When we desire the machine to consult a table of values (i.e., a _function_—see Supplement 2), we punch the table with its arguments and function values on a tape, and we put the tape on a value tape feed mechanism. The instruction to the machine may be: Take the number in register _A_; find the value of the function for this number, and enter it in register _B_. The coding is: —— —— 73 _A_ 7654 61 —— —— 762 —— —— 543 —— —— 75431 841 7654 —— _A_ 763 6421 8762 _B_ 73 —— 8763 7 Without explaining this coding line by line, we can say that this is what happens: The machine reads the argument in register _A_. The machine reads the argument in the table at which the value tape feed is resting. It subtracts them, and thereby determines how far away the desired argument is. The machine then turns the tape that required distance. It checks that the new argument is the wanted argument. It reads the value of the function entered at that point on the function tape. Selecting There is a storage counter in the machine that is called the _selection counter_. The selection counter is counter 70 and has the code 732. It has all the properties of an ordinary storage counter and, in addition, one extra property: depending on the sign of the number stored in the selection counter, it is possible to select whether some other number shall be treated positively or negatively. In other words, addition of a number anywhere in the machine may take place either positively or negatively, if the number stored in the selection counter is positive or negative, respectively. For example, suppose that _x_ is the number in the selection counter. Suppose that _y_ is the number in some other counter _A_. Suppose that _z_ is the number in counter _B_. Suppose that we use the coding: _A_, _B_, 7432 What we get in _B_, because of the 7432 in the third or _C_ field, is _z_ plus _y_ if _x_ is positive or zero, and _z_ minus _y_ if _x_ is negative. In the language of the algebra of logic (see Chapter 9 and Supplement 2), where _T_( ...) is “the truth value of ...,” the number in _b_ equals: _z_ + _y_·_T_(_x_ ≥ 0) - _y_·_T_(_x_ < 0) (The nines complement of 0, namely 999···9 to 24 digits, is treated by the machine as negative.)