Stands for "Arithmetic Logic Unit." An ALU is an integrated circuit within a CPU or GPU that performs arithmetic and logic operations. Arithmetic instructions include addition, subtraction, and shifting operations, while logic instructions include boolean comparisons, such as AND, OR, XOR, and NOT operations.
ALUs are designed to perform integer calculations. Therefore, besides adding and subtracting numbers, ALUs often handle the multiplication of two integers, since the result is also an integer. However, ALUs typically do not perform division operations, since the result may be a fraction, or a "floating point" number. Instead, division operations are usually handled by the floating-point unit (FPU), which also performs other non-integer calculations.
While the ALU is a fundamental component of all processors, the design and function of an ALU may vary between different processor models. For example, some ALUs only perform integer calculations, while others are designed to handle floating point operations as well. Some processors contain a single ALU, while others include several arithmetic logic units that work together to perform calculations. Regardless of the way an ALU is designed, its primary job is to handle integer operations. Therefore, a computer's integer performance is tied directly to the processing speed of the ALU.