Computer Dictionary of Information Technology

Arithmetic and Logic Unit

(ALU or "mill") The part of the central processing unit which performs operations such as addition, subtraction and multiplication of integers and bit-wise AND, OR, NOT, XOR and other Boolean operations. The CPU's instruction decode logic determines which particular operation the ALU should perform, the source of the operands and the destination of the result.

The width in bits of the words which the ALU handles is usually the same as that quoted for the processor as a whole whereas its external busses may be narrower. Floating-point operations are usually done by a separate "floating-point unit". Some processors use the ALU for address calculations (e.g. incrementing the program counter), others have separate logic for this.