Basic Structure and Functional Units of a Computer

Digital Principles and Computer Organization:

Chapter 3: Computer System

 

Basic Structure and Functional Units of a Computer

• A computer structure refers to the way in which different computer components are interconnected and organized. It shows the arrangement of units such as the CPU, memory, input/output devices and system buses.

• A computer function, on the other hand, describes the operations performed by each component as a part of the overall system. In other words, structure explains "how.components are arranged," while function explains "what each component does."

• Every computer performs four basic functions :

1. Data processing – Performing arithmetic and logical operations.

2. Data storage – Holding data and instructions temporarily or permanently.

3. Data movement – Transferring data between the computer and the outside world.

4. Control – Directing and coordinating the activities of all parts of the system.

1. Structural Components of a Computer

• Central Processing Unit (CPU)

■ Often called the brain of the computer.

■ Controls overall operations and performs data processing.

■ Major parts : Arithmetic and Logic Unit (ALU), Control Unit (CU) and Registers.

• Main memory

■ Stores data and instructions that are currently in use.

■ Provides fast, temporary storage directly accessible by the CPU.

• Input/Output (I/O) devices

■ Enable interaction with the external environment.

■ Input examples : Keyboard, mouse, scanner.

■ Output examples : Monitor, printer, speakers.

• System interconnection (Buses)

■ Provides a pathway for communication among CPU, memory and I/O devices.

■ Includes : Data bus, Address bus, and Control bus.

 

2. Functional Unitssive

• The computer consists of five functionally independent units:

■ Input

■ Memory

■ Arithmetic and logic

■ Output and

■ Control units.

• Fig. 3.1.2 (a) and (b) show these five functional units of a computer and its physical locations in the computer.

• The input unit accepts the digital information from user with the help of input devices such as keyboard, mouse, microphone etc.

• The information received from the input unit is either stored in the memory for later use or immediately used by the arithmetic and logic unit to perform the desired operations.

• The program stored in the memory decides the processing steps and the processed output is sent to the user with the help of output devices or it is stored in the memory for later reference.

• All the above mentioned activities are co–ordinated and controlled by the control unit.

• The arithmetic and logic unit in conjunction with control unit is commonly called Central Processing Unit (CPU).

1. Input Unit

• A computer accepts a digitally coded information through input unit using input devices.

• The most commonly used input devices are keyboard and mouse.

• The keyboard is used for entering text and numeric information.

• Mouse is used to position the screen cursor and thereby enter the information by selecting option.

• Apart from keyboard and mouse there are many other input devices are available, which include joysticks, trackball, spaceball, digitizers and scanners.

2. Memory Unit

• The memory unit is used to store programs and data.

• Usually, two types of memory devices are used to form a memory unit : primary storage memory device and secondary storage memory device.

• The primary memory, commonly called main memory is a fast memory used for the storage of programs and active data (the data currently in process).

• The main memory is a semiconductor memory.

• It consists of a large number of semiconductor storage cells, each capable of storing one bit of information.

• These cells are read or written by the central processing unit in a group of fixed size called word.

• The main memory is organized such that the contents of one word, containing n bits, can be stored or retrieved in one write or read operation, respectively.

• To access data from a particular word from main memory each word in the main memory has a distinct address. This allows to access any word from the main memory by specifying corresponding address.

• The number of bits in each word is referred to as the word length of the computer. Typically, the word length varies from 8 to 64 bits.

• The number of such words in the main memory decides the size of memory or capacity of the memory.

• The size of computer main memory varies from few million words to tens of million words.

• An important characteristics of a memory is an access time (the time required to access one word). The access time for main memory should be as small as possible. Typically, it is of the order to 100 nanoseconds. The access time depends on the type of memory. In randomly accessed memories (RAMs), fixed time is required to access any word in the memory. In sequential access memories this time is not fixed.

