<P> For example, if an addition instruction is to be executed, the arithmetic logic unit (ALU) inputs are connected to a pair of operand sources (numbers to be summed), the ALU is configured to perform an addition operation so that the sum of its operand inputs will appear at its output, and the ALU output is connected to storage (e.g., a register or memory) that will receive the sum . When the clock pulse occurs, the sum will be transferred to storage and, if the resulting sum is too large (i.e., it is larger than the ALU's output word size), an arithmetic overflow flag will be set . </P> <P> Hardwired into a CPU's circuitry is a set of basic operations it can perform, called an instruction set . Such operations may involve, for example, adding or subtracting two numbers, comparing two numbers, or jumping to a different part of a program . Each basic operation is represented by a particular combination of bits, known as the machine language opcode; while executing instructions in a machine language program, the CPU decides which operation to perform by "decoding" the opcode . A complete machine language instruction consists of an opcode and, in many cases, additional bits that specify arguments for the operation (for example, the numbers to be summed in the case of an addition operation). Going up the complexity scale, a machine language program is a collection of machine language instructions that the CPU executes . </P> <P> The actual mathematical operation for each instruction is performed by a combinational logic circuit within the CPU's processor known as the arithmetic logic unit or ALU . In general, a CPU executes an instruction by fetching it from memory, using its ALU to perform an operation, and then storing the result to memory . Beside the instructions for integer mathematics and logic operations, various other machine instructions exist, such as those for loading data from memory and storing it back, branching operations, and mathematical operations on floating - point numbers performed by the CPU's floating - point unit (FPU). </P> <P> The control unit of the CPU contains circuitry that uses electrical signals to direct the entire computer system to carry out stored program instructions . The control unit does not execute program instructions; rather, it directs other parts of the system to do so . The control unit communicates with both the ALU and memory . </P>

This holds the actual processing in the cpu