Source : Free On-Line Dictionary of Computing

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 {bit}s 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.
     
        (1995-03-24)