The arithmetic-logic unit (ALU) is an area of the processor responsible for the rigorous processing of mathematical and logical operations required by programs.

Innocuous-looking when separated from the motherboard, the CPU simply looks like a large silicon chip with copper pins radiating from its perimeter or, as with more modular units, a plastic cartridge containing such a chip. The look of the CPU may be deceiving, however; it is made up of a few cooperating parts. One area is called an ALU. It is the ALU that helps smooth the functioning of a high--level spreadsheet and keeps the three dimensional, high--definition graphics of a space war game moving with satisfying fluidity. Even the program that stores, sorts, and prints the family recipes requires the prompt return of calculations from the ALU in order to be competitive with the shoebox of handwritten cards.

An ALU's...