Organization of the General-Purpose Computer
When a man sits down at a desk to work on a computation, he has six things on his desk to work with: a work sheet; a desk calculator, to add, subtract, multiply, and divide; some rules to be followed; the tables of numbers he will need; the data for the problem; and an answer sheet. In his head, he has the capacity to make decisions and to do his work in a certain sequence of steps. These seven subdivisions of calculation are all found in the Bell Laboratories’ general-purpose relay computer. The general-purpose computer is a computing system, in fact, more than it is a single machine. The part of the system which does the actual calculating is called, in the following paragraphs, the computer, or else, since it is in two halves, Computer 1 and Computer 2.
Physical Units
The computing system delivered to the Ballistic Research Laboratories fills a room about 30 by 40 feet and consists of the following:
2 computers: panels of relays, wiring, etc., which add, subtract, multiply, divide, select, decide, control, etc.
4 problem positions: tables each holding 12 mechanisms for feeding paper tape, which read numbers and instructions punched on tape and convert them into electrical impulses.
2 hand perforators: keyboard devices for punching instructions and numbers on paper tape.
1 processor: a table holding mechanisms for feeding 2 paper tapes and punching a third paper tape, used for checking numbers and instructions punched on tape.
2 recorders: each a table holding a teletypewriter, a tape punch, and a tape feed, used for recording answers and, if necessary, consulting them again.
The 2 computers correspond to the work sheet, the desk calculator, and the man’s capacity to make decisions and to carry out a sequence of steps. The 4 problem positions correspond to the problem data, the rules, and the tables of numbers. The 2 recorders correspond to the answer sheet. The 2 hand perforators and the processor are auxiliary machines: they translate the ordinary language of arithmetic into the machine language of punched holes in paper tape.
This is the computing system as organized for the Ballistic Research Laboratories at Aberdeen. The one for the National Advisory Committee for Aeronautics has only 3 problem positions. The computer system may, in fact, be organized with 1 to 10 computers and with 1 to 20 problem positions.
The great bulk of this computing system, like the mechanical brains described in previous chapters, is made up of large numbers of identical parts of only a few kinds. These are: standard telephone relays; wire; and standard teletype transmitters, mechanisms that read punched paper tape and produce electrical impulses.
Numbers
The numbers that the Bell machine contains range from 0.1000000 to 0.9999999 times a power of 10 varying from 10,000,000,000,000,000,000 to 0.000,000,000,000,000,000,1, or, in other words, from 10¹⁹ to 10⁻¹⁹. The machine also contains zero and infinity: zero arises when the number is smaller than 10⁻¹⁹, and infinity arises when the number is equal to or greater than 9,999,999,000,000,000,000. (See Supplement 2.)
The system used in the machine to represent numbers on relays is called biquinary—the bi-, because it is partly twofold like the hands, and the -quinary because it is partly fivefold like the fingers. This system is used in the abacus (see Chapter 2 and Supplement 2). In the machine, for each decimal digit, 7 relays are used. These relays are called the 00 and 5 relays, and the 0, 1, 2, 3, and 4 relays. If, as before, 0 indicates a relay that is not energized and 1 indicates a relay that is energized, then each decimal digit is represented by the positioning of the 7 relays as follows:
| Decimal Digit | Relays | ||
|---|---|---|---|
| 00 | 5 | 0 1 2 3 4 | |
| 0 | 1 | 0 | 1 0 0 0 0 |
| 1 | 1 | 0 | 0 1 0 0 0 |
| 2 | 1 | 0 | 0 0 1 0 0 |
| 3 | 1 | 0 | 0 0 0 1 0 |
| 4 | 1 | 0 | 0 0 0 0 1 |
| 5 | 0 | 1 | 1 0 0 0 0 |
| 6 | 0 | 1 | 0 1 0 0 0 |
| 7 | 0 | 1 | 0 0 1 0 0 |
| 8 | 0 | 1 | 0 0 0 1 0 |
| 9 | 0 | 1 | 0 0 0 0 1 |
Then, for any decimal digit, one and only one of the 00 and 5 relays is energized, and one and only one of the 0, 1, 2, 3, and 4 relays is energized. If more or less than exactly one relay in each set is energized, then the machine knows that it has made a mistake, and it stops dead in its tracks. Thus any accidental failure of a relay is at once caught, and the chance of two compensating failures occurring at the same time is extremely small.