• The main memory consists of only randomly accessed memories. These memories are fast but they are small in capacities and expensive. Therefore, the computer uses the secondary storage memories such as magnetic tapes, magnetic disks for the storage of large amount of data.

Stored program concept

• Today's computer are built on two key principles

1. Instructions are represented as numbers.

2. Programs can be stored in memory to be read or written just like numbers.

• These principles lead to the stored–program concept.

• According to stored–program concept, memory can contain the program (source www.code), the corresponding compiled machine code, editor program and even the compiler that generated the machine code.

3. Arithmetic and Logic Unit

• The arithmetic and logic unit (ALU) is responsible for performing arithmetic operations such as add, subtract, division and multiplication and logical operations such as ANDing, ORing, Inverting etc.

• To perform these operations, operands from the main memory are brought into the high speed storage elements called registers of the processor.

• Each register can store one word of data and they are used to store frequently used operands.

• After performing operation, the result is either stored in the register or memory location.

4. Output Unit

The output unit sends the processed results to the user using output devices such as video monitor, printer, plotter, etc.

• The video monitors display the output on the CRT screen whereas printers and plotters give the hard–copy output.

• Printers are classified according to their printing methodology : Impact printers and non–impact printers.

5. Control Unit

• The control unit co–ordinates and controls the activities amongst the functional units.

• Control unit fetches the instructions stored in the main memory, identify the operations and the devices involved in it and accordingly generate control signals to execute the desired operations.

• It uses control signals or timing signals to determine when a given action is to take place.

 • It controls input and output operations, data transfers between the processor, memory and input/output devices using timing signals.

• The control and the arithmetic and logic units of a computer are usually many times faster than other devices connected to a computer system. This enables them ou to control a number of external input/output devices.

 

3. Interconnection of Components

• The interconnection of components in a computer system is handled by a set of communication pathways known as a System Bus.

• It is the primary communication pathway that connects the main components of a computer : the CPU (Central Processing Unit), the Memoryd Unit, and I/O (Input/Output) devices. It acts as a shared highway for data, addresses, and control signals, enabling all these components to work together seamlessly.

• The system bus is not a single bus but rather a collection of three distinct buses, each with a specific role :

1. Data bus

■ A bidirectional pathway that carries the actual data being transferred between the CPU, memory and I/O devices.

■ When the CPU reads data, it flows from memory (or I/O device) to the CPU. When the CPU writes data, it flows from the CPU to memory (or I/O device).

■ The width of the data bus (e.g., 8, 16, 32, or 64 bits) determines how many bits can be transferred simultaneously. A wider data bus allows more information to move in a single operation, improving performance.

2. Address bus

■ A unidirectional pathway used to specify the address of the memory location or I/O device that the CPU wants to access.

■ The CPU places the desired address on the address bus, and the memory or I/O device with that address responds.

■ The width of the address bus determines the maximum addressable memory. For instance, a 32–bit address bus can directly access 2³² = 4 GB of memory.

3. Control bus

■ A set of lines that carries control signals required to manage and coordinate all operations of the system bus.

■ Examples of control signals include :

➤ Memory read – Instructs memory to place data on the data bus.

➤ Memory write – Instructs memory to store data from the data bus.

➤ I/O read – Reads data from an input device.

➤ I/O write – Sends data to an output device.

 ■ The control bus ensures proper timing and prevents conflicts, so only one device drives the bus at a time.

1. Single Bus Structure

• In a computer system, the interconnection between the CPU, memory, and input/output devices can be achieved in different ways. One common method is the single bus structure.

• In this structure, all components are connected to a common communication path called the system bus. The system bus combines the address bus, data bus and control bus into a single bus, as shown in Fig. 3.1.5.

Working of a single bus structure

• Since all units share the same bus, only two units can communicate at a time (for example, CPU to memory, or CPU to I/O).

• Bus control lines are used to manage access when multiple units request the bus simultaneously.

• Multiplexed address/data lines are often used to reduce the number of pins. In such cases :

■ During the first part of the cycle, the address is placed on the bus.

