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In digital circuits, an adder–subtractor is a circuit that is capable of adding or subtracting numbers (in particular, binary). Below is a circuit that does adding or subtracting depending on a control signal. It is also possible to construct a circuit that performs both addition and subtraction at the same time.

Role in the arithmetic logic unit


Adders are a part of the core of an arithmetic logic unit (ALU). The control unit decides which operations an ALU should perform (based on the op code being executed) and sets the ALU operation. The input to the adder–subtractor above would be one such control line from the control unit.

The adder–subtractor above could easily be extended to include more functions. For example, a 2-to-1 multiplexer could be introduced on each that would switch between zero and ; this could be used (in conjunction with ) to yield the two's complement of since .

A further step would be to change the 2-to-1 mux on to a 4-to-1 with the third input being zero, then replicating this on thus yielding the following output functions:

By adding more logic in front of the adder, a single adder can be converted into much more than just an adder—an ALU.

See also