Computer System Organization Notes – Class 11 CS (083) | NCERT Chapter-1
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Chapter-1 Computer System Notes
Chapter-2 Encoding Scheme and Number System Notes
Computer System
- A computer is an electronic device that can be programmed to accept data as input, process it according to instructions, and produce meaningful information as output.
- A computer along with all the necessary hardware and software components together forms a computer system.
- A computer system mainly consists of:
- Central Processing Unit (CPU)
- Memory
- Input devices
- Output devices
- Storage devices.
Central Processing Unit (CPU)
- The Central Processing Unit (CPU) is the electronic circuitry that carries out the actual processing in a computer and is commonly known as the brain of the computer. It is also called a processor or microprocessor.
- The CPU mainly consists of two components:
- Arithmetic Logic Unit (ALU), which performs arithmetic and logical operations.
- Control Unit (CU), which controls the execution of instructions and manages the flow of data between memory, ALU, and input/output devices.
Input Devices
- The devices through which data and control signals are sent to a computer are called input devices. These devices convert input data into a digital form that can be understood and processed by the computer system.
- Common input devices include keyboard, mouse, scanner, and touch screen. Braille keyboards help visually impaired users, while voice input allows users to enter data using speech.
Output Devices
- The devices that receive data from a computer for display or physical production are called output devices. They convert digital information into a human-understandable form.
- Common examples of output devices include monitor, projector, headphone, speaker, and printer. Braille display monitors help visually challenged users understand textual output generated by computers.
Computer Memory
A computer system requires memory to store data, instructions, and intermediate results during processing.
Types of Memory
Computers mainly use two types of memory —
- Primary Memory
- Secondary Memory
Primary Memory
- Primary memory is the main memory of the computer where data and programs are loaded before processing. The CPU directly interacts with this memory.
- It is of two types:
- RAM (Random Access Memory): A volatile memory used for temporary storage while the computer is working. Its contents are lost when the power is turned off.
- ROM (Read Only Memory): A non-volatile memory used to store permanent instructions such as the startup program or boot loader.
Cache Memory
- Cache memory is a very high-speed memory placed between the CPU and RAM to improve processing speed.
- It stores frequently used data and instructions, reducing the time required for the CPU to access data from the main memory.
Secondary Memory
- Secondary memory is used for permanent storage of data and instructions for future use.
- It is non-volatile, has larger storage capacity, and is slower but cheaper than primary memory.
- Examples of secondary memory devices include Hard Disk Drive (HDD), SSD, CD/DVD, Memory Card, and Pen Drive.
Units of Memory
A computer system uses binary digits called bits to store data, where 4 bits form a nibble, 8 bits form a byte, and bytes are further grouped into larger units such as kilobyte, megabyte, gigabyte, and terabyte.
| Unit | Description |
| KB (Kilobyte) | 1 KB = 1024 Bytes |
| MB (Megabyte) | 1 MB = 1024 KB |
| GB (Gigabyte) | 1 GB = 1024 MB |
| TB (Terabyte) | 1 TB = 1024 GB |
| PB (Petabyte) | 1 PB = 1024 TB |
| EB (Exabyte) | 1 EB = 1024 PB |
| ZB (Zettabyte) | 1 ZB = 1024 EB |
| YB (Yottabyte) | 1 YB = 1024 ZB |
Software
- Software is a set of instructions that helps computer hardware perform specific tasks. Unlike hardware, software cannot be physically touched or seen.
- Based on the mode of interaction with hardware and the functions performed, software is broadly classified into three categories:
- System Software
- Programming Tools
- Application Software
System Software
- System software is the software that manages and controls computer hardware and provides basic functionality for the operation of a computer system.
- It acts as an interface between the hardware and the user or other software.
- Types of System software are:
- Operating Systems
- System Utilities
- Device Drivers
Operating System
- An operating system is the most important system software that manages the overall functioning of a computer.