■ In the second part, the same lines are used for data transfer.

■ To handle this, latches are required to store the address.

Advantages of single bus structure

1. Low cost : Since all devices share a common bus, hardware requirement are reduced.

2. Flexibility : Easy to attach additional peripheral devices.

3. Simplicity : Suitable for small systems where control logic is not very complex.

Limitations of single bus structure

1. Limited communication : Only one pair of devices can use the bus at a time, creating a bottleneck.

2. Bus arbitration required : Additional control logic is necessary to manage multiple requests for the bus.

3. Performance issues in large systems : With many devices connected, data transfer  speed decreases due to contention.

4. Signal quality : Large systems require bus drivers and receivers to maintain signal strength and timing.

2. Multiple Bus Hierarchies

• In a computer system, as the number of devices connected to a single bus increases, the overall performance begins to degrade. This performance drop happens mainly due to two reasons :

1. Bus sharing and propagation delays:

■ When many devices share the same bus, only one device can use the bus at a time

■ The sharing process requires coordination, and when control of the bus is frequently transferred from one device to another, propagation delays become noticeable.

■ These delays reduce the efficiency of the computer system.

2. Bus bottleneck :

■ As the combined data transfer demands of devices approach the maximum capacity of the bus, the bus becomes a bottleneck.

■ To overcome this, either the bus speed must be increased, or the bus must be made wider (more data lines).

■ However, with modern high–speed devices like video controllers and network interfaces, a single bus cannot practically handle all the traffic.

Need for multiple bus hierarchies

• Because of these limitations, modern computer systems use multiple buses organized in a hierarchical structure. This structure reduces contention on a single bus and improves overall system performance.

Traditional bus configuration

• Consists of three types of buses:

1. Local bus – Connects the CPU to cache and memory for high–speed access.

2. System bus – Connects the CPU, main memory and I/O subsystem.

3. Expanded bus – Provides connections for additional peripheral devices (e.g., printers, disk drives).

• This structure reduces the load on a single bus by distributing data transfers across multiple buses.

High–speed bus configuration

• In addition to the local, system, and expanded buses, a high–speed bus is introduced.

• The cache controller is connected to this high–speed bus.

• It supports devices with very high data transfer requirements, such as :

■ High–speed LANs (e.g., Fiber Distributed Data Interface – FDDI).

■ Video and graphics workstation controllers.

■ High–performance I/O controllers (e.g., SCSI, IEEE P1394/FireWire).

• By isolating high–bandwidth devices on a dedicated high–speed bus, the superformance of the overall system is significantly improved.

Advantages of multiple bus hierarchies

1. Reduced bottlenecks : Different buses handle different types of data transfer, preventing congestion.

2. Improved performance : High–speed devices can operate on dedicated buses without affecting slower devices.

3. Scalability : New devices can be added to specific buses without overloading the main system bus.

4. Flexibility : Allows integration of diverse devices with varying speed requirements.

 

Review Questions

1. Define the term CPU. Why is it called the brain of the computer?

2. Explain the structure of a computer with the help of a neat diagram.

3. Explain the main components of the CPU with a neat block diagram.

4. What are the functions of the Arithmetic and Logic Unit (ALU)?

5. State the role of the Control Unit (CU) in a computer.

6. What is the purpose of registers in the CPU? Give one example of a register.

7. Describe the function of the memory unit.

8. Explain different functional units of a digital computer.

9. What is a stored program concept? Explain the functional units of a stored program digital computer, along with a block diagram.

10. What is a system bus?

11. Explain the three types of system buses.

12. Explain why only two units can communicate at a time in a single bus structure.

13. What is the role of multiplexed address/data lines in a single bus structure, and why are latches required?

14. List two limitations of using a single bus structure in large computer systems.

15. Why do modern computer systems use multiple bus hierarchies instead of a single bus?

16. Explain the high–speed bus configuration.