Computer System — NCERT Exercise Solutions for Class 11 Computer Science

Welcome to the complete NCERT exercise solutions for CBSE Class 11 Computer Science Chapter 1: Computer System. Every question from the NCERT textbook is answered below with clear, exam-ready explanations. Use these solutions to verify your own answers, understand the reasoning behind each concept, and prepare thoroughly for your exams.

How to Use These Solutions

1. Attempt the questions yourself first — read the NCERT textbook chapter, then try answering each exercise question on your own before looking at the solutions. 2. Compare your answers — check your answers against the solutions below. If your answer differs, read the explanation to understand why. 3. Focus on the reasoning — the explanations tell you why an answer is correct, which helps you answer variations of the same question in the exam. 4. Revise using the Key Concepts section — before jumping to solutions, quickly review the key concepts below to refresh your memory.

Key Concepts — Quick Review

Before you read the solutions, here is a brief review of the key concepts from this chapter:

• Hardware: Physical components of a computer — CPU, memory, input/output devices, storage.
• Software: Programs and instructions — system software (OS, compilers, drivers), application software (word processors, browsers), utility software (antivirus, backup).
• CPU: The brain of the computer with ALU (arithmetic and logic), Control Unit (directs operations), and Registers (temporary storage).
• Memory: RAM (volatile, temporary), ROM (non-volatile, permanent), Cache (fast, small, between CPU and RAM), Secondary storage (HDD, SSD — long-term, non-volatile).
• I/O Devices: Input sends data in (keyboard, mouse), Output presents data out (monitor, printer). Touchscreen does both.
• Operating System: Manages processes, memory, files, devices, user interface, and security. Types include batch, interactive, real-time, distributed, embedded, and mobile.

Exercise Questions and Answers

Q1. What is a computer? Explain its basic structure.

A computer is an electronic device that accepts data as input, processes it according to a set of instructions (program), and produces meaningful output (information). It performs four basic operations: input, processing, output, and storage.

The basic structure of a computer consists of the following components:

1. Input Unit: Accepts raw data from the user through input devices like keyboard and mouse. It converts the data into a form the computer can understand (binary).

2. Central Processing Unit (CPU): The CPU is the brain of the computer. It processes the input data and executes instructions. The CPU contains:

• Arithmetic Logic Unit (ALU): Performs all arithmetic calculations (addition, subtraction, multiplication, division) and logical comparisons (greater than, less than, equal to).
• Control Unit (CU): Directs and coordinates all computer operations. It fetches instructions from memory, decodes them, and controls execution.
• Registers: Small, high-speed storage inside the CPU that holds data being currently processed.

3. Memory Unit: Stores data, instructions, and results. It includes:

• Primary Memory (RAM and ROM): Directly accessible by the CPU. RAM is volatile (data lost on power off) and ROM is non-volatile (data retained).
• Secondary Memory (HDD, SSD): Non-volatile, long-term storage with large capacity.

4. Output Unit: Presents the processed results to the user through output devices like monitors and printers. It converts the binary results back into a human-readable form.

Q2. Differentiate between hardware and software. Give two examples of each.

Examples of Hardware: 1. CPU (Central Processing Unit) 2. Keyboard

Examples of Software: 1. Windows 11 (Operating System — system software) 2. Microsoft Word (Word processor — application software)

Q3. What are the main components of the CPU? Explain each briefly.

The CPU has three main components:

1. Arithmetic Logic Unit (ALU): The ALU is the part of the CPU that performs all arithmetic and logical operations. Arithmetic operations include addition, subtraction, multiplication, and division. Logical operations include comparisons like greater than, less than, and equal to, as well as logical operations like AND, OR, and NOT. The ALU receives data from registers, processes it, and stores the result back in a register.

2. Control Unit (CU): The Control Unit does not process data itself. Instead, it directs and coordinates the entire computer system. It fetches instructions from memory, decodes them to understand what operation to perform, and then sends control signals to other components (ALU, memory, I/O devices) to execute the instruction. The CU manages the fetch-decode-execute cycle that the CPU continuously performs.

