ALU means Arithmetic Logic Unit and is a digital circuit in the Central Processing Unit (CPU) that performs logic operations e.g AND, OR and arithmetic operations e.g Subtraction, Addition. It also carries out bit-shifting operations. Bit-shifting pertains to the change of position of bits to the left or right. Bit-shifting is considered a division or multiplication operation.
Arithmetic Logic Unit is the calculator of the CPU since it performs all tasks related to arithmetic and logic operations. In some CPUs, it is divided into Logic Unit (LU) and Arithmetic Unit (AU). Modern computers have a powerful Arithmetic Logic Unit. The unit is made powerful to enable it handle even the most complex operations. The basic attributes of ALU are operations/results, algorithms and functional organization.
Functional organization consists of sequencing logic, operations logic and storage registers. ALU has access to input and output devices, control unit and Random Access Memory of computers. Registers store intermediate, input and output data. Therefore, ALU receives input data (operands) from registers. ALU receives instructions from the Control Unit (CU) on what to do with the acquired data.
CU controls all functions in the processor. It does so by giving instructions to all parts of the processor. The Control Unit gives instructions to ALU by sequencing signals on the function that should be performed. ALU stores the result of the data in the output register.
Since ALU is a combinational unit, its output data changes asynchronously in response to change in input data. The Control Unit picks the data from the output register and moves it between the memory, ALU and registers.
All information in computers is handled in binary form that is 0 and 1. A transistor switch is used to handle such data. An open transistor (no current) represent 0 while a closed transistor (has current) represents 1. ALU performs these operations by combining multiple transistors.