Computer System Class 11: Hardware, Memory, Software and the Operating System

What Computer System is about

Weightage: This chapter is part of Unit I, Computer Systems and Organisation, which carries 10 of the 70 theory marks in the school annual exam together with Boolean Logic and Encoding Schemes and Number System.Source: CBSE Computer Science (083) syllabus 2026-27, Class XI distribution of marks, Unit I Computer Systems and Organisation

Key concepts

Computer system and its block diagram

NCERT section NCERT 1.1

A computer is an electronic device that can be programmed to take input, process it and give output. Together with the extra hardware and software it needs, it is called a computer system. Its main parts are the CPU, memory, input devices, output devices and storage devices, all working as one unit. In the block diagram, data enters through an input device, is held in primary memory, is processed by the CPU, can be saved in secondary storage and leaves through an output device. Hardware is the part you can touch (keyboard, RAM, printer); software is the set of instructions and data that makes the hardware do useful work. Neither is useful without the other.

Worked example

A student scans a hand-drawn map, edits it on screen and prints it. Name the device or unit used at each stage: scanning, holding the image while editing, doing the editing calculations, saving it for next week, printing.

Try it first, then show the solutionSolution

1. Scanning brings data in: input device (scanner).
2. While editing, the image is held in primary memory (RAM).
3. The calculations are done by the CPU.
4. Saving for later needs non-volatile secondary storage (hard disk, SSD or pen drive).
5. Printing gives a hard copy: output device (printer).

Answer: Scanner (input), RAM (primary memory), CPU (processing), secondary storage (saving), printer (output).

CPU: ALU, control unit and registers

NCERT section NCERT 1.1.1

The Central Processing Unit (CPU), also called the processor, does the actual processing; it is often called the brain of the computer. It is built on one or more integrated circuits (ICs) made of semiconductor material. The CPU fetches program instructions and data from memory, processes them and stores the results back in memory. It has three parts:

• Arithmetic Logic Unit (ALU): does arithmetic (+, -, ×, ÷) and logical operations (comparisons such as greater than, AND, OR).
• Control Unit (CU): interprets instructions, makes them run in sequence and directs the flow of data between memory, the ALU and the input-output devices. It does no calculation itself.
• Registers: very small, very fast storage locations inside the CPU chip that hold the data, instruction or intermediate result currently being worked on. They are limited in size and number.

Worked example

A program checks whether a student's marks are at least 33 and, if so, adds 5 grace marks. Which part of the CPU (a) compares the marks with 33, (b) adds 5, (c) decides that the addition instruction runs only after the comparison, (d) holds the marks while this happens?

Try it first, then show the solutionSolution

1. Comparison is a logical operation: ALU.
2. Addition is arithmetic: ALU.
3. Sequencing and interpreting instructions: Control Unit.
4. The value being worked on sits in a register inside the CPU.

Answer: (a) ALU (b) ALU (c) Control Unit (d) a register.

Input and output devices

NCERT section NCERT 1.1.2, 1.1.3

An input device sends data and control signals into the computer and converts them into digital form: keyboard, mouse, scanner, touch screen, microphone (voice input), and braille keyboards for visually impaired users. An output device takes digital data from the computer and converts it into a form people can understand: monitor, projector, speaker, headphone, printer, and braille display. A printer gives a hard copy; a file on a disk is a soft copy. Common printers are inkjet, laser and dot matrix; a 3D printer builds a physical object from a digital 3D design and is used to make product prototypes. Some devices do both jobs: a touch screen takes touches as input and shows output.

Worked example

Classify as input, output or both: (i) webcam (ii) 3D printer (iii) touch screen (iv) braille display (v) microphone.

Try it first, then show the solutionSolution

1. Webcam and microphone capture data and send it in: input.
2. 3D printer and braille display give results out: output.
3. A touch screen reads touches and shows images: both.

Answer: Input: (i), (v). Output: (ii), (iv). Both: (iii).

Primary memory: RAM and ROM

NCERT section NCERT 1.3.2 (A)

Programs and data must be loaded into primary (main) memory before the CPU can process them; the CPU reads and writes primary memory directly. It has two types:

• RAM (Random Access Memory): volatile, so its contents are lost when the power goes off. It holds the programs and data in use while the computer works. It is faster than secondary storage and is usually what people mean by the computer's memory.
• ROM (Read Only Memory): non-volatile, so contents stay without power. It is small and holds contents that rarely change, such as the start-up program (boot loader) that loads the operating system into RAM when the computer is switched on.

Worked example

Riya types an essay for 20 minutes without saving and the power fails. When power returns, the computer starts normally but the essay is gone. Explain both facts using RAM and ROM.

Try it first, then show the solutionSolution

1. The unsaved essay was only in RAM, which is volatile: it was erased when power failed.
2. The start-up program is in ROM, which is non-volatile, so it was still there to load the operating system.

Answer: The essay was lost because RAM is volatile; the computer could still start because the boot loader is kept in non-volatile ROM.

Cache and secondary memory

NCERT section NCERT 1.3.2 (B), (C)

RAM is faster than storage but slower than the CPU, so the CPU would often wait. Cache is a very high-speed memory placed between the CPU and RAM. It keeps copies of data from frequently used RAM locations. When the CPU needs data it looks in the cache first; if the data is there it is read quickly, otherwise RAM is accessed. This cuts the average access time.

Secondary memory (storage) is non-volatile, has a much larger capacity, is slower and cheaper per byte than RAM, and cannot be accessed directly by the CPU: its contents are first brought into RAM. Examples: hard disk drive (HDD), solid state drive (SSD, much faster than an HDD), CD/DVD, memory card, pen drive.

The order by speed and cost per byte is: registers > cache > RAM > secondary storage; capacity runs the other way.

Worked example

Arrange registers, secondary storage, cache and RAM in increasing order of (a) access speed and (b) capacity in a typical laptop.

Try it first, then show the solutionSolution

1. Speed: the closer to the CPU, the faster; secondary storage is slowest, registers fastest.
2. Capacity: storage is largest, registers smallest.

Answer: (a) Secondary storage < RAM < cache < registers. (b) Registers < cache < RAM < secondary storage.

Units of memory

NCERT section NCERT 1.3.1, Table 1.1

Computers store everything as bits (binary digits 0 and 1). Bits are grouped: 4 bits make a nibble (for example 1010) and 8 bits, or two nibbles, make a byte (for example 01000110). Bigger units go up in steps of 1024 (2¹⁰):

• 1 KB (kilobyte) = 1024 bytes
• 1 MB (megabyte) = 1024 KB
• 1 GB (gigabyte) = 1024 MB
• 1 TB (terabyte) = 1024 GB
• 1 PB (petabyte) = 1024 TB

To go to a smaller unit, multiply by 1024 at each step; to go to a bigger unit, divide. Remember the bit-to-byte step is 8, not 1024.

Worked example

How many bits are there in 2 KB? How many MB are there in 3 GB?

Try it first, then show the solutionSolution

1. 2 KB = 2 × 1024 bytes = 2048 bytes; 2048 × 8 = 16384 bits.
2. 3 GB = 3 × 1024 MB = 3072 MB.

Answer: 2 KB = 16384 bits; 3 GB = 3072 MB.

System software

NCERT section NCERT 1.7.1, 1.7.2

Software is grouped by how it works with the hardware: system software, programming tools and application software. System software gives the basic functionality to run the computer and works directly with the hardware. It includes:

• Operating system: the most basic system software; without it no other software can run (Windows, Linux, Ubuntu, macOS, Android, iOS).
• System utilities: software for maintaining and configuring the computer. Some come with the OS (disk defragmenter, formatting utility, system restore); others are added to improve performance (anti-virus, disk cleaner, disk compression).
• Device drivers: each handles one particular device at the hardware level and acts as an interface between that device and the operating system, hiding the device's details. A new printer usually needs its driver installed before the OS can use it.

Worked example

Classify: (i) anti-virus (ii) printer driver (iii) Android (iv) disk defragmenter (v) touchpad driver.

Try it first, then show the solutionSolution

1. Android is an operating system.
2. Anti-virus and defragmenter maintain the system: utilities.
3. Printer and touchpad drivers control one device each: device drivers.

Answer: OS: (iii). Utilities: (i), (iv). Device drivers: (ii), (v). All five are system software.

Programming languages and translators

NCERT section NCERT 1.7.3

Low-level languages are machine dependent. Machine language is written in 0s and 1s and runs directly, but it is hard to write and debug. Assembly language uses English-like short words (mnemonics) and symbols, but code written for one CPU type does not run on another. High-level languages (Python, C++, Java) are machine independent and close to English, but need translation.

The program as written is the source code; the translated machine form is the object code. Three translators:

• Assembler: assembly language to machine language.
• Compiler: translates the whole high-level program at once; once translated, the compiler is no longer needed to run it.
• Interpreter: translates and runs one line at a time, stopping at the first line with an error; it is needed every time the program runs. Python is run by an interpreter.

An editor is used to type source code; an IDE (Integrated Development Environment) combines an editor, build tools and a debugger (a tool to find and fix errors). Python's IDLE is an IDE.

Worked example

A program has 50 lines and an error on line 30. What happens when it is (a) interpreted (b) compiled?

Try it first, then show the solutionSolution

1. An interpreter translates and runs line by line: lines 1 to 29 run, then it stops at line 30 and reports the error.
2. A compiler translates the whole program first: it reports the error and produces no object code, so no line runs.

Answer: (a) Lines 1-29 run, then an error at line 30. (b) Nothing runs; the compiler lists the error first.

Application software and software licences

NCERT section NCERT 1.7.4, 1.7.5

Application software works on top of system software to do a user's task. General purpose software is ready-made for a wide audience (spreadsheet, image editor, web browser). Customised software is tailor-made for one organisation or person (a school's fee and attendance system, a shop's billing software, a company's website), like getting clothes stitched to measure. A computer can work without application software but not without system software.

By licence: Free and Open Source Software (FOSS) gives the software and its source code freely so others can study and improve it (Ubuntu, Python, LibreOffice, Firefox). Freeware is free to use but the source code is not given. Proprietary software must be bought from the copyright holder (Microsoft Windows, Tally).

Worked example

A hospital hires a firm to build software for its own patient-token and billing system. Its staff also use a ready-made spreadsheet for monthly reports. Classify both and give one advantage of each.

Try it first, then show the solutionSolution

1. Built for one hospital's own needs: customised application software; it fits their exact workflow.
2. Ready-made for everyone: general purpose application software; it is available at once and cheaper.

Answer: Token and billing system: customised (fits the hospital's needs). Spreadsheet: general purpose (ready-made and low cost).

Operating system: user interfaces and functions

NCERT section NCERT 1.8

The operating system is a resource manager: it manages the CPU, RAM, disks, network and input-output devices, controls applications and drivers, and handles security and user access. It has two aims: to provide services for running programs (loading them into memory, giving them CPU time, deciding their order) and to give the user an interface.

User interfaces: command-based (typed commands, keyboard; MS-DOS, Unix), graphical (GUI) (icons, menus, windows with mouse and keyboard; Windows, Ubuntu, macOS), touch-based (Android, iOS), voice-based (voice assistants) and gesture-based (waving, tilting, eye movement).

Functions:

• Process management: a program in execution is a process; the OS shares the CPU among processes and allocates resources.
• Memory management: allocates RAM to running processes and frees it when they finish, tracking which locations are free or occupied.
• File management: creating, updating, deleting and protecting files on secondary storage, so users cannot open files not shared with them.
• Device management: manages the input-output devices through their drivers, lets users configure them and restricts access to authorised users.

Worked example

Name the OS function at work: (a) a music app keeps playing while you browse; (b) closing a heavy game frees space for another app; (c) a classmate cannot open your password-protected folder on a shared PC; (d) a new webcam starts working after plugging in.

Try it first, then show the solutionSolution

1. Sharing CPU time between running programs: process management.
2. Taking back and reallocating RAM: memory management.
3. Protecting files on storage: file management.
4. Recognising and running a device through its driver: device management.

Answer: (a) Process (b) Memory (c) File (d) Device management.

Computer System in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• CPU, Memory Hierarchy and Units of Memory with Conversions
• Software, Programming Languages, Translators and the Operating System

Computer System: syntax and key facts

Bits and bytes

1 nibble = 4 bits; 1 byte = 8 bits

The only steps of 4 and 8; every larger step is 1024.

Memory units (NCERT Table 1.1)

1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB; 1 PB = 1024 TB

1024 = 2^10. Bigger to smaller: multiply; smaller to bigger: divide.

Unit jump

n steps up the KB-MB-GB-TB-PB ladder = a factor of 1024^n

1 GB = 1024 × 1024 KB = 1048576 KB.

Memory hierarchy

Speed and cost per byte: registers > cache > RAM > secondary storage; capacity: the reverse

The CPU cannot read secondary storage directly.

Translators

Assembler: assembly to machine code; compiler: whole program at once; interpreter: line by line

Python uses an interpreter.

OS functions

Process, memory, file and device management

Remember the four as P-M-F-D.

How the school exam asks Computer System

The question patterns to expect, and the method that works for each.

Tricks to remember Computer System

Short memory aids. Say each one aloud once, then use it on a question the same day.

• RAM Runs Away

  RAM's data runs away when the power goes; ROM Remains.

  Use it when: Volatile versus non-volatile.

• Eight then 1024

  Bit to byte is 8; every jump after that (KB, MB, GB, TB, PB) is 1024. Order: Kind Men Give Their Patience.

  Use it when: Unit conversion and the order of units.

• Compiler = whole book, interpreter = live translator

  A compiler translates the whole book before anyone reads it; an interpreter translates a speech sentence by sentence and stops when one sentence makes no sense.

  Use it when: Compiler versus interpreter questions.

• P-M-F-D

  Please Manage Files and Devices: Process, Memory, File, Device management.

  Use it when: Listing the four OS functions.

Computer System: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Computer System

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using 1000 instead of 1024 between units

  The NCERT units (Table 1.1) use 1024; answers based on 1000 do not match.

  Wrong working

  2 GB = 2 × 1000 MB = 2000 MB

  Right working

  2 GB = 2 × 1024 MB = 2048 MB

  ✓Do this: Use 1024 for every step from KB upwards.

  Usually costs: 1 mark

• ✗Mistake: Using 1024 between bits and bytes

  A byte is 8 bits, not 1024 bits.

  Wrong working

  1 KB = 1024 × 1024 bits

  Right working

  1 KB = 1024 bytes = 1024 × 8 = 8192 bits

  ✓Do this: Bits to bytes: divide by 8. Bytes to KB: divide by 1024.

  Usually costs: 1 mark

• ✗Mistake: Calling ROM 'memory that can never be changed' or calling cache a kind of storage device

  The key property of ROM is that it is non-volatile; cache is a fast memory between CPU and RAM, not storage.

  Wrong working

  Cache is a secondary storage that stores files permanently.

  Right working

  Cache is a very fast memory between the CPU and RAM that keeps copies of frequently used data.

  ✓Do this: Define each memory by position, speed and volatility.

  Usually costs: 1 mark

• ✗Mistake: Saying a compiler runs the program line by line

  That describes an interpreter.

  Wrong working

  A compiler translates one line at a time.

  Right working

  A compiler translates the whole program in one go; an interpreter translates and runs one line at a time.

  ✓Do this: Tie 'whole program' to compiler and 'line by line' to interpreter.

  Usually costs: 1-2 marks

• ✗Mistake: Listing an anti-virus or a device driver as application software

  Both maintain or run the computer itself, so they are system software.

  Wrong working

  Application software: MS Word, anti-virus, printer driver.

  Right working

  Application: MS Word. System software: anti-virus (utility), printer driver (device driver).

  ✓Do this: Ask: does it serve the computer or the user's own task?

  Usually costs: 1 mark

NCERT exercise guide: Computer System

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Computer System

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which of these is non-volatile? (a) RAM (b) Cache (c) Registers (d) ROM

   Hint

   Keeps its contents without power.

   Answer

   (d) ROM

2. Q2 · 1 mark · Very short answerExam level

   Which unit of the CPU interprets instructions and controls their sequence?

   Hint

   It does no arithmetic.

   Answer

   The Control Unit (CU).

3. Q3 · 2 marks · Short answerHarder exam level

   Arrange in increasing order: 1 TB, 1048576 KB, 2^34 bytes, 1500 MB.

   Hint

   Convert all to MB.

   Answer

   1048576 KB = 1024 MB; 1500 MB; 2^34 bytes = 2^14 MB = 16384 MB; 1 TB = 1048576 MB. Order: 1048576 KB < 1500 MB < 2^34 bytes < 1 TB.

4. Q4 · 1 mark · MCQHarder exam level

   Assertion (A): Secondary storage is needed even though RAM is faster. Reason (R): RAM is volatile and has a limited capacity. Choose: (a) both true, R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.

   Hint

   Why can RAM not replace a hard disk?

   Answer

   (a) Both true and R explains A.

5. Q5 · 2 marks · Short answerExam level

   Give one example each of a general purpose and a customised application software and say what makes the second customised.

   Hint

   Who was it made for?

   Answer

   General purpose: a web browser or spreadsheet. Customised: a school's attendance software, made to the specific needs of one organisation.

6. Q6 · 2 marks · Short answerHarder exam level

   An assembly program written for one processor does not run on a computer with a different processor family, but a Python program does. Explain why.

   Hint

   Machine dependent versus machine independent.

   Answer

   Assembly language is low-level and tied to one CPU's instruction set, so its machine code is not portable. Python is high-level and machine independent; the interpreter on each computer translates it for that machine.

7. Q7 · 2 marks · Short answerHOTS / competency

   A computer has a large SSD but only 2 GB RAM. When many browser tabs are open, it becomes slow. Which OS function is most stressed, and why?

   Hint

   Every tab needs main memory.

   Answer

   Memory management: each running process needs RAM, and with only 2 GB the OS must keep allocating and freeing memory among many processes, so the system slows.

8. Q8 · 2 marks · Short answerHarder exam level

   How many 700 MB files can be stored completely on a 4 GB pen drive?

   Hint

   Change 4 GB to MB first.

   Answer

   4 GB = 4096 MB; 4096 ÷ 700 = 5.85, so 5 complete files.

9. Q9 · 3 marks · Short answerExam level

   Write any three differences between system software and application software.

   Hint

   Purpose, dependence, examples.

   Answer

   Purpose: system software runs and manages the computer; application software does the user's task. Dependence: application software runs on top of system software; a computer cannot work without system software but can work without applications. Examples: Windows, drivers, utilities versus spreadsheets, browsers, billing software.

10. Q10 · 2 marks · Short answerExam level

    Why is the operating system called a resource manager? Name four resources it manages.

    Hint

    What does every program need?

    Answer

    Because it decides how the computer's resources are shared among programs and users so that each is used well: CPU, main memory, storage (files) and input-output devices.

11. Q11 · 3 marks · Short answerHOTS / competency

    Suppose a voice assistant on a phone hears 'Call Mum'. List the input device, output device and two OS functions involved until the call starts.

    Hint

    Speech in, call screen and speaker out.

    Answer

    Input: microphone. Output: screen and speaker. OS: voice-based interface takes the command; process management starts the phone app; device management uses the microphone, speaker and modem through their drivers (any two functions).

12. Q12 · 2 marks · Short answerHarder exam level

    Python's IDLE is called an IDE. What three things does an IDE combine, and how does a debugger help a learner?

    Hint

    Editor, build tools, debugger.

    Answer

    An editor to type code, tools to translate and run it, and a debugger. The debugger helps find and correct errors, for example by running a program step by step.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What are the three parts of a CPU?Show answer

   ALU, control unit and registers.

2. 2.Which memory holds the boot loader?Show answer

   ROM.

3. 3.How many bits are in a nibble?Show answer

   4.

4. 4.1 TB = how many GB?Show answer

   1024 GB.

5. 5.Which translator converts assembly language to machine code?Show answer

   An assembler.

6. 6.What is a program in execution called?Show answer

   A process.

7. 7.Name two operating systems with a touch-based interface.Show answer

   Android and iOS.

8. 8.Is Python FOSS, freeware or proprietary?Show answer

   FOSS (free and open source).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Computer: takes input, processes it, gives output; with extra hardware and software it is a computer system.
• ☐CPU = ALU (arithmetic and logic) + CU (control, sequencing) + registers (tiny fast storage).
• ☐RAM volatile, holds programs in use; ROM non-volatile, holds the boot loader.
• ☐Cache sits between CPU and RAM; secondary storage is non-volatile, large, slow, not directly read by the CPU.
• ☐4 bits = nibble, 8 bits = byte, then ×1024: KB, MB, GB, TB, PB.
• ☐System software: OS, utilities, drivers. Application software: general purpose or customised.
• ☐Assembler (assembly), compiler (whole program), interpreter (line by line).
• ☐OS: resource manager; interfaces command, GUI, touch, voice, gesture; functions process, memory, file, device management.

Computer System: frequently asked questions