Performing the arithmetic or logic operation:

• Generally ALU is used inside CPU to perform arithmetic and logic operation. ALU is a combinational logic circuit which does not have any internal storage.

Therefore, to perform any arithmetic or logic operation (say binary operation) both the input should be made available at the two inputs of the ALU simultaneously. Once both the inputs are available then appropriate signal is generated to perform the required operation.

We may have to use temporary storage (register) to carry out the operation in ALU .

The sequence of operations that have to carried out to perform one ALU operation depends on the organization of the CPU. Consider an organization in which one of the operand of ALU is stored in some temporary register Y and other operand is directly taken from CPU internal bus. The result of the ALU operation is stored in another temporary register Z. This organization is shown in the Figure 5.7.