**The arithmetic logic unit (ALU**) is a digital building block capable of performing both arithmetic as well as logic operations. Arithmetic logic units that can perform a variety of arithmetic operations such as addition, subtraction, etc., and logic functions such as ANDing, ORing, EX-ORing, etc., on two four-bit numbers are usually available in IC form. The function to be performed is selectable from function select pins. Some of the popular type numbers of ALU include 74181, 74381, 74382, 74582 (all from the TTL logic family) and 40181 (from the CMOS logic family). Functional details of these ICs are given in the latter part of the chapter under the heading of Application-Relevant Information. More than one such IC can always be connected in cascade to perform arithmetic and logic operations on larger bit numbers.