3. Registers: Registers are small, extremely fast storage locations inside the CPU. They hold data and instructions that the CPU is currently processing. Important registers include the Accumulator (stores intermediate results), Program Counter (holds the address of the next instruction), Instruction Register (holds the current instruction), Memory Address Register (holds the address to access in memory), and Memory Data Register (holds data being transferred to/from memory).

Q4. Differentiate between RAM and ROM.

Q5. List five input devices and five output devices with their functions.

Input Devices:

1. Keyboard: Used to enter text, numbers, and commands into the computer by pressing keys. It is the most commonly used input device.

2. Mouse: A pointing device used to move the cursor on the screen, select items, click buttons, and drag objects. Types include optical mouse, mechanical mouse, and wireless mouse.

3. Scanner: Converts physical documents, photographs, and images into digital format that the computer can store and process.

4. Microphone: Converts sound waves into electrical signals, allowing the user to input voice, record audio, and use voice commands.

5. Barcode Reader: Scans barcode labels on products and converts the pattern into digital data, commonly used in retail and inventory systems.

Output Devices:

1. Monitor: Displays text, images, and video on a screen. Types include LCD, LED, and OLED monitors. It is the most commonly used output device.

2. Printer: Produces a hard copy (paper output) of documents, images, and photos. Types include inkjet, laser, and dot matrix printers.

3. Speaker: Converts electrical signals into sound waves, producing audio output for music, alerts, and notifications.

4. Projector: Projects the computer display onto a large screen or wall, used for presentations in classrooms and meetings.

5. Headphones: Personal audio output device that delivers sound directly to the user's ears without disturbing others.

Q6. What is an operating system? List its main functions.

An operating system (OS) is system software that acts as an intermediary between the user and the computer hardware. It manages all hardware and software resources of the computer and provides an environment in which users can run application programs. Without an operating system, a computer cannot function.

Main functions of an operating system:

1. Process Management: Creates, schedules, and terminates processes. Allocates CPU time to each process and manages multitasking so multiple programs can run simultaneously.

2. Memory Management: Allocates and deallocates memory space to processes. Tracks which memory areas are in use and which are free. Uses techniques like paging and segmentation for efficient memory use.

3. File Management: Organizes files into directories and subdirectories. Provides operations like create, delete, open, close, read, write, and rename. Manages file permissions and attributes.

4. Device Management: Manages all input/output devices through device drivers. Controls device allocation, deallocation, and communication. Handles buffering and spooling.

5. User Interface: Provides a way for users to interact with the computer. Offers Command Line Interface (CLI) where users type commands, and Graphical User Interface (GUI) where users interact through icons, menus, and windows.

6. Security and Access Control: Prevents unauthorized access by managing user accounts, passwords, and file permissions. Provides firewall and encryption features to protect the system.

7. Error Detection and Handling: Monitors the system for errors, displays error messages, maintains error logs, and handles system crashes gracefully.

Q7. Explain the different types of software with examples.

Software is classified into three main types:

1. System Software: Software that manages the computer hardware and provides a platform for running application software. The user does not directly use system software for everyday tasks. Types of system software include:

• Operating System: Manages all hardware and software resources. Examples: Windows 11, macOS, Linux (Ubuntu), Android, iOS.
• Language Translators: Convert programming code into machine language. Examples: C compiler, Python interpreter, Java assembler.
• Device Drivers: Allow the OS to communicate with hardware devices. Examples: printer driver, graphics card driver, network adapter driver.

2. Application Software: Software designed to perform specific tasks for the user. These are the programs people use daily. Types include:

• General Purpose Software: Used for a wide range of tasks. Examples: Microsoft Word (word processing), Google Chrome (web browsing), Microsoft Excel (spreadsheets).
• Specific Purpose Software: Designed for a particular task or industry. Examples: Tally (accounting), AutoCAD (design), railway reservation system.
• Custom Software: Developed specifically for an organization. Examples: hospital management system, banking software, school ERP system.

