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What Is an Arithmetic Unit?

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  • Written By: John B Landers
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 April 2014
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    2003-2014
    Conjecture Corporation
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The arithmetic unit, also called the arithmetic logic unit (ALU), is a component of the central processing unit (CPU). It is often referred to as the “engine” of the CPU because it allows the computer to perform mathematical calculations, such as addition, subtraction, and multiplication. The ALU also performs logic operations, like “AND,” “OR,” and “NOT.” The arithmetic unit works along with the register array, which holds data, when processing any of these operations. The arithmetic unit is comprised of many interconnected elements that are designed to perform specific tasks.

Some central processing units are comprised of two components, an arithmetic unit and a logic unit. Other processors may have an arithmetic unit for calculating fixed-point operations and another AU for calculating floating-point computations. Some PCs have a separate chip known as the numeric coprocessor. This coprocessor contains a floating-point unit for processing floating-point operands. The coprocessor increases the operating speed of the computer because of the coprocessor ability to perform computation faster and more efficiently.

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Operations are processed when data is loaded into the arithmetic logic unit from one of the CPU’s input or processor registers. The register is a part of the computer’s overall memory apparatus that stores data. The data contained in these registers can be retrieved much quicker than from other storage locations. The control unit is part of the internal workings of the CPU and transmits instructions to the arithmetic control unit. The instructions are typically comprised of several parts. Generally, there is an operation code, at least one operand, and in some cases, a format code.

The operation code, or opcode, instructs the arithmetic logic unit on which operation to execute. The operands tells the ALU where the data is located and the address where the information can be retrieved. It also has the various functions that are to be used in the calculations, such as subtraction or a logic comparison. The format code is typically used with the operation code and may inform the ALU floating-point or fixed-point machine instruction word.

Typical tasks performed by arithmetic logic unit include addition and subtraction, multiplication and division, and logical tests. The ALU also performs comparisons and bit shifting operations. Bit shifting is the process involved in moving the smallest amounts of information (bit) handled by the computer. Bits are grouped together to form words from 32 to 64-bit in size.

Arithmetic units can be designed to perform virtually any computation. More expensive ALUs are needed for more complex operations. These arithmetic units are usually larger and require more space and power. Smaller arithmetic units can perform the calculation, but will take a longer time to complete the execution due to the multiple steps involved in processing the formula.

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