The central processing unit(CPU) is the brain of any computer. It carries out all the processing in the computer. Central Processing Unit itself consists of three main subsystems. The first one is Control Unit, the second is Registers, and the third is Arithmetic and Logic Unit(ALU).
A CPU works in a fetch execute cycle. On power on, the CPU fetches the first instruction from a location specified by the program counter. This instruction is brought into instruction register which is decoded by the control unit. Based on the instruction, the control unit would either fetch the operand and or carry out arithmetic or logical operations on it, or store the result of such an operation into a specified memory location. After one instruction is executed the next instruction is fetched by the processor and executed. This process goes on till the processor does not come to an halt instruction. A real life processor would have large number of registers, sophisticated microprogram control unit and a sophisticated arithmetic and logic unit. Most powerful processors currently popular are from from Intel, Pentium III and Pentium IV.