Dictionary.net

(ALU or "mill") The part of the centralprocessing unit which performs operations such as addition,subtraction and multiplication of integers and bit-wiseAND, OR, NOT, XOR and other Boolean operations. TheCPU's instruction decode logic determines which particularoperation the ALU should perform, the source of the operandsand the destination of the result.The width in bits of the words which the ALU handles isusually the same as that quoted for the processor as a wholewhereas its external busses may be narrower. Floating-pointoperations are usually done by a separate "floating-pointunit". Some processors use the ALU for address calculations(e.g. incrementing the program counter), others haveseparate logic for this.