How Information Goes Into the Machine
In order to put a problem into this machine—just as with the other machines—first a mathematician who knows how the problem is to be solved, and who knows how to organize it for the machine, lays out the scheme of calculation. Then, a girl goes to one of the hand perforators. Sitting at the keyboard, she presses keys and punches out feet or yards of paper tape expressing the instructions and numbers for the calculation. Each character punched—digit, letter, or sign—has one or more of a maximum of 6 holes across the tape. Another girl, using the other hand perforator, also punches out the instructions and numbers for the calculation. If she wishes to erase a wrong character, she can press an erase key that punches all 6 holes, and then the machine will pass by this row as if it were not there.
Three kinds of tapes are produced for the machine:
Problem tapes, which contain information belonging to the particular problem.
Table tapes, which contain tables of numbers to be referred to from time to time.
Routine tapes, which contain the program, or routine, or sequence of steps that the machine is to carry out.
In each of these tapes one character takes up ⅒ of an inch along the tape. In the case of a table tape, however, an ordinary 1-digit number requires 4 characters on the tape, and a 7-digit number requires 11 characters on the tape. On a table tape there will be on the average about 1 inch of tape per number.
The Processor
The two paper tapes prepared on the perforator should agree. But whether or not they agree, a girl takes them over to the processor and puts them both in. The processor has two tape feeds, and she puts one tape on each and starts the machine. The processor compares them row by row, making sure that they agree, and punches a new tape row by row. If the two input tapes disagree, the processor stops. You can look to see which tape is right, and then you can put the correct punch into the new tape with a keyboard mounted on the processor. As the processor compares the two input tapes, it also converts any number written in the usual way into machine language. For example, the processor will automatically translate 23,188 into +.231 8800 × 10⁺⁵. The processor also puts in certain safeguards. If you want it to, the processor will also make a printed record of a tape. Also, when a tape becomes worn from use in the machine, you can put it into the processor and make a fresh copy.
The Problem Positions
Next, the girl takes the punched tape made by the processor over to a problem position that is idle. Two of the problem positions are always busy guiding the two computers. The other two problem positions stand by, ready to be loaded with problems.
A problem position looks like a large covered-over table. Under the covers are 12 tape feeds, or tape transmitters. All these transmitters look exactly alike except for their labels and consist of regular teletype transmitters. Six-hole paper tape can be fed into any transmitter. Six metal fingers sense the holes in the paper tape and give out electrical impulses at proper times. At the front of the problem position is a small group of switches that provide complete control over the problem while it is on the machine. These are switches for starting, disconnecting, momentary stop, etc.
One tape transmitter is the problem tape transmitter. It takes in all the data for the problem such as the starting numbers. The first thing it does at the start of a problem is to check (by comparing tape numbers) that the right tapes are in the right feeds.
Five transmitters are routine tape transmitters. Each of these takes in the sequence of computing steps. The routine tapes also contain information for referring to table tapes and instructions for printing and punching tape. The machine can choose according to instructions between the five routine tapes and can choose between many different sections on each tape. Therefore, we can use a large number of different routines in a calculation, and this capacity makes the machine versatile and powerful.
Six transmitters are table tape transmitters. They read tables of numbers when directed to. A table tape can be as long as 100 feet and will hold numbers at the rate of 1 inch per number, so that about 1200 numbers of seven decimal digits can be stored on a table tape.
When we look up a number in a table, such as the following,
| 2½ | 3 | 3½ | ··· | |
| 1 | 1.02500 | 1.03000 | 1.03500 | |
| 2 | 1.05063 | 1.06090 | 1.07123 | |
| 3 | 1.07689 | 1.09273 | ··· | |
| 4 | 1.10381 | ··· | ··· | |
| 5 | 1.13141 | ··· | ||
| 6 | 1.15969 | |||
| 7 | ··· | |||
| 8 | ··· | |||
| 9 | ··· | |||
| 10 | ||||
| ··· | ··· | |||
we look along the top and down the side until we find the column and row of the number we are looking for. These are called the arguments of the tabular value that we are looking for (see Supplement 2). Now when we put this table on a tape to go into the Bell Laboratories machine, we write it all on one line, one figure after another, and we punch it as follows:
2-½ 1-5 1.02500 1.05063 1.07689 1.10381 1.13141
6-10 1.15969 ··· 11-15 ··· ··· ··· 3 1-5
1.03000 1.06090 ··· ··· 3½ 1-5 1.03500 ···
You will notice that the column labels 2½, 3, 3½ have been put on the tape, each in front of the group of numbers they apply to. The row labels 1 to 5, 6 to 10, ··· have also been put on the tape, each in front of the group of numbers they apply to. The appropriate column and row numbers, or arguments, must be put often on every table tape, so that it is easy for the machine to tell what part of the table tape it is reading.
In the Bell Laboratories machine, we do not need to put equal blocks of arguments like 1-5, 6-10 ··· on the table tape. Instead we can put individual arguments like 1, 2, 3, 4 ···, or, if we wish, we can use blocks of different sizes, like 1-3, 4-15, 16-30···. For some tables, such as income tax tables, it is very useful to have varying-sized blocks of arguments. The machine, when hunting for a certain value in the table, makes a comparison at each block of arguments.
The machine needs about 6 seconds to search a foot of tape. If we want to set up a table economically, therefore, we need to consider the average length of time needed for searching.