ALU

Arithmetic Logic Units (ALUs)

An arithmetic and logic unit (ALU) is a combinational circuit that can perform any of a number of different arithmetic and logical operations on a pair of b-bit operands. The operation to be performed is specified by a set of function-select inputs. Legacy MSI ALUs have 4-bit operands and three to five function-select inputs, allowing up to 32 different functions to be performed. CPUs (Central Processing Units) contain ALUs.

The logic symbols for the 74x381 and 74x382 legacy MSI ALUs are shown in the figure below in (a) and (b), respectively. They each provide the eight different functions as detailed in the table below. Note that the identifiers A, B, and F in the table refer to the 4-bit words A3–A0, B3–B0, and F3–F0; and the symbols ·, +, and ⊕ refer to bit-by-bit logical AND, OR and XOR operations.