How Information Is Manipulated in the Machine
The main part of the computing system consists of 27 large frames loaded with relays and wiring, called the computer, or Computer 1 and Computer 2. In this “telephone central station,” all the “phone calls” from one number to another are attended to. There are 8 types of these frames in the computer:
| Frames | Number |
|---|---|
| Storing register frames | 6 |
| Printer frames | 2 |
| Problem frames | 2 |
| Position frames | 2 |
| Calculator frames | 6 |
| Control frames | 2 |
| Routine frames | 4 |
| BTL (Block-Trig-Log) frames | 2 |
| Permanent table frames | 1 |
| Total | 27 |
In most but not quite all respects, the two halves, Computer 1 and Computer 2, can compute independently. The storing register frames hold enough relays to store 30 numbers. The registers for these numbers are named A, B, C, D, ···, M, N, O in two groups of 15 each. One group belongs to Computer 1 and the other to Computer 2. In each Computer, the calculator frames hold enough relays for storing two numbers (held in the X and Y registers) and for performing addition, subtraction, multiplication, division, and square root. In each Computer, the problem frame stores the numbers that are read off the problem tape and the table tapes, and the printer frame stores the numbers that are read into the printer. The printer frame also stores indications, for example, the signs of numbers, plus or minus, for purposes of combining them. These frames also hold the relays that control the printer, the problem tape, and the table tapes. Jointly for both Computers, the position frames connect a problem in some problem position to a Computer that becomes idle. For example, one problem may finish in the middle of the night; the machine automatically and unattended switches to another problem position and proceeds with the instructions there contained. A backlog of computing on hand can be stored in two of the problem positions, while the other two control the two Computers. In each Computer, the routine frames hold the relays that make the Computer follow the routine instructions. Jointly for both Computers, the remaining frames—the control frames, the BTL frames, and the permanent table frames—hold the relays that control: the alarms and lights for indicating failures; some circuits called the BTL controls; the tape processor; and the mathematical tables that are permanently wired into the machine. The permanent table frames hold the following mathematical functions (see Supplement 2): sine, cosine, antitangent, logarithm, and antilogarithm.
Storing
Numbers can be stored in the machine in the 30 regular storing registers of both Computers together. They can also be stored, at the cost of tying up some machine capacity, in the other registers: the 4 calculator registers, the 2 problem registers, the 2 table registers, and the 2 printer registers. Numbers can also be punched out on tape, in either of the two printers, and later read again from the tape. Labels identifying the numbers can also be punched and read again from the tape.
Each register in the machine stores a number in the biquinary notation, as explained above. In programming the machine, after mentioning a register it is necessary—as a part of the scheme for checking—to tell the machine specifically whether to hold the number in the register or to clear it.
Addition and Subtraction
The calculator frames can add two numbers together, if so instructed in the routine tape. Suppose that the two numbers are in the registers B and D and that we wish to put the sum in register F. Suppose that we wish to clear the D number but hold the B number after using them. The code on the routine tape is B H + D C = F. H and C coming right after the names of the registers always designate “hold” and “clear,” respectively.
The calculator frames can, likewise, subtract a number. The routine instruction B H-D C = F means:
Take the number in register B (hold it); subtract the number in D (clear it); put the result in F
Multiplication and Division
The calculator frames perform multiplication by storing the digits of the multiplier, adding the multiplicand over and over, and shifting, until the product is obtained. However, if the multiplier is 1989, for example, the calculator treats it as 2000-11. This short-cut applies to digits 6, 7, 8, 9 and cuts the time required for multiplying. The routine instruction is B H × D C = F.
The calculator performs division by repeated subtraction. The routine instruction is B H ÷ D C = F. The operation signs +,-, ×, ÷ actually appear on the keyboard of the perforator and on the printed tape produced by the printer.
Discrimination
Discrimination is the term used in the Bell Laboratories computer for what we have previously called selection, or comparison, or sequencing. The discriminator is a part of the calculator that compares or selects or decides—“discriminates.” The discriminator can decide whether a number is zero or not zero. In the language of the algebra of logic (see Chapter 9 and Supplement 2), if a is a number, the discriminator can find T(a = 0). The discriminator can also decide whether a number is positive or negative. In the language of logic, it can find T(a > 0) or T(a < 0). The actions that a discriminator can cause to be taken are:
Stop the machine.
Stop the problem, and proceed to another problem.
Stop the routine going on, and proceed with a new routine.
Permit printing, or prevent printing; etc.
In this way the discriminator can:
Distinguish between right and wrong results.
Tell that a certain result is impossible.
Recognize a certain result to be the answer.
Control the number of repetitions of a formula.
Change from one formula to another formula.
Check a number against a tolerance; etc.