Arithmetic logic unit

(Arithmetic Logic Unit) The high-speed CPU circuit that does calculating and comparing. Numbers are transferred from memory into the ALU for calculation, and the results are sent back into memory. Alphanumeric data are sent from memory into the ALU for comparing. The results are tested by GOTOs; for example, IF ITEM EQUALS ITEM GO TO UPDATE ROUTINE.

Some chips have multiple ALUs that allow for simultaneous calculations. For example, Chromatic Research's Mpact media processor chip had 450 of them. It allowed audio, video and other multimedia processes to be performed simultaneously. See Mpact chip.

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

Thirty Years Later
The ALU embedded within this 386 chip would fit on the end of a pencil eraser with room to spare. (Image courtesy of Intel Corporation.)

In computing, an arithmetic logic unit (ALU) is a digital circuit that performs arithmetic and logical operations. The ALU is a fundamental building block of the central processing unit (CPU) of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers. The processors found inside modern CPUs and graphics processing units (GPUs) have inside them very powerful and very complex ALUs; a single component may contain a number of ALUs.

Mathematician John von Neumann proposed the ALU concept in 1945, when he wrote a report on the foundations for a new computer called the EDVAC.

A typical schematic symbol for an ALU: A & B are the data (registers); R is the output; F is the Operand (instruction) from the Control Unit; D is an output status.