The Full form of ALU is Arithmetic Logic Unit. An ALU (Arithmetic Logic Unit) is a digital circuit in a computer processor used to perform arithmetic and logic operations. It represents the fundamental building block of the CPU (Central Processing Unit) of a computer. Modern CPUs contain complex and very powerful ALUs. Modern CPUs also contain a CU (control unit) in addition to ALUs. One or more ALUs perform most of the operations of a CPU, which load data from input registers.

A register is storage available in a small amount as part of a CPU. The control unit instruct the Arithmetic Logic Unit what operation to perform on that data, and the ALU stores the result in an output register. As it performs basic arithmetic and logic operations, so the examples of arithmetic operations include addition, multiplication, subtraction, and division and the examples of logic operations include comparisons of values such as AND, NOT, and OR. All information is manipulated and stored in a computer in the form of binary numbers, i.e. 0 and 1.

To manipulate binary numbers, transistor switches are used since there are only two possible states of a switch: open or closed. An open transistor represents a 0, through which there is no current. A closed transistor represents a 1, through which there is a current.