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Definition: ALU

(Arithmetic Logic Unit) The high-speed circuit in the CPU that does the calculating and comparing. Numbers are transferred from RAM (memory) into the ALU for calculation, and the results are sent back to RAM. Alphanumeric data are sent from RAM into the ALU for comparing. The results of the compare are tested and may cause the computer to go to another part of the program; for example, If ItemA equals ItemB GoTo UpdateRoutine.

Floating Point Operations
A division may result in a fraction, and while some ALUs handle floating point operations, which support fractions, others do not and require a separate circuit (see math coprocessor). See DSP.

Multiple ALUs
Some chips have multiple ALUs that allow for simultaneous calculations. In an extreme case, Chromatic Research's MPACT media processor had 450 ALUs. It allowed audio, video and other multimedia processes to be performed simultaneously (see MPACT chip). See computer, control unit and half adder.

An ALU in 1957
An arithmetic logic unit you have to sit back and admire. This floor-standing ALU was part of Honeywell's Datamatic 1000 computer. (Image courtesy of Honeywell, Inc.)

Thirty Years Later
In 1987, the ALU embedded within this 386 chip would fit on the end of a pencil eraser with room to spare. Today, an ALU takes up less space than the tip of the pencil. (Image courtesy of Intel Corporation.)