Arithmetic logic unit

This article is about computer arithmetic units. An alternative meaning of ALU is Alu sequence.

An arithmetic[-]logic unit (ALU) is a core component of all computer CPUs. ALUs are capable of calculating the results of a wide variety of basic arithmetical computations, most commonly the following ones:

• integer arithmetic operations (addition, subtraction, multiplication)
• bitwise logic operations (AND, NOT, OR, XOR)
• bit-shifting operations (shifting or rotating a word by a specified number of bits to the left or right, with or without sign extension)

A standard ALU typically does not handle integer division or any floating point operations. For these calculations separate components, such as dividers and floating point units (FPUs), may be used, or a microcode program may use the ALU to emulate these operations.

The ALU takes as inputs the data to be operated on (called operands) and a code from the control unit indicating which operation to perform. The output is the result of the computation.

In many designs the ALU also takes/generates as inputs/outputs a set of condition codes from/to a status register; typically these codes are used to indicate cases such as carry-in or carry-out, overflow, divide-by-zero, etc.

See also

• ALU adder
• multiplication ALU
• execution unit
• 7400 series (chip # 74181)