- It controls application programs, manages hardware resources, and provides security and access to users.
- Examples of operating systems are Windows, Linux, Ubuntu, Android, iOS, and Macintosh.
System Utilities
- System utilities are software tools used for maintenance, management, and improvement of computer performance.
- Examples include disk cleanup tools, antivirus software, formatting utilities, and system restore tools.
Device Drivers
- A device driver is software that helps the operating system communicate with hardware devices.
- It acts as an interface between the operating system and the device, ensuring proper functioning of hardware components.
Programming Tools
Programming tools are software used to write, translate, test, and debug computer programs. They help programmers give instructions to the computer in an easy and understandable way.
Classification of Programming Languages
Programming languages are mainly classified into two categories:
- Low-level Languages
- High-level Languages
Low-level languages
Low level languages are machine-dependent languages. They include machine language and assembly language and are closer to hardware.
- Machine Language: A low-level language that uses 1s and 0s, directly understood by the computer. It is used for direct execution by hardware but is difficult to write and debug.
- Assembly Language: A low-level language that uses English-like symbols instead of 1s and 0s. It simplifies coding but is machine-dependent and works only on specific CPUs.
High-level languages
High level languages are machine-independent and easier to write. They use English-like statements and follow rules similar to natural languages. However, they are not directly understood by the computer, so translators are required to convert high-level language into machine language. Examples include C++, Java, and Python.
Language Translators
A language translator is software that converts source code written in assembly or high-level language into machine language (object code) because a computer understands only machine language.
The three types of language translators are assembler, compiler, and interpreter.
- Assembler: Converts assembly language into machine language and is specific to a particular processor.
- Compiler: Converts the entire high-level program into machine code at once; once compiled, it can be executed without the compiler.
- Interpreter: Converts and executes high-level program code line by line and is required every time the program runs.
Program Development Tools
- Integrated Development Environment (IDE): Is a software that combines a text editor, build tools, and a debugger, allowing programmers to write, compile, and debug programs in one place.
- Text Editor: A text editor is used to write program instructions and save them as source code, which is later converted into object code using a translator for execution.
- Debugger: Is a tool used to identify and correct errors in the source code. Examples of IDEs include Python IDLE, NetBeans, Eclipse, Atom, and Lazarus.
Application Software
- Application software is designed to perform specific tasks for users and works on top of system software to meet different user requirements.
- It is mainly of two types: general purpose software and customised software.
- General Purpose Software: These are ready-made software used for common tasks by many users, such as Calc (LibreOffice), Adobe Photoshop, GIMP, Mozilla Firefox, and iTunes.
- Customised Software: These are tailor-made software developed for specific users or organisations according to their requirements, such as school management systems, accounting software, and websites.
Operating System
An Operating System (OS) is a system software that acts as a resource manager by controlling and managing computer hardware resources such as CPU, RAM, disk, network, and input/output devices. It also manages application software, device drivers, system security, and user access.
Functions of Operating System
- Process Management: Manages running processes, allocates CPU time, and ensures efficient execution of multiple tasks.
- Memory Management: Allocates and frees main memory for processes and keeps track of used and free memory.
- File Management: Handles creation, deletion, modification, and protection of files in secondary storage.
- Device Management: Controls input/output devices through device drivers and manages their secure access.
OS User Interface
- Command-based Interface: Users type commands using the keyboard to perform tasks. It is less interactive. Examples: MS-DOS, Unix.
- Graphical User Interface (GUI): Users interact using icons, menus, and windows with mouse/keyboard. Examples: Windows, Ubuntu, Mac.
- Touch-based Interface: Users operate devices using touch gestures on screens. Examples: Android, iOS.
- Voice-based Interface: Users control the system using voice commands. Examples: Siri, Google Assistant.
- Gesture-based Interface: Users interact using movements like waving or tilting. Used in smartphones and gaming systems.
Chapter-1 Computer System Notes
