Arithmetic Logic Units (ALU)

Arithmetic logic units (ALU) perform arithmetic and logic operations on binary data inputs. In some processors, the ALU is divided into two units: an arithmetic unit (AU) and a logic unit (LU). In processors with multiple arithmetic units, one AU may be used for fixed-point operations while another is used for floating-point operations. In some personal computers (PCs), floating-point operations are performed by a special floating-point AU that is located on a separate chip called a numeric coprocessor. Typically, arithmetic logic units have direct input and output access to the processor controller, main memory and input/output (I/O) devices. Inputs and outputs flow along an electronic path called a bus. Each input consists of a machine instruction word that contains an operation code, one or more operands, and sometimes a format code. The operation code determines the operations to perform and the operands to use. When combined with a format code, it also indicates whether the operation is fixed-point or floating-point. ALU outputs are placed in a storage register. Generally, arithmetic logic units include storage points for input operands, operands that are being added, the accumulated result, and shifted results.