A computer performs all calculation and comparison (decision-making) operations in the ALU. During processing of a job, the computer transfers data and instructions stored in its primary storage to ALU as and when needed. ALU does the processing and the computer temporarily transfers the intermediate results generated there back to primary storage unit needed later. Hence, data may move back and forth several times between primary storage and ALU before processing of the job is over.
The engineering design of a computer's ALU determines the type and number of arithmetic and logic operations that a computer can perform. However, almost all ALUs are designed to perform the four basic arithmetic operations (add, subtract, multiple, and divide) and logic operations or comparisons such as, less than, equal to and greater than.
An arithmetic logic unit (ALU) is a digital circuit used to perform arithmetic and logic operations. It represents the fundamental building block of the central processing unit (CPU) of a computer. Modern CPUs contain very powerful and complex ALUs. In addition to ALUs, modern CPUs contain a control unit (CU). Most of the operations of a CPU are performed by one or more ALUs, which load data from input registers. A register is a small amount of storage available as part of a CPU. The control unit tells the ALU what operation to perform on that data and the ALU stores the result in an output register. The control unit moves the data between these registers, the ALU, and memory.