CS 10 · Unit 1 · Computer System — Athens Academy Islamabad

Athens Academy · FBISE

Computer Systems

SubjectComputer Science

GradeGrade 10

UnitUnit 1

Exam SessionExam 2027

Weight★★★ High

1Definitions to Reproduce Exactly

These are the wordings that earn full marks. Learn them verbatim.

Definition — Operating System (Examinable) Essential software that manages a computer's memory, processes, hardware and software, enabling user interaction. It loads into RAM when the computer is turned on and is necessary to run any program.

Definition — Process (Examinable) A program in execution; a dynamic instance of a program during its execution in RAM. It represents the basic unit of work and uses resources such as CPU time, files, I/O devices and memory.

Definition — 2's Complement (Examinable) Obtained by taking the 1's complement of a binary number and then adding 1 to the least significant bit (LSB).

Definition — Unicode (Examinable) An international character-encoding system designed to facilitate the electronic exchange, processing and display of written texts from diverse modern and classical languages.

All examinable definitions

Term	Definition to write in the paper
Bit	A binary digit having one of two states, 0 or 1; the fundamental unit of information in a computer.
Number system	A way of writing and showing numbers using specific digits or symbols, which helps organise numbers and perform arithmetic operations.
Positional value	The value of a digit depends on its position; each position carries a weight equal to a power of the base.
MSB / LSB	Most Significant Bit — leftmost, greatest weight. Least Significant Bit — rightmost, least weight.
Overflow	Occurs when a calculation produces a result larger than the maximum value that can be stored with the available number of bits.
Underflow	Occurs when a value is too small to be represented properly within the available number of bits.
1's complement	Obtained by inverting (toggling) all bits of a binary number: every 1 becomes 0 and every 0 becomes 1.
2's complement	Obtained by taking the 1's complement of the number and then adding 1 to the LSB.
Unsigned number	A binary number with no sign bit, containing magnitude only; range $0$ to $2^n-1$ .
Signed number	A binary number using the leftmost bit as a sign bit: 0 for positive, 1 for negative.
ASCII	American Standard Code for Information Interchange — a standard coding scheme that assigns standard numeric values to alphabets, numerals, punctuation marks and other characters.
Unicode	An international character-encoding system designed to facilitate the electronic exchange, processing and display of written texts from diverse modern and classical languages.
Operating system	Essential software that manages a computer's memory, processes, hardware and software, enabling user interaction. It loads into RAM when the computer is turned on.
Program	Executable code stored on disk as a text file.
Process	A program in execution; a dynamic instance of a program during its execution in RAM. It is the basic unit of work.
Thread	A basic ordered sequence of instructions within a process that cannot be executed independently.
Process scheduler	Part of the OS that manages the execution order of processes by the CPU, ensuring efficient and fair use of CPU time.
Process synchronisation	A method used to coordinate multiple processes or threads that share resources, preventing race conditions.
Interrupt	A signal that alerts the CPU to an event needing immediate attention; the CPU saves its state and runs an Interrupt Service Routine (ISR).
Deadlock	Occurs when two or more processes are stuck, each waiting for a resource held by the other.
Metadata	Information about files and directories such as names, sizes, types, permissions and timestamps.
Software	A collection of programs, data and instructions that tell a computer how to perform specific tasks or functions.
Application patch	A piece of software designed to update, fix or improve an existing program or its supporting data.
Software hosting	The process of deploying, managing and providing access to software applications on servers.
Compiler	Translates code written in a high-level programming language into machine code the computer can execute.
IDE	Integrated Development Environment — a comprehensive tool combining a text editor, debugger and compiler in one interface.

