ALU

An arithmetic logic unit (ALU) is a digital circuit that performs arithmetic operations such as addition and subtraction, as well as logical operations such as AND, OR, and XOR, and certain shift and rotate operations. It is a fundamental building block of the central processing unit (CPU) in most computer architectures. The ALU receives operands from registers or memory, executes the operation specified by the control unit, and returns a result to a register file. It may also generate status flags, such as carry/borrow, zero, sign, and overflow, which are used for conditional branching and multi-step operations.

Common ALU operations include addition, subtraction, bitwise operations, shift left/right, rotate, and comparisons. Some ALUs perform multiply and divide either directly or via dedicated units; many systems implement multiply and divide in separate functional units due to their higher latency. The exact set of operations is determined by the instruction set architecture and the microarchitectural design.

ALUs are implemented using combinational logic, often employing adders, multiplexers, and logic gates. In modern CPUs, multiple ALUs may operate in parallel, and separate integer and floating-point units may exist to handle different classes of operations. The width of an ALU (for example, 8-, 16-, 32-, or 64-bit) determines the size of data it can process in a single operation; wider ALUs process more data per cycle but require more area and power.

Historically, the ALU has evolved from simple adders in early machines to complex, pipelined units with integrated status flags and interaction with the control unit. In contemporary designs, the ALU is integrated with other execution units in the CPU’s pipeline and supports the instruction set architecture through coordinated control signals.