1.3 CENTRAL PROCESSING UNIT (CPU)

 In a human body, the brain takes all major decisions and the other parts of the body function as directed by the brain. Similarly, in order to work a computer, also need some sort of “brain” or “calculation capacity”. In a computer system, there is a device roughly the size of a large postage stamp, known as the central processing unit (CPU). In a computer system, all major calculations and comparisons are made inside the CPU . It is also responsible for activating and controlling the operations of other units of a computer system.

The CPU resides inside a box known as the system unit, along with various support devices and tools for storing information. Just think of the system unit as a container for the CPU. The system unit case, that is the metal case itself, usually sits on top of your desk, often underneath the screen (as shown in Figure-1), or it can be taller and narrow, in which case it generally sits underneath your desk and is referred to as a tower case (as shown in Figure-2).

CPU reads and executes program instructions, performs calculations, and makes decisions. The CPU is mainly responsible for storing and retrieving information on disks and other media. It also handles information from one part of the computer to another and directs the flow of traffic throughout the computer system. Below you can see meaning of each word of a Control Processing Unit.

A block diagram of a modern computer is shown in Figure 6:
Basically the Control Unit (CU) and the Arithmetic-Logic-Unit (ALU) of a computer system are jointly known as the Central Processing Unit (CPU). Logically we can say:

CPU = CU + ALU

Thus the CPU has mainly two parts:
• Control Unit (CU) ; and
• Arithmetic-Logic Unit (ALU).

The Control Unit (CU) controls the entire operation of a computer. It is responsible of carrying out program instructions and telling the rest of the computer system what to do. It manages and coordinates the entire computer system.

It obtains instructions from the program stored in the main memory, interprets the instructions, and issues signals that cause other units of the system to execute them. Although, it does not perform any actual processing on the data, the control unit acts as a central nervous system for the other components of the computer. It manages and coordinates the entire computer system.

The Arithmetic-Logic-Unit (ALU) of a computer system is the place where the actual execution of the program instructions takes place during the processing operation.

The internal architectural design of computers differs from one system to another. However, the basic organization remains the same for all computer system. A block diagram of the basic computer organization is shown in Figure 6. Now let us look at each part of CPU in detail.

1.3.1 Control Unit (CU)

How does the input device know that it is the time for it to feed data into the storage unit (or memory)? How does the ALU know what should be done with the data once those are received?

And how the final results are sent to output device (monitor/screen or printer etc.), not the intermediate result? All this is possible because of the control unit (CU) of the computer system. By selecting, interpreting and seeing to the execution of the program instructions, the control unit is able to maintain order and direct the operation of the entire system. The control unit (CU) basically does the following things:

• It manages and coordinates the entire computer system. 
• The CU is responsible of carrying out program instructions and telling the rest of the computer system what to do.
• It obtains the instructions from the program stored in the main memory, interprets the instructions and issues signals that cause other units of the system to execute them.
• The Instruction Register contains a current instruction once it has been fetched from the primary memory. The control unit uses the instruction contained in the Instruction Register to decide which circuits need to be activated (see Figure 5). Program counter contains the address of the next instruction to be fetched for execution (Figure 5).

• It communicates with both the arithmetic logic unit and main memory. 

• 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.

• The CU instructs the arithmetic logic unit which arithmetic operations (such as addition, subtraction etc.) or logical operation (comparison between two number) is to be performed through control logic unit shown in Figure 7. 


 Specialized electronic circuits (fetch and decode unit) in the control unit is designed to fetch and decode program instructions held in the main memory. Each instruction is read from the memory into the instruction register. The process of reading an instruction is often referred to as the fetch-execute process.

1.3.2 Arithmetic-Logic Unit (ALU)

The arithmetic Logic unit (ALU) of a computer system is the place where the actual execution takes place.

• All the Arithmetic calculations such as addition, subtraction, multiplication, and division are performed and all comparisons (decisions) are made in the ALU.

• Relational operator (=, <, >), i.e., equal to, less-than and greater-than are used to describe the comparison operations by the ALU.

• ALU carries out arithmetic operations on integer (whole number) and real (with a decimal point) operands. It performs simple logical tests for integers operands only.

• The data and instructions, stored in the primary memory prior to the processing, are transferred as and when need to the ALU, when processing takes place.

• No processing is done in the primary storage unit (or memory).

• Intermediate results generated in the ALU are temporarily transferred back to the primary storage unit until needed at later time.

• Data may thus move from primary storage to ALU and back again to storage many times before the processing is over.

• After the completion of processing, the final results, which are stored in the storage unit, are released to an output device (monitor/screen or printer).

• The accumulator is used to accumulate results. It is the place where the answers of many operation are stored temporarily before being sent to the computer's memory (see Figure 6). Only the final result is transmitted to the memory unit for storage.

To understand the operation of CU, ALU etc., let us execute the statement (or command) R=n1+n2. This statement adds two numbers n1 and n2 and places the result in a location X (refer the Figure 8). 



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