CENTRAL PROCESSING UNIT

CPU is an acronym for a central processing unit, which is considered to be the brain of a computer. The part of a computer that interprets and carries out instructions provided by the software. It tests and manipulates data, and transfers information to and from other components, such as the working memory, disk drive, monitor, and keyboard. The central processing units of personal computers are generally implemented on a single chip, called a microprocessor.

The CPU is the most important part of a computer of any size. It controls everything from the operating system and software to games and hardware. The CPU is housed in the computer’s motherboard and is responsible for instructing the computer what to do and when, such as connect to the Internet and write documents. It performs all analytical, computational and logical functions that occur within THE computer. The CPU interprets instructions from each program and piece of hardware and processes the data.

CPU speed accounts for the amount of cycles that a CPU performs per second, which controls how quickly it accomplishes its task of doling out instructions of what to do. The more cycles it perform, also known as clock speed, the more instructions it can execute, which in the end determines how fast a computer operates. The first Intel CPU operated at a speed of 108 KHz, which is equivalent to 108,000 cycles per second. Today’s CPUs are much faster, with speeds as fast as 3.5 GHZ, or 3.5 billion cycles per second.

 

HOW THE CPU WORKS

The CPU is centrally located on the motherboard. Since the CPU carries out a large share of the work in the computer, data pass continually through it. The data come from the RAM and the units (keyboard, drives, etc.). After processing, the data is sent back to the RAM and the units.

The CPU continually receives instructions to be executed. Each instruction is a data processing order. The work itself consists mostly of calculations and data transport.

 

THE INSTRUCTION-EXECUTION CYCLE

Many types of personal computers can execute instructions in less than one-millionth of a second; supercomputers can execute instructions in less than one-billionth of a second.

The CPU performs four steps in executing an instruction:

1. The control unit gets the instruction from memory.
2. The control unit decides what the instruction means and directs the necessary data to be moved from the memory to the arithmetic logic unit.
3. The arithmetic logic unit performs the actual operation on the data.
4. The result of the operation is stored in memory or a register.

The first two instructions make up what is called the instruction time. The last two instructions make up what is called the execution time.

The combination of these two is called a machine cycle.

Each central processing unit has an internal clock (or system clock), which produces pulses at a fixed rate to synchronise all computer operations. A single machine cycle instruction is made up of a number of sub – instructions, each of which must take at least one clock cycle.

Each type of CPU is designed to understand a specific group of instruction called the instruction set.

 

COMPONENT OF CENTRAL CONTROL UNITS

The components of the central processing units are:

• Arithmetic logic unit.
• Memory or storage unit.
• Control unit.

 

A) Arithmetic logic unit: This unit consists of two sub – section namely:

1. Arithmetic section.
2. Logic Section.

 

1) Arithmetic section: The function of Arithmetic section is to perform arithmetic operations like addition, subtraction, multiplication and division. All complex operations are done by making repetitive use of above operations.

2) Logic Section: Function of logic section is to perform logic operations such as comparing, selecting, matching and merging of data.

 

Functions of Arithmetic Logic Unit (ALU)

1. The arithmetic logic unit executes arithmetic and logical operations.
2. Arithmetic operations include addition, subtraction, multiplication and division.
3. Logical operations compare numbers, letters and special characters.
4. The arithmetic logic unit performs logic functions such as AND, OR and NOT.
5. The accumulator is used to accumulate results. It is the place where the answers from many operations are stored temporarily before being put out to the computer’s memory.
6. The other general-purpose registers hold data on which operations are to be performed by the arithmetic logic unit.

 

 

B) Memory Unit: This unit can store instruction, data and intermediate results. This unit supplies information to the other units of the computer when needed. It is also known as internal storage unit or main memory or primary storage or Random access memory (RAM).

Its size affects speed, power and capability. There are primary memory and secondary memory two types of memories in the computer.

 

Functions of Memory Units

1. The Memory Unit is the part of the computer that holds data and instructions for processing.
2. Memory associated with the CPU is also called primary storage, primary memory, main storage, internal storage and main memory.
3. When we load software from a floppy disk, hard disk or CD-ROM, it is stored in the Main Memory.
4. It stores all the data to be processed and the instructions required for processing.
5. It stores intermediate results of processing.
6. It stores final results of processing before these results are released to an output device.
7. All inputs and outputs are transmitted through main memory.

 

 

C) Control Unit: The control unit coordinates the components of computer system. It fetches the code of all the instructions in the program. It directs the operation of the other units by providing timing and control signals. All computer resources are managed by the control unit. Control units direct the flow of data between the central processing unit and the other devices. The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it.

 

Functions of the control units

1. The control unit directs the entire computer system to carry out stored program instructions.
2. The control unit must communicate with both the arithmetic logic unit and main memory.
3. The control unit uses the instruction contained in the Instruction Register to decide which circuit need to be activated.
4. The control unit co-ordinates the activities of the other two units as well as all peripheral and auxiliary storage devices linked to the computer.
5. The control unit instructs the arithmetic logic unit which arithmetic operations or logical operation is to performed.
6. It is responsible for controlling the transfer of data and instructions among other units of a computer.
7. It manages and coordinates all the units of the computer.
8. It obtains the instructions from the memory, interprets them and directs the operation of the computer.
9. It communicates with Input / Output devices for transfer of data or results from storage.