3. Utility Software: Software that helps maintain, manage, and optimize the computer system. Examples: Windows Defender (antivirus), Disk Cleanup, Disk Defragmenter, WinZip (file compression), backup software.

Q8. What is the memory hierarchy? Explain cache memory and its levels.

The memory hierarchy arranges different types of computer memory in order of speed, cost, and capacity. From fastest and most expensive (smallest) to slowest and least expensive (largest), the hierarchy is:

Registers > Cache (L1, L2, L3) > RAM > SSD > HDD

The purpose of the memory hierarchy is to provide the CPU with fast access to frequently used data (through cache and registers) while also providing large, affordable long-term storage (through HDD and SSD).

Cache Memory:

Cache memory is a small, extremely fast memory located between the CPU and RAM. Its purpose is to store frequently accessed data and instructions so the CPU can retrieve them quickly without waiting for the slower RAM. When the CPU needs data, it first checks the cache. If the data is found in the cache (cache hit), it is retrieved quickly. If not found (cache miss), the CPU fetches it from RAM and also stores a copy in the cache for future use.

Levels of Cache:

1. L1 (Level 1) Cache: The smallest (8-64 KB) and fastest cache. Located directly inside each CPU core. Since it is closest to the CPU, access time is the lowest. Each CPU core has its own L1 cache.

2. L2 (Level 2) Cache: Larger than L1 (256 KB to 1 MB) but slightly slower. May be located inside or just outside the CPU core. Each CPU core typically has its own L2 cache, though some designs share it.

3. L3 (Level 3) Cache: The largest cache level (2 MB to 64 MB) but the slowest among cache levels. Shared across all CPU cores. Still much faster than RAM. L3 cache acts as a shared buffer for all cores.

Q9. What are the different types of operating systems? Explain briefly.

The main types of operating systems are:

1. Batch Operating System: Users submit jobs to the operator, who batches similar jobs together. The OS processes each batch sequentially without user interaction. Advantage: efficient resource use. Disadvantage: no interaction, long waiting time. Example: early IBM mainframe systems.

2. Interactive (Time-Sharing) Operating System: Multiple users interact with the computer simultaneously. The CPU switches rapidly between users, giving each a small time slice. Advantage: quick response. Disadvantage: reliability concerns. Example: Unix, Linux.

3. Real-Time Operating System: Used when strict time constraints exist. Must process data within guaranteed time limits. Two types: Hard Real-Time (missing a deadline causes failure, e.g., airbag systems) and Soft Real-Time (missing a deadline degrades performance, e.g., video streaming). Example: VxWorks, RTLinux.

4. Distributed Operating System: Multiple computers on a network work together as a single system. Resources are shared across the network. Advantage: fault tolerance and resource sharing. Disadvantage: network dependency and security concerns. Example: LOCUS, Amoeba.

5. Embedded Operating System: Designed for devices with limited resources (small memory, low power). Found in washing machines, microwaves, digital watches. Advantage: compact and efficient. Disadvantage: limited functionality. Example: Embedded Linux, FreeRTOS.

6. Mobile Operating System: Designed for smartphones and tablets with touch input and wireless connectivity. Example: Android (Google), iOS (Apple).

Q10. Differentiate between compiler and interpreter.

Related Chapters

• [CBSE Class 11 Computer Science Chapter 2 — Encoding Schemes and Number Systems](/class/11/computer-science/cbse-encoding-number-systems)
• [Encoding Schemes and Number Systems — Exercise Solutions](/class/11/computer-science/solutions/cbse-encoding-number-systems-exercise-solutions)
• [CBSE Class 11 Computer Science Chapter 1 — Computer System (Chapter Guide)](/class/11/computer-science/cbse-computer-system)