2Machine-Level Data Representation

All data is stored as binary digits. Four data types must be known:

Data type	Representation	Key figures
Integer	Fixed number of bits; MSB indicates sign.	8-bit signed $-128$ to $127$ ; unsigned $0$ to $255$
Floating-point	Positive or negative number with a decimal point.	$15.5$ , $0.45$ , $-203.345$
Character	Encoded by ASCII or Unicode; each character has a unique binary code.	'A' = 65 = 01000001
Boolean	Single bit.	0 = false, 1 = true

Must memorise

$2^7=128$ · $2^8=256$ · $2^{10}=1024$ · 8-bit unsigned max $=255$ · 8-bit signed range $=-128$ to $127$

3Number Systems

System	Base	Digits	Bits/digit	Weights	Example
Decimal	10	0–9	—	powers of 10	964₁₀
Binary	2	0, 1	1	powers of 2	1011₂
Octal	8	0–7	3	powers of 8	672₈
Hexadecimal	16	0–9, A–F	4	powers of 16	3AF₁₆

Trap

Hex A–F are digits, not letters: A=10, B=11, C=12, D=13, E=14, F=15.

3.1Any base → Decimal

Multiply each digit by its positional weight and add.

Worked examples

1011.101_2 = 8+0+2+1+0.5+0+0.125 = 11.625_{10}

672_8 = 6{\times}64 + 7{\times}8 + 2 = 442_{10}

25.6_8 = 16 + 5 + 0.75 = 21.75_{10}

3AF_{16} = 3{\times}256 + 10{\times}16 + 15 = 943_{10}

Fig 1 — Positional weights.

3.2Decimal → Any base

Integer part: divide by the base, record remainders, read upward. Fractional part: multiply by the base, record integer parts, read downward.

179.257_{10}

→ binary

÷2	Quotient	Rem
179	89	1 ← LSB
89	44	1
44	22	0
22	11	0
11	5	1
5	2	1
2	1	0
1	0	1 ← MSB

$179_{10} = 10110011_2$

Fraction: $0.257{\times}2$ repeatedly → digits 0,1,0,0,0,0,0 → $0.0100000_2$

$179.257_{10} \approx 10110011.0100000_2$

Decimal → octal and hexadecimal

165 \div 8

: rem 5, 4, 2 →

245_8

331 \div 8

: rem 3, 1, 5 →

513_8

739 \div 16

: rem 3, 14(E), 2 →

2E3_{16}

Trap — direction

Remainders read upward; fraction digits read downward. Reversing either gives a wrong answer. Division climbs, multiplication falls.

3.3Octal ↔ Binary (groups of 3) · Hex ↔ Binary (groups of 4)

Octal	0	1	2	3	4	5	6	7
Binary	000	001	010	011	100	101	110	111

Hex	0	1	2	3	4	5	6	7
Binary	0000	0001	0010	0011	0100	0101	0110	0111
Hex	8	9	A	B	C	D	E	F
Binary	1000	1001	1010	1011	1100	1101	1110	1111

Worked examples

7301_8

→ 111 011 000 001 =

111011000001_2

1100111000101_2

→ 001 100 111 000 101 =

14705_8

D73A1_{16}

→ 1101 0111 0011 1010 0001 =

11010111001110100001_2

110011100100110101_2

→ 0011 0011 1001 0011 0101 =

33935_{16}

Trap

Always group from the right; pad zeros on the left only.

3.4Octal ↔ Hexadecimal

No direct method. Method 1: via decimal. Method 2: via binary (faster — regroup 3s into 4s).

6457_8

→ hexadecimal

Via decimal:

6457_8 = 3072+256+40+7 = 3375_{10}

;

3375 \div 16

→ rem 15(F), 2, 13(D) =

D2F_{16}

Via binary: 110 100 101 111 → regroup 1101 0010 1111 =

D2F_{16}

Exam technique

Always show the binary middle step — it protects method marks.

4Binary Arithmetic

Addition	Result	Subtraction	Result
0 + 0	0	0 − 0	0
0 + 1	1	1 − 0	1
1 + 0	1	1 − 1	0
1 + 1	10 (carry 1)	0 − 1	1 (borrow 1)

Worked additions

   11              111  11
   10010  (18)      11101010  (234)
 +   111  ( 7)    + 10011011  (155)
 ────────         ──────────
   11001  (25)     110000101  (389)

Verify by converting to decimal and adding.

