The microprocessor, (or CPU), is the brain of the computer. We can say that the heart of all personal computers and most workstations sits.The first microprocessors used a word size of only 4 bits, so that the transistors of its logic circuits would fit onto a single part. One or more microprocessors typically serve as the processing elements of a computer system.The first Microprocessors made possible in the mid-1970s. Before that, CPUs were typically made from small-scale integrated circuits containing the equivalent of only a few transistors.Now,it has become the most prevalent implementation of the CPU, almost completely replacing all other forms. In the world of personal computers, the terms microprocessor and CPU are used interchangeably . Microprocessors control almost all the logic of digital devices.

Three basic characteristics differentiate microprocessors:

• Instruction set : The set of instructions that the microprocessor can execute.

• bandwidth : The number of bits processed in a single instruction.

• clock speed :The clock speed determines how many instructions per second the processor can execute in in megahertz (MHz).

The higher the value of thdese three basic characteristics , the more powerful the CPU,for example:32-bit microprocessor that runs at 50MHz is more powerful than a 16-bit microprocessor that runs at 25MHz.

There are two type microprocessors :

1.RISC (reduced instruction set computer)

2.CISC (complex instruction set computer).

RISC : As per its name,it is designed to perform a smaller number of types of computer instructions so that it can operate at a higher speed(MIPS) . Since each instruction type that a computer must perform requires additional transistors and circuitry, a larger list or set of computer instructions tends to make the microprocessor more complicated and slower in operation.The PowerPC microprocessor, used in IBM’s RISC System/6000 workstation and Macintosh computers, is a RISC microprocessor.

CISC : As per its name,it is designed to perform with a full set of computer instructions that were intended to provide needed capabilities in the most efficient way.Intel’s Pentium microprocessors are CISC microprocessors. RISC takes each of the longer, more complex instructions from a CISC design and reduces it to multiple instructions that are shorter and faster to process.