Understand the purpose of a primary key and identify one

IGCSE Computer Science (0478) – Concise Revision Notes

How to use these notes

• Read each section once for a quick overview.
• Use the bullet‑point “Key facts for the exam” boxes to focus revision.
• Practice the example SQL, algorithm and programming snippets – they are the exact style asked in Paper 2.
• Keep the summary tables handy for quick recall of conversions, data‑type sizes and normalisation rules.

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1. Data Representation

1.1 Number systems

System	Base	Digits used	Typical use
Binary	2	0, 1	All computer data
Octal	8	0‑7	Compact binary view (group 3 bits)
Decimal	10	0‑9	Human‑readable numbers
Hexadecimal	16	0‑9, A‑F	Compact binary view (group 4 bits)

Conversion cheat‑sheet

• Binary → Decimal: add powers of 2 for each ‘1’.
• Decimal → Binary: divide by 2, record remainders (read upwards).
• Binary ↔ Hex: group bits in sets of 4 (add leading 0s if needed).

Two’s‑complement (signed integers)

• Positive numbers = normal binary.
• Negative number = invert all bits, add 1.
• Range for n‑bit word: –2ⁿ⁻¹ … 2ⁿ⁻¹ – 1.

Logical shifts

• << (left shift) = multiply by 2ⁿ (fill with 0s).
• >> (right shift) = integer division by 2ⁿ (fill with sign bit for arithmetic shift).

Key facts for the exam

• Remember the binary‑to‑hex mapping (0000 = 0, 0001 = 1, … 1111 = F).
• Two’s‑complement overflow occurs when the sign bit changes unexpectedly.
• Logical shift left by 1 is equivalent to “multiply by 2”.

1.2 Text, sound and images

Data type	Common encoding	Key parameters
Text	ASCII (7‑bit), Unicode (UTF‑8/16)	1 char = 1 byte (ASCII) or 1‑4 bytes (Unicode)
Sound	Pulse‑code modulation (PCM)	Sample rate (Hz) × Bit depth (bits) × Channels ÷ 8 = KB/s
Images	Bitmap (pixel‑by‑pixel)	Resolution (pixels) × Colour depth (bits) ÷ 8 = KB per line

Example calculation – A 44 kHz, 16‑bit stereo audio file lasting 10 s:

44 000 samples/s × 16 bits × 2 channels = 1 408 000 bits/s
1 408 000 ÷ 8 = 176 000 bytes/s ≈ 172 KB/s
172 KB/s × 10 s = 1 720 KB ≈ 1.68 MB

Compression

• Lossless (e.g., ZIP, PNG) – original data can be perfectly reconstructed.
• Lossy (e.g., MP3, JPEG) – some data discarded; smaller files but quality loss.

Key facts for the exam

• 1 byte = 8 bits; 1 KB = 1024 bytes; 1 MB = 1024 KB, etc.
• File‑size = (sample rate × bit depth × channels × duration) ÷ 8.
• Colour depth of 24‑bit = 16 777 216 possible colours.

1.3 Data storage units & prefixes

• Bit (b) – smallest unit.
• Byte (B) – 8 bits.
• Kibi‑byte (KiB) = 2¹⁰ = 1024 B; Mebi‑byte (MiB) = 2²⁰ B; Gibi‑byte (GiB) = 2³⁰ B.
• Decimal prefixes (kB, MB, GB) are used by hard‑disk manufacturers; remember the conversion difference for exam questions.

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2. Data Transmission

2.1 Network basics

• Packet‑switched network – data broken into packets, each routed independently.
• Circuit‑switched network – a dedicated path is reserved for the duration of a call.
• Common topologies: star, bus, ring, mesh.

2.2 Common transmission media

Medium	Typical speed	Pros	Cons
Copper (twisted‑pair, coax)	10 Mbps – 10 Gbps	Cheap, easy to install	Susceptible to EMI, limited distance
Fiber‑optic	100 Mbps – 100 Gbps+	Very high bandwidth, immune to EMI	Expensive, fragile
Wireless (Wi‑Fi, Bluetooth, 4G/5G)	11 Mbps – 10 Gbps	Mobility, no cables	Interference, security concerns

2.3 Error detection & correction

• Parity bit – simple even/odd check; detects single‑bit errors.
• Checksum – sum of data bytes; detects many errors but not all.
• CRC (Cyclic Redundancy Check) – polynomial division; widely used in networking.