5Overflow and Underflow

Overflow: result too large for the available bits. Underflow: value too small to represent.

8-bit unsigned overflow —

255 + 1

   11111111   (255)
 +        1
 ──────────
  100000000   (256 — needs 9 bits)
  ↑ 9th bit lost; register holds 00000000

Stored result is 0, not 256. Overcome by using a 16-bit register.

Handling overflow and underflow

Use larger data types — e.g. 8-bit → 16-bit integer.
Perform range checking before the operation.
Use libraries or built-in functions that detect and handle these conditions.

6Complements (1's and 2's)

Used in digital logic to perform logical operations, especially subtraction.

Forming complements

1's:  01101    → 10010          (invert all bits)

2's:  01010001 → 10101110       (1's complement)
                      + 1       (add 1 to LSB)
                 ────────
                 10101111       (2's complement)

6.1Subtraction using complements

Situation	1's complement	2's complement
Carry produced	Add the carry back into the LSB (end-around carry).	Ignore the carry; remaining bits are the answer.
No carry	Take 1's complement of result; answer is negative.	Take 2's complement of result; answer is negative.

111000_2 - 100110_2

— both methods

1's COMPLEMENT              2's COMPLEMENT
100110 → 011001             100110 → 011010

    111000                      111000
  + 011001                    + 011010
  ────────                    ────────
  1 010001                    1 010010
  ↑ carry → add to LSB        ↑ ignore carry

    010001
  +      1
  ────────
    010010                      010010

Both give $010010_2 = 18_{10}$ ; check: $56-38=18$ ✓

7Signed and Unsigned Representation

Feature	Unsigned	Signed
Sign bit	✗ None	✓ Leftmost bit: 0 = +ve, 1 = −ve
Values	Magnitude only, all positive	Both positive and negative
Range ( $n$ bits)	$0$ to $2^n-1$	$-2^{n-1}$ to $2^{n-1}-1$
8-bit range	$0$ to $255$	$-128$ to $127$
Method	Straight binary conversion	Two's complement

Negative numbers in 8-bit 2's complement

−13:  13 = 00001101 → 11110010 → +1 → 11110011
−55:  55 = 00110111 → 11001000 → +1 → 11001001
−43:  43 = 00101011 → 11010100 → +1 → 11010101

Check: number + its 2's complement = 00000000 (carry discarded).

Trap

Sign bit 1 does not mean "read normally and add a minus" — take the 2's complement to find the magnitude.

8Floating-Point Representation (IEEE 754)

Fig 2 — 32-bit single precision: 1 + 8 + 23 bits.

Scope

Only the three components and the 1 / 8 / 23 split are required. Encoding a number into IEEE 754 is beyond Grade 10 scope.

9Coding Schemes — ASCII and Unicode

Why coding schemes are used

Reason	Point to write
Data representation	Maps human-readable characters to binary codes so computers can store and manipulate text.
Standardisation	Consistent representation across platforms and devices, ensuring compatibility.
Efficiency	Fixed-length binary representation optimises storage and processing.
Communication	Enables different systems to communicate using a common representation.

ASCII vs Unicode

Point	ASCII	Unicode
Stands for	American Standard Code for Information Interchange	Universal coding system
Introduced	Approved 17 June 1963	First version 1991
Published by	American standard body	Unicode Consortium (multilingual software manufacturers)
Bits	ASCII-7 = 7 bits; ASCII-8 = 8 bits	Originally 2 bytes (16-bit); version 3 is 4 bytes (32-bit)
Characters	$2^7=128$ or $2^8=256$	Over 100,000
Languages	English and limited symbols	All major written languages, plus emoji and technical symbols
Encodings	Single fixed scheme	UTF-7, UTF-8, UTF-16, UTF-32 — conversion is lossless
Compatibility	—	Version 3 is fully compatible with ASCII

Facts examiners ask

ASCII-8 became popular in 1981 with IBM's PC · 32 code combinations in ASCII-8 are control commands (start of text, carriage return, form feed) · Microsoft's 8-bit encoding is ANSI · IBM initially used EBCDIC instead of ASCII · UTF = Unicode Transformation Format.

Character codes

'A' = 65 = 01000001 · 'Z' = 90 = 01011010

10Operating System

Loads into RAM at startup; necessary to run any program. Examples: Windows, macOS, Linux, iOS, Android.

Six functions of an OS

Function	What to write
Process management	Allocates resources such as CPU time to various processes in memory.
Memory management	Controls main memory; allocates space to programs loaded for execution, tracks freed memory and updates memory status.
File management	Manages files and folders on storage devices; allows create, copy, move, rename, delete, search, read, write, open and close.
Device management	Controls input and output devices such as keyboards, mouse, printers and network interfaces.
Network management	Monitors and manages network resources, creates user groups, assigns privileges, detects and fixes network problems.
Security management	Creates user accounts, enforces security policies, controls access, protects from unauthorised access and malware.

Process scheduling — order affects average time

Processes A = 5 ms, B = 2 ms, C = 1 ms, all arriving at time 0. Each process waits for every process ahead of it to finish, so the order alone changes the averages.

Dry run — order A, B, C

Process	Burst (ms)	Starts at	Finishes at	Waiting	Turnaround
A	5	0	5	0	5
B	2	5	7	5	7
C	1	7	8	7	8

Average waiting

=\dfrac{0+5+7}{3} = 4.00

ms · average turnaround

=\dfrac{5+7+8}{3} = 6.67

Dry run — order B, C, A

Process	Burst (ms)	Starts at	Finishes at	Waiting	Turnaround
B	2	0	2	0	2
C	1	2	3	2	3
A	5	3	8	3	8

Average waiting

=\dfrac{0+2+3}{3} = 1.67

ms · average turnaround

=\dfrac{2+3+8}{3} = 4.33

Case 2 is more efficient. Running the shortest jobs first lowers both averages: waiting falls from 4.00 to 1.67 ms and turnaround from 6.67 to 4.33 ms. Total time is 8 ms either way — only the order changes.

⚠ Exam Trap — waiting vs turnaround

Waiting time is how long a process sits before it starts (its start time). Turnaround time is how long until it finishes. They are different numbers: read the question carefully and state which one you are averaging.

Types of operating system

Type	Working	Used in
Batch processing	Groups similar tasks into batches, executes sequentially.	Payroll, bank statements
Multiprogramming	Multiple programs in main memory; CPU switches when one waits for I/O, so CPU is rarely idle.	Maximising CPU utilisation
Multitasking	Multiple tasks concurrently on one CPU by rapid switching.	PCs, mobile devices
Time-sharing	Divides CPU time into time slices, each program gets a slice in turn.	Banks, universities, large organisations
Real-time (RTOS)	Immediate response within strict time constraints and deadlines.	Medical equipment, traffic control, industrial automation
Multiprocessor	Controls multiple CPUs in one system, distributing tasks.	Data centres, servers, scientific research
Distributed	Manages a network of computers, balancing load; users need not know which machine runs the task.	Cloud computing, networks
Embedded	Built into a specific device's hardware for specialised tasks.	Microwave, TV, camera

Trap — asked almost every year

Multiprogramming switches when a program waits for I/O (keeps CPU busy). Multitasking switches rapidly and continually (makes tasks appear simultaneous to the user).

11Process Management

Five states of a process

State	Meaning
Start / New	First state when the process is created.
Ready	Ready for execution but waiting to be assigned to the processor.
Running	Being executed by the processor.
Blocked / Waiting	Not executing; waiting for a resource to become available.
Terminated	Has completed its execution.

Fig 3 — Process states. Blocked returns to Ready, never straight to Running.

Process vs thread — five differences

#	Process	Thread
1	An executing instance of a program.	A subset of the process.
2	Has its own copy of the data segment of the parent process.	Has direct access to the data segment of its process.
3	Change in a process does not affect other processes.	Change in a thread may affect other threads of the process.
4	Runs in separate memory space.	Runs in shared memory space.
5	Controlled by the operating system.	Controlled by the programmer in a program.

Scheduler · synchronisation · interrupts · deadlock

Concept	Point to write
Process scheduler	Manages execution order of processes by the CPU using scheduling algorithms.
Synchronisation	Coordinates processes sharing resources; prevents race conditions.
Interrupts	CPU stops current task, saves its state, runs the ISR, then resumes.
Deadlock	P1 holds R1 and needs R2; P2 holds R2 and needs R1 — neither can proceed.

Fig 4 — Deadlock: solid = holds, dashed = waits for.

Four system resources managed by an OS

CPU (scheduling, prioritising, deadlock resolution, interrupts) · Memory (allocation and deallocation, memory hierarchy) · Storage (file system, read/write, data integrity) · I/O devices (device communication, buffering, drivers).

12File System

Components: Files (smallest unit of data storage, a collection of bytes) · Directories/folders (containers holding files and other directories) · Metadata (name, size, type, permissions, timestamps).

Fig 5 — Three directory structures.

Allocation method	How files are stored	Effect
Contiguous	Files occupy consecutive blocks.	✓ Faster access ✗ Causes fragmentation
Linked	Files stored in linked lists.	✓ Flexible ✗ Slower — pointer traversal
Indexed	An index points to the actual data blocks.	✓ Faster access, balanced storage efficiency

File system	Features
FAT	File Allocation Table — older, used by DOS and Windows. Simple but limited in features and scalability.
NTFS	New Technology File System — Windows NT onward. Supports large files, security features, disk quotas.
ext3 / ext4	Linux. ext4 offers improved performance and journaling, which helps recover from crashes.

File operations: Create · Open · Read · Write · Delete · Rename

13Computer Software

Type	Purpose and function	Examples
System software	Manages hardware and system resources; controls overall system operations and provides a platform for applications.	Windows, macOS, Linux, Android
Application software	Performs specific tasks for the user, such as writing, streaming or browsing.	MS Word, Spotify, Google Chrome
Programming software	Provides tools for writing, testing and debugging code; translates programming languages into machine-readable code.	Eclipse, Visual Studio, GitHub
Driver software	Acts as a bridge between the operating system and hardware devices.	Printer, NVIDIA graphics, Wi-Fi drivers

Offline vs online applications

Type	Description	Examples
Offline	Run without an internet connection, relying on the device's local resources.	MS Word, Adobe Photoshop, VLC Media Player
Online	Require internet as they access resources or services on remote servers.	Google Docs, Spotify, Gmail

Application patch — four key functions

Fix bugs (resolve glitches or errors) · Improve security (patch vulnerabilities) · Enhance performance (optimise the program) · Add features (new functionality).

Examples: Windows security patches · game updates (World of Warcraft, Call of Duty) · Adobe Acrobat patches.

Software hosting — four types

Type	Description	Pros	Cons
On-premises	Installed and run on servers within the organisation's own premises.	✓ Complete control over hardware and data security ✓ Customisable to compliance needs	✗ High initial and ongoing cost ✗ Needs in-house expertise ✗ Limited scalability
Shared	Multiple websites share the same server resources of a hosting company.	✓ Cost-effective ✓ Easy to set up and manage	✗ Limited control ✗ Performance issues ✗ Security risk from other sites
Dedicated	An entire physical server for one user or organisation.	✓ High performance and reliability ✓ Full control ✓ Enhanced security	✗ High cost ✗ Requires technical expertise
Cloud	Network of virtual and physical servers giving resources on demand (AWS, Google Cloud, Azure).	✓ Highly scalable and flexible ✓ Pay-as-you-go ✓ Reliable, global reach	✗ Depends on internet connectivity ✗ Data security and compliance concerns

