THE PROCESSOR - CPU( Central Processing Unit)

CPU(CENTRAL PROCESSING UNIT)


   The main unit inside the computer is the CPU. This Unit is responsible for all events inside the computer. It controls all internal and external devices, performs arithmetic and logic operation. The CPU is the device that interprets and executes instructions.

  The operations, a microprocessor performs are called the instruction set of this processor. Processors differs from one to another by the instruction set. If the same program can run on two different computer brands they are said to be compatible. Programs written for IBM compatible computers will not run on apple computers because these two architectures are not compatible.

                        Mainframes and early minicomputers contained circuit boards full of integrated circuits cards that implemented the central processing unit. Today's single-chip central processing units, called microprocessors, make personal computers and workstations possible. The CPU has the ability to fetch, decode, and execute instructions and transfer information to and from other resources over the computer's main data-transfer path, the bus. By definition, the CPU is the chip that functions as the "brain" of a computer. In some instances, however the term encompasses both the processor and the computer's memory or, even more broadly, the main computer console.
         
The CPU is  composed of several units:-



THE CONTROL UNIT (CU)   directs and controls the activities of the internal and external devices. It interprets the instructions obtained into the computer, determines what data are needed where it is stored, where t store the results of the operation, and sends the control signals to the devices involved in the execution of the instructions.

THE ARITHMETIC AND LOGIC UNIT(ALU) is the part where actual computations take place. It  consists of circuits which perform arithmetic operations (e.g. addition, subtraction, multiplication, division) over data received from memory and capable to compare numbers.
                                  While performing these operations the ALU takes data from the temporary storage are inside the CPU needed REGISTERS. Registers are a group of cells used for memory  addressing, data manipulation and processing.  Some of the registers are general purpose and some are reserved for certain functions. It is a high speed memory, which holds only data for the next instruction, they are sent back to the main memory and registers are occupied be the new data used in the next instruction.

All the activities in the computer system are composed of thousands of individual steps. This steps should follow in some order in fixed intervals of time. These interval are generated by the CLOCK UNIT. Every operation within the CPU takes place at the clock pulse. No operation, regardless of how simple  can performed in less time than transpires between ticks of this clock. But some operations require more than one clock pulse. The faster clock runs, the faster the computer performs. The clock  rate is measured in megahertz(MHz) or million ticks per second. In most modern microprocessors the clock is usually incorporated within the CPU.