Browse Definitions :
Definition

arithmetic-logic unit (ALU)

Contributor(s): David Brown and Theuns Kruger

An arithmetic-logic unit (ALU) is the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words. In some processors, the ALU is divided into two units, an arithmetic unit (AU) and a logic unit (LU). Some processors contain more than one AU - for example, one for fixed-point operations and another for floating-point operations. (In personal computers floating point operations are sometimes done by a floating point unit on a separate chip called a numeric coprocessor.)

Typically, the ALU has direct input and output access to the processor controller, main memory (random access memory or RAM in a personal computer), and input/output devices. Inputs and outputs flow along an electronic path that is called a bus. The input consists of an instruction word (sometimes called a machine instruction word) that contains an operation code (sometimes called an "op code"), one or more operands, and sometimes a format code. The operation code tells the ALU what operation to perform and the operands are used in the operation. (For example, two operands might be added together or compared logically.) The format may be combined with the op code and tells, for example, whether this is a fixed-point or a floating-point instruction. The output consists of a result that is placed in a storage register and settings that indicate whether the operation was performed successfully. (If it isn't, some sort of status will be stored in a permanent place that is sometimes called the machine status word.)

In general, the ALU includes storage places for input operands, operands that are being added, the accumulated result (stored in an accumulator), and shifted results. The flow of bits and the operations performed on them in the subunits of the ALU is controlled by gated circuits. The gates in these circuits are controlled by a sequence logic unit that uses a particular algorithm or sequence for each operation code. In the arithmetic unit, multiplication and division are done by a series of adding or subtracting and shifting operations. There are several ways to represent negative numbers. In the logic unit, one of 16 possible logic operations can be performed - such as comparing two operands and identifying where bits don't match.

The design of the ALU is obviously a critical part of the processor and new approaches to speeding up instruction handling are continually being developed.

This was last updated in September 2005

Join the conversation

2 comments

Send me notifications when other members comment.

Please create a username to comment.

What is the power consumed by an ALU?
Cancel

To be honest and quite clear, out of all information about ALU, this is the BEST explanation ever! Great Job to the Editor!

Cancel

-ADS BY GOOGLE

File Extensions and File Formats

Powered by:

SearchCompliance

  • risk management

    Risk management is the process of identifying, assessing and controlling threats to an organization's capital and earnings.

  • compliance as a service (CaaS)

    Compliance as a Service (CaaS) is a cloud service service level agreement (SLA) that specified how a managed service provider (...

  • data protection impact assessment (DPIA)

    A data protection impact assessment (DPIA) is a process designed to help organizations determine how data processing systems, ...

SearchSecurity

  • quantum key distribution (QKD)

    Quantum key distribution (QKD) is a secure communication method for exchanging encryption keys only known between shared parties.

  • identity theft

    Identity theft, also known as identity fraud, is a crime in which an imposter obtains key pieces of personally identifiable ...

  • cybercrime

    Cybercrime is any criminal activity that involves a computer, networked device or a network.

SearchHealthIT

SearchDisasterRecovery

  • disaster recovery plan (DRP)

    A disaster recovery plan (DRP) is a documented, structured approach that describes how an organization can quickly resume work ...

  • business continuity plan (BCP)

    A business continuity plan (BCP) is a document that consists of the critical information an organization needs to continue ...

  • disaster recovery team

    A disaster recovery team is a group of individuals focused on planning, implementing, maintaining, auditing and testing an ...

SearchStorage

  • logical unit number (LUN)

    A logical unit number (LUN) is a unique identifier for designating an individual or collection of physical or virtual storage ...

  • NVMe over Fabrics (NVMe-oF)

    NVMe over Fabrics, also known as NVMe-oF and non-volatile memory express over fabrics, is a protocol specification designed to ...

  • CIFS (Common Internet File System)

    CIFS (Common Internet File System) is a protocol that gained popularity around the year 2000, as vendors worked to establish an ...

Close