Programming software

Component	Function	Examples
Text editors	Allow programmers to write and edit code.	Notepad++, Sublime Text, VS Code
IDEs	Combine text editor, debugger and compiler in one interface.	IntelliJ IDEA, Eclipse, Visual Studio
Compilers	Translate high-level code into machine code.	GCC for C/C++, javac for Java

How it helps: syntax highlighting · code completion · debugging (step through code, inspect variables) · real-time error checking · compiling and optimisation · project management.

14"Hello World!" Project — HTML, CSS, JavaScript

Setup: create folder HelloWorldProject, open in Visual Studio Code, create three files — index.html (structure), styles.css (appearance), script.js (behaviour). View in a browser.

<!-- index.html -->
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Change Message</title>
  <link rel="stylesheet" href="styles.css">
</head>
<body>
  <div id="message">Hello World</div>
  <button id="change-message-button">Change Message</button>
  <script src="script.js"></script>
</body>
</html>

/* styles.css */
body {
  font-family: arial, sans-serif;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: #f0f0f0;
}
button {
  padding: 10px 20px;
  font-size: 1em;
  cursor: pointer;
  border-radius: 5px;
}

// script.js
document.addEventListener("DOMContentLoaded", function() {
  const messageDiv = document.getElementById('message');
  const button = document.getElementById('change-message-button');

  button.addEventListener("click", function() {
    if (messageDiv.textContent === 'Hello World') {
      messageDiv.textContent = 'I Love my country, Pakistan';
    } else {
      messageDiv.textContent = 'Hello World';
    }
  });
});

Trace table — messageDiv.textContent

Event	Before	Condition true?	Branch	After
Load	—	—	—	Hello World
Click 1	Hello World	Yes	if	I Love my country, Pakistan
Click 2	I Love my country, Pakistan	No	else	Hello World
Click 3	Hello World	Yes	if	I Love my country, Pakistan
Click 4	I Love my country, Pakistan	No	else	Hello World

Output: page shows "Hello World" and a button; clicking alternates the message with "I Love my country, Pakistan".

Trap

=== is comparison; = is assignment. Writing = inside the if is the most common error.

Likely exam question

Separation of concerns: HTML = structure, CSS = appearance, JavaScript = behaviour. Deleting script.js leaves the page looking identical but the button does nothing.

15Rapid Revision — Facts and Figures

Item	Value to reproduce
Powers of 2	$2^7=128$ · $2^8=256$ · $2^{10}=1024$
8-bit ranges	Unsigned $0$ – $255$ · Signed $-128$ to $127$
Bits per digit	Octal = 3 bits · Hexadecimal = 4 bits
Hex letter values	A=10, B=11, C=12, D=13, E=14, F=15
ASCII approved	17 June 1963
ASCII-8 popular	1981, with IBM PC
ASCII capacities	ASCII-7 = 128 · ASCII-8 = 256 · 32 control codes
Unicode	1991 · Unicode Consortium · over 100,000 characters · UTF-7/8/16/32
Character codes	'A' = 65 = 01000001 · 'Z' = 90 = 01011010
IEEE 754 (32-bit)	1 sign + 8 exponent + 23 mantissa
OS functions	Process, memory, file, device, network, security management
OS types	Batch, multiprogramming, multitasking, time-sharing, real-time, multiprocessor, distributed, embedded
Process states	Start, Ready, Running, Blocked/Waiting, Terminated
System resources	CPU, memory, storage, I/O devices
File allocation	Contiguous, linked, indexed
File systems	FAT, NTFS, ext3/ext4
File operations	Create, open, read, write, delete, rename
Software types	System, application, programming, driver
Hosting types	On-premises, shared, dedicated, cloud
Scheduling algorithms	FCFS, SJN/SJF, priority, round robin

💻 Chapter 1 · Computer Systems