2.4 Basic encryption concepts

• Symmetric key – same key for encryption and decryption (e.g., AES).
• Public‑key (asymmetric) – pair of keys; one public, one private (e.g., RSA).
• For the IGCSE you only need to know the purpose: protect confidentiality and verify integrity.

Key facts for the exam

• Packet = header + payload + trailer.
• Parity can only detect an odd number of flipped bits.
• Wi‑Fi uses the 2.4 GHz or 5 GHz radio bands; Bluetooth ≈ 2.4 GHz but lower power.

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3. Data Processing & Computer Architecture

3.1 The CPU

• ALU (Arithmetic Logic Unit) – performs arithmetic and logical operations.
• Control Unit – fetches, decodes and executes instructions.
• Registers – very fast storage inside the CPU (e.g., ACC, PC, IR).
• Cache – small, high‑speed memory that stores frequently used data.

3.2 Memory hierarchy

Level	Typical size	Speed (relative)	Cost per MB
Registers	few KB	fastest	Very high
Cache (L1/L2)	tens‑to‑hundreds KB	very fast	High
RAM (Main memory)	4 GB – 64 GB	fast	Medium
Secondary storage (HDD/SSD)	250 GB – 4 TB	slow	Low

3.3 Input/Output and buses

• Bus – set of electrical pathways that transfer data between CPU, memory and peripherals.
• Interrupt – signal that temporarily halts the CPU’s current task to service a device.
• Common I/O devices: keyboard, mouse, scanner, printer, monitor (output), microphone (input).

Key facts for the exam

• CPU fetch‑decode‑execute cycle repeats for each instruction.
• Cache reduces the average time to access data from main memory.
• Interrupts improve efficiency by allowing the CPU to respond only when needed.

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4. Computer Systems & Software

4.1 System components

• Motherboard – holds CPU, RAM, expansion slots, and connectors.
• Power Supply Unit (PSU) – converts mains AC to low‑voltage DC.
• BIOS/UEFI – firmware that performs POST and boots the OS.
• Peripheral devices – connected via USB, HDMI, Ethernet, etc.

4.2 Types of software

Category	Purpose	Examples
System software	Manage hardware & provide platform	Operating System (Windows, macOS, Linux)
Application software	Perform specific user tasks	Word processor, spreadsheet, web browser
Utility software	Maintain, protect & optimise the system	Antivirus, backup, disk‑defragmenter

4.3 Security basics

• Authentication – verifying identity (passwords, biometrics).
• Authorization – defining what an authenticated user may do.
• Encryption – converting data to unreadable form without a key.
• Malware – viruses, worms, trojans; protect with anti‑virus and regular updates.

Key facts for the exam

• BIOS/UEFI runs before the OS and performs the Power‑On Self‑Test (POST).
• Operating systems manage multitasking, memory allocation and file systems.
• Strong passwords should be at least 8 characters, mix of letters, numbers and symbols.

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5. Algorithms & Problem Solving

5.1 What is an algorithm?

A finite, ordered set of instructions that solves a problem or performs a task.

5.2 Flowchart symbols (exam‑required)

Symbol	Name	Meaning
▭	Process	Action or operation (e.g., calculation)
▱	Input/Output	Read or display data
◆	Decision	Yes/No test (branch)
⭮	Loop/Connector	Repeat or link parts of the chart

5.3 Basic algorithmic constructs

• Sequence – one step after another.
• Selection – IF … THEN … ELSE … (decision).
• Iteration – WHILE or FOR loops.

5.4 Example: Find the largest of three numbers

START
READ a, b, c
SET max = a
IF b > max THEN SET max = b
IF c > max THEN SET max = c
OUTPUT max
END

5.5 Searching & sorting (paper 2 basics)

• Linear search – examine each record until a match is found (O(n)).
• Binary search – repeatedly halve a sorted list (O(log n)).
• Bubble sort – repeatedly swap adjacent out‑of‑order items (O(n²)).

Key facts for the exam

• All algorithms must terminate – no infinite loops.
• Complexity notation: O(n) = linear, O(log n) = logarithmic, O(n²) = quadratic.
• Flowcharts must have a single start and a single end.

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6. Programming (Python‑style pseudocode)

6.1 Variables & data types

• INTEGER – whole numbers.
• REAL – numbers with a decimal point.
• STRING – text enclosed in quotes.
• BOOLEAN – TRUE or FALSE.

6.2 Control structures

# Selection
IF condition THEN
    …          # statements
ELSE
    …          # statements
END IF

# Loop – count controlled
FOR i FROM 1 TO 10
    …          # statements
END FOR

# Loop – condition controlled
WHILE condition
    …          # statements
END WHILE

6.3 Common operators

Operator	Name	Example
+	Addition / Concatenation	5 + 3 = 8; "Hi" + "!" = "Hi!"
-	Subtraction	10 - 4 = 6
*	Multiplication	7 * 2 = 14
/	Division (real result)	7 / 2 = 3.5
//	Integer division	7 // 2 = 3
%	Modulo (remainder)	7 % 2 = 1
==, !=, <, >, <=, >=	Comparison	a == b
AND, OR, NOT	Logical	(a > 0) AND (b < 5)

6.4 Example program – calculate average mark

READ mark1, mark2, mark3
SET total = mark1 + mark2 + mark3
SET average = total / 3
OUTPUT "Average =", average

Key facts for the exam

• Indentation is not required in the exam, but logical nesting must be clear.
• All variables must be declared before they are used (Paper 2).
• Remember the difference between integer division (//) and real division (/).

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7. Databases – Primary Keys, Foreign Keys & SQL

7.1 What is a database?

A structured collection of related data stored in tables. Each table holds records (rows) and fields (columns).

7.2 Primary key – purpose & properties

• Uniqueness – no two records share the same key value.
• Non‑null – every record must contain a value.
• Immutability – the value should never change once assigned.
• Reference point – other tables link to it via foreign keys.
• Performance – DBMS automatically creates an index on the primary key.

7.3 Choosing a good primary key

Criterion	Good example	Poor example
Unique & permanent	StudentID (auto‑increment integer)	Full name (may duplicate)
Simple & minimal	ISBN (13‑digit)	Combination of title + author + publisher
Immutable	EmployeeNumber	Address (can change)

7.4 Composite (compound) primary keys

Used when a single field cannot guarantee uniqueness.

CREATE TABLE Enrolments (
    StudentID   INT NOT NULL,
    CourseCode  CHAR(6) NOT NULL,
    EnrolDate   DATE NOT NULL,
    PRIMARY KEY (StudentID, CourseCode)   -- composite key
);

Here a student may enrol in many courses, but the pair (StudentID, CourseCode) is unique.

7.5 Surrogate (system‑generated) keys

• Auto‑increment integer or UUID.
• Used when no natural field satisfies all primary‑key rules.

7.6 Foreign keys – linking tables

ALTER TABLE Enrolments
ADD CONSTRAINT FK_Student
FOREIGN KEY (StudentID) REFERENCES Students(StudentID);

Diagram (textual):

+----------------+          +----------------+
|   Students     |          |   Enrolments   |
|----------------|          |----------------|
| PK StudentID   |◄───────► | FK StudentID   |
| FirstName      |          | PK CourseCode  |
| LastName       |          | EnrolDate      |
+----------------+          +----------------+

7.7 Basic SQL statements (Paper 2)

Command	Purpose	Example
SELECT … FROM …	Retrieve data	SELECT FirstName, LastName FROM Students;
WHERE	Filter rows	SELECT * FROM Students WHERE Class = '10A';
ORDER BY	Sort results	… ORDER BY LastName ASC;
GROUP BY	Group rows for aggregation	SELECT Class, COUNT(*) FROM Students GROUP BY Class;
SUM, COUNT, AVG, MIN, MAX	Aggregate functions	SELECT SUM(Marks) FROM Results;
INSERT INTO … VALUES …	Add a new record	INSERT INTO Students VALUES (101, 'Ada', 'Lovelace', '2005-12-10', '10A');
UPDATE … SET … WHERE …	Modify existing records	UPDATE Students SET Class='11B' WHERE StudentID=101;
DELETE FROM … WHERE …	Remove records	DELETE FROM Enrolments WHERE StudentID=101 AND CourseCode='CS101';

7.8 Normalisation – 1NF & 2NF (exam level)

• First Normal Form (1NF) – each field contains atomic (indivisible) values; no repeating groups.
• Second Normal Form (2NF) – table is in 1NF and every non‑key field depends on the whole primary key (eliminate partial dependency).

Example – splitting a non‑normalised Orders table:

Orders (OrderID, CustomerName, CustomerPhone, Product1, Product2, …)
→
Customers (CustomerID, CustomerName, CustomerPhone)
Orders    (OrderID, CustomerID, OrderDate)
OrderLines(OrderID, ProductCode, Quantity)

Key facts for the exam

• Primary key = unique identifier; must be NOT NULL and immutable.
• Composite key = two or more fields together uniquely identify a record.
• Foreign key = field (or fields) that reference a primary key in another table.
• SQL statements required for Paper 2: SELECT, FROM, WHERE, ORDER BY, GROUP BY, INSERT, UPDATE, DELETE, plus aggregate functions.
• Normalization reduces redundancy; remember the “one fact per table” rule.

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8. Impacts of Digital Technology

8.1 Ethical & legal issues

• Copyright – creators own the exclusive right to reproduce their work.
• Data protection – personal data must be stored securely and used only with consent (GDPR‑style principles).
• Computer misuse – unauthorised access, hacking, phishing.

8.2 Environmental considerations

• E‑waste – hazardous chemicals; importance of recycling.
• Energy consumption – servers, data centres; green computing practices.

8.3 Social & economic impact

• Digital divide – unequal access to technology.
• Automation – can increase productivity but may reduce certain jobs.
• Globalisation – easier communication, but also raises security concerns.

Key facts for the exam

• Give one advantage and one disadvantage of a technology (e.g., “smartphones: advantage = instant communication; disadvantage = privacy risk”).
• Explain why strong passwords and regular backups are part of good digital citizenship.

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9. Quick Revision Checklist

Topic	Must‑know items
Number systems & binary arithmetic	Conversions, two’s‑complement range, logical shifts.
Text, sound, images	ASCII vs Unicode, sample rate & bit depth, pixel resolution, lossless vs lossy compression.
Data storage units	Bit, byte, KiB, MiB, GiB; difference between decimal and binary prefixes.
Transmission media & error detection	Packet structure, parity, checksum, CRC, basic encryption purpose.
CPU & memory hierarchy	ALU, control unit, registers, cache, RAM, secondary storage.
System software	BIOS/UEFI, OS functions, utility software, security basics.
Algorithms	Flowchart symbols, sequence/selection/iteration, O‑notation, example searches/sorts.
Programming	Variables, data types, operators, IF/ELSE, FOR/WHILE loops, basic I/O.
Databases	Primary key rules, composite vs surrogate keys, foreign keys, basic SQL syntax, 1NF/2NF.
Impacts of technology	Ethical/legal issues, environmental effects, social/economic consequences.

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10. Final Tips for the IGCSE Exam

• Read the question carefully – note which part of the syllabus is being tested (Paper 1 theory vs Paper 2 practical).
• When writing SQL or pseudocode, keep it tidy: one command per line, proper indentation, and end each statement with a semicolon (SQL) or END (pseudocode).
• For diagram questions (e.g., ER diagrams, flowcharts), label every element clearly and use the standard symbols.
• Time‑management: allocate roughly 15 minutes per short‑answer question, leaving 10 minutes at the end for review.
• Practice past papers under timed conditions – the more you write, the more comfortable you’ll become with the exam language.