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: