Computer System

Overview

Learning outcomes

• Students will be able to define a computer system and explain the input-process-output cycle with an everyday example.
• Students will be able to draw and label the block diagram of a computer and state the role of the CPU, ALU, CU, registers and memory.
• Students will be able to name the system buses and common ports, and classify devices as input, output or storage.
• Students will be able to describe key milestones in the evolution of computing devices, including the stored-program concept and Moore's Law.
• Students will be able to convert between memory units from bit to yottabyte using powers of 2.
• Students will be able to compare RAM, ROM, cache and secondary memory on speed, cost, capacity and volatility.
• Students will be able to explain how deleted data can be recovered and suggest safeguards for data security and safe disposal of devices.
• Students will be able to classify software as system software or application software, and application software as general purpose or customised.

Lesson plan

Notes

Key terms

Computer system

A computer together with the hardware and software needed to accept input, process it, store it and produce output.

Hardware

The physical parts of a computer that can be seen and touched.

Software

A set of programs and data that tells the hardware what to do.

CPU

Central Processing Unit; the processor that fetches and executes instructions, made of the ALU, CU and registers.

ALU

Arithmetic Logic Unit; the part of the CPU that performs calculations and comparisons.

Control Unit

The part of the CPU that interprets instructions and directs the flow of data and control signals.

Register

A tiny, very fast storage location inside the CPU for data being processed right now.

Bus

A set of electrical paths that carries data, addresses or control signals between components.

Port

A socket on a computer through which external devices are connected, such as USB or HDMI.

Stored-program concept

Von Neumann's idea that instructions and data are both kept in the computer's memory.

Moore's Law

The observation that the number of transistors on a chip doubles about every two years.

Bit

A binary digit, 0 or 1; the smallest unit of data.

Nibble

A group of 4 bits.

Byte

A group of 8 bits; enough to store one English character.

Volatile memory

Memory that loses its contents when power is switched off, such as RAM.

Cache memory

Very fast memory between the CPU and RAM that holds frequently used data and instructions.

Boot loader

A start-up program stored in ROM that loads the operating system into RAM.

Data recovery

Retrieving deleted, corrupted or lost data from a storage device.

Device driver

System software that lets the operating system control a particular hardware device.

Customised software

Application software developed to meet the specific needs of one user or organisation.

MCQs

Tap an option to check it, or use Show answers above to reveal them all.

1. Which part of the CPU interprets instructions and controls the flow of data between memory and the ALU?

   1. Cache
   2. Register
   3. Data bus
   4. Control Unit
   Answer

   (d) Control Unit

   The Control Unit decodes instructions and sends control signals to coordinate all parts.

2. How many bits make one nibble?

   1. 8
   2. 2
   3. 4
   4. 16
   Answer

   (c) 4

   A nibble is 4 bits; two nibbles make a byte of 8 bits.

3. Which of the following memories is volatile?

   1. DVD
   2. SSD
   3. ROM
   4. RAM
   Answer

   (d) RAM

   RAM loses its contents when the power is switched off.

4. 2 GB is equal to:

   1. 2048 KB
   2. 2048 MB
   3. 2,000,000 KB
   4. 2000 MB
   Answer

   (b) 2048 MB

   1 GB = 1024 MB, so 2 GB = 2 x 1024 = 2048 MB.

5. Which device is used at a supermarket billing counter to capture product details?

   1. Plotter
   2. Speaker
   3. Barcode reader
   4. Projector
   Answer

   (c) Barcode reader

   A barcode reader captures the product code in digital form; the others are output devices.

6. Who designed the Analytical Engine, considered the basis of modern computers?

   1. Herman Hollerith
   2. Alan Turing
   3. Blaise Pascal
   4. Charles Babbage
   Answer

   (d) Charles Babbage

   Charles Babbage designed the Analytical Engine with input, processing, storage and output.

7. The idea that both program and data are stored in the computer's memory was given by:

   1. John von Neumann
   2. Gordon Moore
   3. Herman Hollerith
   4. Blaise Pascal
   Answer

   (a) John von Neumann

   This is the von Neumann stored-program concept, used in EDVAC and later computers.

8. Moore's Law predicts that the number of transistors on a chip will:

   1. double every ten years
   2. halve every year
   3. stay constant
   4. double about every two years
   Answer

   (d) double about every two years

   Gordon Moore (1965) observed that transistor counts roughly double every two years.

9. Cache memory is placed between:

   1. Keyboard and CPU
   2. RAM and hard disk
   3. CPU and main memory
   4. ROM and hard disk
   Answer

   (c) CPU and main memory

   Cache speeds up the CPU by holding frequently used data from main memory.

10. Which of the following is NOT secondary storage?

    1. Register
    2. Pen drive
    3. Memory card
    4. Hard disk
    Answer

    (a) Register

    Registers are tiny storage locations inside the CPU.

11. The start-up program that loads the operating system into RAM is stored in:

    1. Cache
    2. Pen drive
    3. Registers
    4. ROM
    Answer

    (d) ROM

    The boot loader/firmware is kept in non-volatile ROM so it is available when power comes on.

12. 1 TB is equal to how many bytes?

    1. 2^40
    2. 2^30
    3. 2^20
    4. 10^12
    Answer

    (a) 2^40

    1 TB = 1024^4 bytes = 2^40 = 1,099,511,627,776 bytes.

13. When a file is deleted, the operating system usually:

    1. overwrites every bit with 0
    2. sends it to the printer
    3. marks its space as free without erasing the data
    4. moves it to ROM
    Answer

    (c) marks its space as free without erasing the data

    Only the entry is removed and the space marked free, which is why recovery is possible until overwritten.

14. Which bus carries the location of the memory cell the CPU wants to access?

    1. Address bus
    2. Data bus
    3. USB
    4. Control bus
    Answer

    (a) Address bus

    The address bus carries addresses one way from the CPU to memory/devices.

15. Which of these is an example of system software?

    1. VLC media player
    2. School fee management software
    3. LibreOffice Calc
    4. Device driver
    Answer

    (d) Device driver

    Device drivers help the OS control hardware, so they are system software.

16. Software developed for the fee and attendance rules of one particular school is:

    1. A system utility
    2. General purpose software
    3. Customised software
    4. An operating system
    Answer

    (c) Customised software

    Tailor-made software for one organisation's needs is customised (specific purpose) software.

17. A phone has 32 GB free. How many 8 MB videos can be stored?

    1. 256
    2. 4096
    3. 40,960
    4. 4000
    Answer

    (b) 4096

    32 x 1024 = 32,768 MB; 32,768 / 8 = 4096 videos.

18. Which device is both an input and an output device?

    1. Touch screen
    2. Printer
    3. Scanner
    4. Microphone
    Answer

    (a) Touch screen

    A touch screen displays output and also accepts touch input.

19. Which is the correct order from fastest to slowest?

    1. Cache, registers, HDD, RAM
    2. Registers, cache, RAM, HDD
    3. RAM, cache, registers, HDD
    4. HDD, RAM, cache, registers
    Answer

    (b) Registers, cache, RAM, HDD

    The memory hierarchy is registers > cache > RAM > secondary storage.

20. A disk defragmenter is an example of:

    1. a device driver
    2. a system utility
    3. general purpose application software
    4. customised software
    Answer

    (b) a system utility

    Utilities like defragmenters, disk cleaners and anti-virus maintain and optimise the system.

Assertion–Reason

1. Assertion (A): RAM is called volatile memory.

   Reason (R): The contents of RAM are lost when the power supply is switched off.

   Answer

   (A)

   Volatility means losing data without power, which is exactly what R states, so R explains A.

2. Assertion (A): Charles Babbage's Analytical Engine is considered the basis of modern computers.

   Reason (R): Blaise Pascal invented a mechanical calculator called the Pascaline in 1642.

   Answer

   (B)

   Both statements are true historical facts, but Pascal's invention does not explain why Babbage's design is the basis of modern computers.

3. Assertion (A): Cache memory increases the speed of processing.

   Reason (R): Cache memory is placed between the CPU and the hard disk.

   Answer

   (C)

   A is true, but cache sits between the CPU and main memory (RAM), not the hard disk, so R is false.

4. Assertion (A): The CPU reads data directly from secondary storage while processing.

   Reason (R): Secondary memory is non-volatile.

   Answer

   (D)

   Data from secondary storage must first be loaded into RAM, so A is false. R is true.

5. Assertion (A): A file deleted using Shift + Delete can often be recovered.

   Reason (R): Deletion usually marks the file's space as free without erasing the stored bits.

   Answer

   (A)

   Because the bits remain until overwritten, recovery tools can retrieve the file, so R explains A.

Short answer

1. 2 marks

   Differentiate between data and information with an example.

   Answer

   Data is raw, unprocessed facts, for example the marks 72, 85, 90 of a student. Information is processed, meaningful data, for example 'Aman's average is 82.3 and he has scored grade A'. Information is the output of processing data.

2. 2 marks

   Name the two main components of the CPU and state one function of each.

   Answer

   • ALU: performs arithmetic (addition, subtraction etc.) and logical operations (comparisons).
   • Control Unit: interprets instructions and controls the flow of data and signals among memory, ALU and I/O devices.
3. 2 marks

   What is a bus? Name its three types.

   Answer

   A bus is a set of electrical paths that carries signals between components of a computer. The three types of the system bus are the data bus, the address bus and the control bus.

4. 2 marks

   Convert 3 GB into KB. Show the steps.

   Answer

   3 GB = 3 x 1024 MB = 3072 MB

   3072 MB = 3072 x 1024 KB = 3,145,728 KB

5. 2 marks

   Write two differences between RAM and ROM.

   Answer

   1. RAM is volatile; ROM is non-volatile.
   2. RAM allows read and write and holds running programs; ROM is mainly read-only and holds start-up (boot) instructions.
6. 2 marks

   Why is cache memory used? Where is it placed?

   Answer

   RAM is slower than the CPU, so the CPU would have to wait. Cache is a very fast memory that stores copies of frequently used data and instructions so the CPU can get them quickly. It is placed between the CPU and main memory (RAM).

7. 2 marks

   Give two differences between primary and secondary memory.

   Answer

   1. The CPU accesses primary memory directly; secondary memory is accessed through primary memory.
   2. Primary memory is faster but smaller and costlier; secondary memory is slower but larger, cheaper and non-volatile.
8. 2 marks

   What is a device driver? Why is it needed?

   Answer

   A device driver is system software that lets the operating system communicate with and control a specific hardware device such as a printer. It is needed because each device works differently and the OS alone cannot handle the details of every device; the driver hides those details.

9. 2 marks

   Name two input devices and two output devices that help persons with visual impairment.

   Answer

   Input: braille keyboard, microphone (voice input / speech-to-text).

   Output: refreshable braille display, speaker/headphones with screen-reader (text-to-speech) software.

10. 2 marks

    What is meant by data recovery? When is it possible?

    Answer

    Data recovery is the process of retrieving deleted, corrupted or lost data from a secondary storage device using recovery tools. It is possible only if the storage space of the deleted data has not yet been overwritten by new data.

11. 2 marks

    Classify the following as system software or application software: (i) Ubuntu (ii) MS Excel (iii) Anti-virus (iv) Railway reservation software.

    Answer

    (i) Ubuntu: system software (operating system)

    (ii) MS Excel: application software (general purpose)

    (iii) Anti-virus: system software (utility)

    (iv) Railway reservation software: application software (customised)

12. 2 marks

    State Moore's Law. How has it affected computing devices?

    Answer

    Moore's Law (Gordon Moore, 1965) states that the number of transistors on a chip doubles about every two years while cost falls. As a result, computers and phones have become smaller, faster, cheaper and more powerful over time.

Long answer

1. 5 marks

   Draw a neat labelled block diagram of a computer system and explain the function of each component.

   Answer

   Diagram (draw boxes and arrows): Input Device -> [CPU: Control Unit, ALU, Registers] <-> Primary Memory; Primary Memory <-> Secondary Storage; CPU -> Output Device. Show data-flow arrows and control-signal arrows from the CU to every unit.

   Functions:

   • Input device: accepts data and instructions and converts them to digital form (keyboard, mouse).
   • CPU: processes data. Its ALU performs arithmetic and logical operations, its CU interprets instructions and controls all units, and registers hold data currently being processed.
   • Primary memory: holds programs and data in current use (RAM) and boot instructions (ROM).
   • Secondary storage: stores data permanently (HDD, SSD).
   • Output device: presents results in human-readable form (monitor, printer).
2. 3 marks

   Explain the memory hierarchy of a computer. Why can't a computer be built using only the fastest memory?

   Answer

   From top to bottom: registers, cache, RAM (main memory), secondary storage (SSD/HDD), removable/offline storage (pen drive, DVD, tape).

   Going down the hierarchy, speed and cost per GB decrease while capacity increases. Registers and cache are very fast but very costly and small; secondary storage is slow but cheap and huge.

   A computer cannot use only the fastest memory because it would be extremely expensive for large capacities, and registers/cache/RAM are volatile, so data would be lost when power goes off. The hierarchy gives a balance: frequently used data stays in fast memory and everything else is kept cheaply and permanently in storage.

3. 3 marks

   A school wants to store scanned answer sheets. Each sheet is 750 KB and the school has a 1 TB external disk. (a) How many sheets can be stored? (b) If each student has 6 answer sheets, how many students' sheets fit? Show working.

   Answer

   (a) 1 TB = 1024 GB = 1024 x 1024 MB = 1,048,576 MB = 1,048,576 x 1024 KB = 1,073,741,824 KB.

   Number of sheets = 1,073,741,824 / 750 = 1,431,655.77, so 1,431,655 sheets (round down).

   (b) Students = 1,431,655 / 6 = 238,609.17, so 238,609 students.

4. 5 marks

   Describe the evolution of computing devices from the abacus to the integrated circuit, with one contribution of each milestone.

   Answer

   • Abacus (~500 BC): bead frame for simple arithmetic.
   • Pascaline (1642, Blaise Pascal): mechanical calculator for addition and subtraction; multiplication/division by repetition.
   • Analytical Engine (1834, Charles Babbage): mechanical design with input, processing, storage and output; basis of modern computers.
   • Tabulating Machine (1890, Herman Hollerith): summarised punched-card data; early step towards programming.
   • Turing Machine (1937, Alan Turing): theoretical general-purpose programmable machine.
   • EDVAC/ENIAC (1945): built on von Neumann's stored-program concept.
   • Transistor (1947, Bell Labs): replaced vacuum tubes; smaller, cooler, more reliable.
   • Integrated Circuit (1970 in NCERT timeline): whole circuit on a silicon chip; led to microprocessors and much smaller computers.
5. 3 marks

   Explain the security concerns related to data deletion and recovery. Suggest measures to address each.

   Answer

   Concern 1: unauthorised deletion or modification of data by a person, hacker or malware.
   Measures: restrict access to computers, use strong passwords for accounts and files, encrypt sensitive files, install and update anti-virus software, and keep regular backups.

   Concern 2: unwanted recovery of data from discarded, sold or repaired devices. Since deletion only marks space free, someone can recover personal data (photos, marks, bank details).
   Measures: use data-shredding or secure-erase tools that overwrite data before disposal, encrypt the device and factory-reset phones, and physically destroy faulty disks that cannot be wiped.

6. 3 marks

   What is system software? Explain its three types with examples.

   Answer

   System software operates and controls the computer hardware and provides a platform for other software.

   1. Operating System: manages hardware, memory, files and programs and provides a user interface and security. Without it the computer cannot be used. Examples: Windows, Ubuntu, Android.
   2. System utilities: maintain, configure and optimise the system. Examples: disk defragmenter, formatting and backup tools (with OS); anti-virus, disk cleaner, compression tools (installed separately).
   3. Device drivers: let the OS control specific devices by hiding hardware details. Examples: printer driver, graphics driver, webcam driver.
7. 3 marks

   Differentiate between general purpose and customised application software. Give two examples of each and suggest which type a small medical store should use for billing, with reason.

   Answer

   General purpose software is ready-made for a wide range of users with common needs, is cheaper and easily available, but may not suit special requirements. Examples: LibreOffice Calc, MS Word.

   Customised software is developed for the specific needs of one organisation or user; it fits exactly but is usually costlier and takes time to build. Examples: a school management system, a hospital patient-record system.

   For a small medical store, a ready-made billing package (general purpose) is usually suitable because it is affordable and handles standard GST billing and stock. If the store has special needs (e.g. integration with a chain's central stock system), customised software would be better.

Case-based

Activities

PART A: Ports (model answer)
Port                Shape / look                   Device connected
------------------  -----------------------------  ---------------------------------
USB Type-A          flat rectangle                 keyboard, mouse, pen drive, printer
USB Type-C          small oval, reversible         phone, charger, external SSD
HDMI                trapezoid, wide flat           monitor, projector, smart TV
VGA                 blue D-shape, 15 pins          older monitor or projector
Ethernet (RJ-45)    square with clip               LAN cable to switch / router
3.5 mm audio jack   small round hole               headphones, speakers, mic
Power socket        round or 3-pin                 power adapter / mains cable

PART B: Device classification
Device               Type
-------------------  ----
Keyboard             I
Webcam               I
Projector            O
Touch screen         B
Pen drive            S
Laser printer        O
Microphone           I
Headset with mic     B
Scanner              I
External hard disk   S
Speaker              O
Biometric scanner    I

1. 5 KB = 5 x 1024 = 5120 bytes = 5120 x 8 = 40,960 bits
2. 6144 MB = 6144 / 1024 = 6 GB
3. 3 GB = 3 x 1024 = 3072 MB = 3072 x 1024 = 3,145,728 KB
4. 2 TB = 2 x 1024 = 2048 GB = 2048 x 1024 = 2,097,152 MB
5. 32 nibbles = 32 x 4 = 128 bits = 128 / 8 = 16 bytes
6. 16,777,216 bytes / 1024 = 16,384 KB / 1024 = 16 MB
7. Free space = 128 - 20 = 108 GB = 108 x 1024 = 110,592 MB
   Songs = 110,592 / 5 = 22,118.4  ->  22,118 songs (round down)
8. Needed = 1200 x 3 = 3600 MB
   Available = 4 GB = 4 x 1024 = 4096 MB
   Yes, they fit. Space left = 4096 - 3600 = 496 MB

# Memory unit converter (binary units, 1 KB = 1024 bytes)
units = ["B", "KB", "MB", "GB", "TB", "PB"]

def to_bytes(value, unit):
    power = units.index(unit)
    return value * (1024 ** power)

def convert(value, from_unit, to_unit):
    b = to_bytes(value, from_unit)
    return b / (1024 ** units.index(to_unit))

print("2 GB in KB    :", convert(2, "GB", "KB"))
print("4096 MB in GB :", convert(4096, "MB", "GB"))
print("1 TB in bytes :", to_bytes(1, "TB"))
print("3 KB in bits  :", to_bytes(3, "KB") * 8)

# How many 4 MB photos fit in a 64 GB phone?
photos = to_bytes(64, "GB") // to_bytes(4, "MB")
print("4 MB photos in 64 GB:", photos)

2 GB in KB    : 2097152.0
4096 MB in GB : 4.0
1 TB in bytes : 1099511627776
3 KB in bits  : 24576
4 MB photos in 64 GB: 16384

SYSTEM SOFTWARE
  Operating system : Windows 11, Android, Ubuntu
  Utility          : anti-virus, disk defragmenter, file compression tool
  Device driver    : printer driver, graphics card driver

APPLICATION SOFTWARE
  General purpose  : LibreOffice Impress, Chrome browser, VLC, GIMP
  Customised       : IRCTC reservation system, school fee management software,
                     bank's core banking software, website for the sweet shop

Expected observations (model answer)
Step 2: The Desktop file goes to the Recycle Bin and is restored to its
        original location with one click.
Step 3: The pen-drive file does not go to any bin, but the recovery tool
        finds it (often with a generic name like f0001234.txt) and restores it.

Why recovery worked: deletion only removed the file's entry and marked its
space as free; the bits were still on the drive.

What stops recovery: new data overwriting that space, a secure-erase or
shredding tool, full (not quick) format with overwrite, or physical damage.

Lesson: stop using a drive immediately after accidental deletion; before
giving a drive away, wipe it with a secure-erase tool.

Sample record (answers vary by machine)
Processor      : Intel Core i5-1235U, 10 cores, 12 MB cache
RAM            : 8 GB
Storage        : 512 GB SSD
Operating sys. : Windows 11 Home, 64-bit

Primary memory    : the 8 GB RAM (holds running programs; volatile)
Secondary storage : the 512 GB SSD (stores files permanently; non-volatile)
System software   : Windows 11 (operating system)
The 12 MB cache is part of the processor and sits between CPU and RAM.

Common mistakes

• Calling the whole cabinet (tower) the 'CPU'. The CPU is the processor chip inside it; the cabinet also holds RAM, storage, power supply and the motherboard.

• Using 1000 instead of 1024 in memory conversions. In CBSE answers, 1 KB = 1024 bytes and every step up is x1024. Only bits to bytes uses 8.

• Writing 1 nibble = 8 bits or 1 byte = 4 bits. Correct: 1 nibble = 4 bits, 1 byte = 8 bits = 2 nibbles.

• Saying cache lies between the CPU and the hard disk. Cache sits between the CPU and main memory (RAM).

• Thinking a file deleted with Shift + Delete is gone forever. Its space is only marked free; the data can often be recovered until it is overwritten.

• Classifying device drivers or anti-virus as application software. Both are system software (a driver and a utility respectively).

• Saying secondary memory is accessed directly by the CPU. Data must first be loaded from secondary storage into RAM.

• Rounding up in 'how many files fit' problems. Always round down, because a partly stored file is not usable.