Adders & Subtractors

Adder and Subtractor circuits are used to perform basic calculations in computer systems. Adders, as their name suggests, add binary digits. Since binary numbers consist of only two digits, 1 and 0, it is almost always necessary to carry a digit to the next higher-order column when adding. For example 1 + 1 = 0 carry 1. There are always 3 inputs into a full adder; the 2 digits being added and the carry from the adjacent lower order column (A, B & CI). Figure 15 shows the circuit for a "Full-adder" function.

Full-Adder Circuit

Subtractor circuits are again a combination of basic gates with the inputs A, B and BRI. BRI is the borrowed digit from the subtraction in the adjacent lower-order column (if applicable). The outputs are D, the difference between the digits in the subtraction, and BRO, the digit borrowed from the adjacent higher-order column (if applicable).