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. PROBLEMS