine from teleprint tapes and out of it onto other teleprint tapes and onto a printer. The input mechanism provides a further departure from the principles of the `reduced machine', which was capable of taking only a finite number of states, although this number was quite large, and each state was completely determined by the last one. We now have to regard the state as determined by the last state in combination with the character which is momentarily in the reading head.

The apparatus required consists of a tape reader together with a punch, and a printer. The tape in the input only affects the behaviour of the machine when `special magnetic function O' occurs. The character on the tape is then superimposed (`or' combination) on the five most significant digits of the accumulator, and the tape moves forward so that the next character is in the reading head. The latter process takes a certain time, (about 5 ms) and arrangements are therefore made to prevent another reading process from occurring until the next character is in position. There is no interference with the other instructions, and if another such reading instruction is given prematurely it is not `lost', but merely held up. This process is described somewhat incompletely below in the equation ${\tt\bf A}'={\tt\bf A}\lor 2^{75}{\tt\bf T}$, where T represents the content of the reading head.

In order to facilitate the changing of tapes a switch is provided which inhibits this special function.

The other special magnetic functions are concerned with output. Of these T is most essential. Its effect is to punch the character given in the five most significant digits of the accumulator. This character will also be printed by the printer unless it is in the figures-shift position or the printer is switched off. If it is in the figures-shift position the corresponding figure is obtained. These figures are given in the table below

/	E	@	A	:	S	I	U	¼	D	P	M	F
0	1	2	3	4	5	6	7	8	9	¼	-	.

No guarantee can be given concerning what will happen when other characters are printed on figure shift. There may be only a smudge.

A normal teleprinter responds differently to the stunt characters /, @, :, ¼, ", £ producing respectively no effect, line feed, space, carriage return, figure shift, letter shift. The printer associated with the machine prints these characters, but arrangements are also made to do the stunt operations. These are provided by other special magnetic functions as shown below. No effect is produced on the punch in these cases.

When any one of the special magnetic functions is being obeyed, any other special magnetic function, other than B, is held up until the first is complete. Ordinary instructions, and ordinary magnetic instructions and the special function B may be obeyed during such a period.

The special function B, if obeyed within 10 ms of the special function T superimposes the character set up for punching on digits 35 to 39 of the accumulator. Its effect at a later time cannot be guaranteed.

		Time in beats
T	${\tt\bf P}'=\{{\tt\bf A}_+\}_{75}^{79}$ (and possibly print $\{{\tt\bf A}_+\}_{75}^{79}$)	about 62; 38 if printer off
Z	Space
L	Carriage return
W	Line feed	about 62; 4 if printer off
H	Figure shift
Y	Letter shift
O	${\tt\bf A}' = {\tt\bf A}\lor \{2^{75}{\tt\bf T}_+\}_0^{79}$, Input tape moves.	about 28.
B	${\tt\bf A}' = {\tt\bf A}\lor \{2^{75}{\tt\bf P}_+\}_0^{79}$	5
£	no effect	4

[P in these equations evidently represents the punch buffer.] Writing operations [to magnetic storage?] take 258 beats and reading and checking each take 183 beats. It is important to observe that no provisions for carriage return are made other than L above, consequently even if there is no intention of `page printing' results it is still necessary to provide carriage returns to prevent the margin stop being reached.

The `or' ($\lor$) connection is particularly suitable for these processes. It is very easy to provide the apparatus to do it, and also, if the operation is applied several times the same effect occurs as if it were applied only once.

Connected with the printer there is also a keyboard with which one can `break in' to print additional remarks.

[Examples]

The magnetic coding has now been completely described and we therefore choose this point to include some examples and exercises.

Examples of magnetic instructions

SSFG	means	Track 165L [i.e., 165 left] to be checked with
		page 13 (i.e., columns G, ") in a write-like
		manner i.e. ignoring sixty-fifth lines.
@/RS	means	Pages 2, 3 (columns :, S, I, U) to be written
		(not reversed) on track 2, i.e. columns
		:, S on 2L; I, U on 2R.
COWS and LAWN	both mean	Line feed.

Exercises

1. Interpret ABCD as a magnetic instruction.
2. Make up a magnetic instruction to mean `read track 19L [i.e., 19 (left)] onto page 4'.
3. Make a routine to compare electronic pages 3 and 4 and to give a loop stop with hoot if they differ.

[Library input routines]

The general user of the machine will be more concerned with the properties of the input and output routines which are available than these actual properties of the machine itself. The output routines have not yet been decided on but we shall describe here the arrangements intended for the input routine.

The material on the tapes used for input is divided into `meaningful sequences and `intermediate rubbish'. Each meaningful sequence begins with a special character called a `warning character'. These warning characters are chosen from those which are not very frequent in English and do not include / or £. The purpose of the former restriction is to enable one to include English words or sentences in the rubbish. The character / must not be a warning because one wishes to leave blank spaces at the beginning of the tapes, and to start the tape at some point in this blank space without particular care. If £ is not a warning character it is possible to remove meaningful sequences by punching additional holes in any warning characters which occur in them. The characters which it is at present intended should be warnings are J, K, Z, Q, ", X. The length of the meaningful sequence and its treatment depends on which warning character is used.

Warning character J

The length of the sequence is 11 characters. No action is taken unless the last character has 0 for its last digit. In this case two lines of columns ¼ and D are altered, namely those whose address has its first character given as the sixth character of the sequence. The second third fourth and fifth characters form a line which is copied into the appropriate line of column ¼ and the seventh eighth ninth and tenth go into column D. For example the effect of the sequence JABCDUFGHIJ is to put ABCD into U¼ and FGHI into UD. This apparently somewhat involved process is designed to make the hand punching of material as simple as possible. It is only necessary to imagine a page of material flanked with a column of J's on either side and then punch each row straight across. The purpose of the second J is to facilitate the correction of errors. If it is noticed that a mistake has been made before the second J has been punched the line may be nullified by replacing the last J by one of the characters T, Z, ..., £.

Warning character K

This permits the writing of material into any successive sequence of lines. The length of the meaningful sequence is determined by the fourth character of the sequence; if the value of this character ($+$ convention) is $n$ the length of the sequence is $4n+4$. The last $4n$ characters are to be regarded as divided into consecutive groups of four, which are to be written into consecutive lines. The address of the first of these lines is given by the second and third characters of the sequence. Thus for example the effect of the sequence KZSAVKTAVST/E:TC is to put VKTA into ZS, VST/ into LS, and E:TC into WS. It need hardly be mentioned that extravagant effects are to be anticipated if this facility is used to write into lines which contain the input routine itself, i.e. into columns /, E, @ or A. It is permitted however to write into any of the other columns including C and K.

Warning character Z

One character sequence, i.e. there is only the warning character itself. The instruction contained in CS is obeyed. The effect of this under normal circumstances is to `enter the routine changing sequence' and is described on p. .

Warning character Q

The second character (plus convention) gives the length of the sequence, reduced by two. The effect is to punch all the characters of the sequence, including the first two. Its main purpose is to enable titles of input tapes to be recorded in the output.

Warning character "

This is used for the input of numbers in decimal form. It continues from the warning character up to and including the first occurrence of one of the three characters P, M, £ after the warning character and the two immediately following it. In other words its length is the shortest consistent with being at least of length four and ending with P, M or £. It may be divided as follows

Warning character ("), Two characters of address, Content characters, End character (P, M or £).

If the end character is £ there is no effect, just as if the whole sequence were intermediate rubbish. If it is P or M, and if the content characters are all chosen from /, E, @, A, :, S, I, U, ¼, D the effect is to alter the (long) line described by the address part. If the content is given by characters $\delta_1, \delta_2, \ldots, \delta_n$ the new value of the line is $\left\{\pm\sum_{r=1}^n 10^{n-r} \{\delta_r\}_+ \right\}_0^{39}$ the plus or minus sign being taken according as the final character is P(lus) or M(inus). If however it should happen that there is a character not included in these ten, it is to be considered that the tape has been incorrectly punched, and the machine stops with a continuous hoot (middle C).

Warning character X

Five character sequence. The last four characters form an instruction which is obeyed shortly after reading the last character of the sequence. Before doing so however the accumulator is filled from the long lines HK and PK, the former filling the least significant half. After the instruction has been obeyed it is emptied back into those lines. They therefore in effect act like an accumulator and are collectively called the `pseudo accumulator'.

[Input routine timing]

The speed of the input routine is mainly limited by the speed of the input process itself, i.e. special magnetic function T. With decimal input [``binary'' in the ms.] the speed is about half this.

Tape handling equipment

It is essential that some further equipment be provided for the handling of tapes independently of the computer. The first essential is a punch controlled from a keyboard. Almost equally important are means for copying tapes and means for converting material on tapes into a typewritten form. These are standard teletype apparatus. The provisions mentioned below are not intended to be final but merely represent our intentions at the time of writing.

It is intended that there shall be two keyboard perforators. These will have 32 keys with the characters /,E,...,£ engraved, punching the corresponding combinations when depressed. At the same time the teleprint signal corresponding to the character in question is transmitted along a teleprint line. There are also keys engraved with [0,]1,2,3,4,5,6,7,8,9,¼,-,. whose effects are respectively duplicates of /,E,@,A,:,S,I,U,¼,D,P,M,F. There will also be a reperforator which accepts signals from a teleprint line and punches the corresponding combinations of holes. There will be two tape readers which accept an input of tape and provide an electrical output of teleprint signals. These units may be coupled in various ways. For instance one may connect both a reader and a keyboard perforator to a reperforator. This enables one to copy a tape with interpositions of new material, and possibly with omissions. In another arrangement the tape from the reperforator may be threaded through the reader, which is electrically connected to the reperforator. Under these circumstances a given sequence of characters can be repeated indefinitely often on a tape.

There will also be a mechanism for the printing out of the contents of tapes. In the letter shift position this will give the standard characters used in this handbook; on figure shift the equivalents

0	1	2	3	4	5	6	7	8	9	¼	-	.
/	E	@	A	:	S	I	U	¼	D	P	M	F

will apply. The changes from figure to letter shift and back are manually controlled.