In digital circuits and electronics, an adder/subtractor is a circuit that is capable of adding or subtracting numbers, typically 4-bit binary numbers. The circuit has a mode switch that allows the user to choose between adding and subtracting. It is also possible to build a circuit that adds and subtracts at the same time.

 

To perform binary addition, you simply add the zeroes and ones. With binary addition one plus one equals zero, one plus zero equals one, and zero plus zero equals zero. The 4-bit adder using full-adder circuits is capable of adding two 4-bit numbers resulting in a 4-bit sum and a carry digit. For binary subtraction, two’s complement is used to make things simple. In practice, each digit of the number being subtracted is flipped, and then the number one is added to it. This two’s complement number is then added to the first number. Whether you are adding or subtracting, the last step is always adding numbers.

 

In many computers and other types of processors, adder/subtractors are used not only in the arithmetic logic units, but also in other parts of the processor, where they are used to calculate addresses, table indices, increment and decrement operators, and comparable operations.