Arithmetic and Logic Unit (ALU)
Arithmetic and Logic Unit is the part of a computer processor which is used to perform mathematical and boolean operations. The operations performed in this unit include adding, subtracting, multiplying, dividing, counting, shifting, complementing, and comparing.
In some processors the ALU can be divided into two units, an arithmetic unit (AU) and a logic unit (LU). Some processors contain more than one AU. In such processors, one AU is used for fixed-point operations and another AU is used for floating-point operations. Some Personal computers have a seperate chip called numeric coprocessor for floating point operations.
The ALU functions under the direct control of the control unit. The control unit determines when the services of the ALU are needed, and it provides this unit with the data that it will need. The data which is required to perform the operation are inputs, and the result of the opeartion is output. Inputs and outputs flow through an electronic path that is called a bus.
The ALU has data inputs and control inputs. Numbers are transferred from memory as inputs into the ALU for calculation, and the results are sent back into memory as outputs. Alphanumeric data are sent from memory into the ALU for comparing. The input consists of an instruction word one or more operands, and sometimes a format code.
The operation code tells the ALU what operation to perform and the operands are used in the operation. The output consists of a result and settings. The settings indicate whether the operation was performed successfully. The results are stored in the storage place called registers. The ALU includes storage places for input operands, operands that are being added, the accumulated result, and shifted results.