IGCSE Computer Science (0478) – Comprehensive Revision Notes

1. Learning Objectives

• Recall key definitions and concepts across all syllabus units (AO1).
• Apply knowledge to solve calculations, design simple circuits, write pseudocode and SQL queries (AO2).
• Evaluate technologies, choose appropriate solutions and justify decisions in exam‑style questions (AO3).

2. Syllabus Alignment

Syllabus Unit	Key Topics Covered in These Notes
1 – Data Representation	Binary, octal, hexadecimal, two’s‑complement, signed/unsigned numbers, overflow, logical/arithmetical shifts.
2 – Data Transmission	Packet structure, USB, Bluetooth, error detection (parity, checksum, CRC), ARQ, symmetric/asymmetric encryption.
3 – Hardware	CPU, FDE cycle, cores, cache, instruction set, embedded systems, diagram of a Von Neumann architecture.
4 – Software	System vs. application software, OS functions, interrupts, high‑/low‑level languages, IDEs, compilers & interpreters.
5 – The Internet & Digital Technologies	Internet vs. WWW, URLs, HTTP/HTTPS, browsers, cookies, DNS, digital currency & blockchain basics, cyber‑security threats & counter‑measures.
6 – Automated & Emerging Technologies	Sensors & actuators, robotics, AI, expert systems, machine learning, ethical considerations.
7‑10 – Algorithms, Programming & Logic	Program‑development cycle, flowcharts, pseudocode, basic algorithms (search, sort), data types, arrays, file handling, SQL, Boolean logic.
3.3 – Data Storage – Cloud vs Local	Definitions, comparison table, technical details (bandwidth, latency, redundancy, encryption), cost models, advantages & disadvantages, evaluation checklist, case study.

3. Core Concepts (AO1)

3.1 Data Representation

• Binary (base‑2): 0 and 1 only. Example: 1011₂ = 11₁₀.
• Octal (base‑8) and Hexadecimal (base‑16) – convenient for grouping binary digits (3 bits per octal digit, 4 bits per hex digit).
• Two’s‑complement for signed integers:
  • Invert bits, add 1 to obtain the negative.
  • Range for an 8‑bit byte: –128 to +127.
• Overflow: occurs when a calculation exceeds the representable range; the result “wraps round”.
• Shifts:
  • Logical shift – fills vacated bits with 0.
  • Arithmetic shift – preserves the sign bit for signed numbers.

3.2 Data Transmission

• Packet structure: Header (source/destination address, error‑check), Payload (data), Trailer (checksum/CRC).
• USB & Bluetooth: Serial communication, plug‑and‑play, typical speeds (USB 3.0 ≈ 5 Gb/s, Bluetooth 5 ≈ 2 Mbps).
• Error‑detection:
  • Parity bit – simple but can detect only odd numbers of errors.
  • Checksum – sum of data bytes, used in TCP.
  • CRC – polynomial division, provides strong detection.
• ARQ (Automatic Repeat reQuest) – retransmit corrupted frames (Stop‑and‑wait, Go‑back‑N, Selective Repeat).
• Encryption:
  • Symmetric (same key – e.g., AES‑256).
  • Asymmetric (public/private key – e.g., RSA 2048‑bit).

3.3 Hardware

• CPU – Central Processing Unit; performs the fetch‑decode‑execute (FDE) cycle.
• Core – independent processing unit; multi‑core CPUs run several threads simultaneously.
• Cache – small, fast memory (L1, L2, L3) that stores frequently accessed data.
• Instruction set – the set of operations a CPU can perform (e.g., ADD, SUB, LOAD, STORE).
• Embedded systems – computers built into devices (e.g., microwave, traffic light controller).

Diagram – Simplified Von Neumann Architecture

3.4 Software

• System software – operating system (OS) that manages hardware, provides services (file system, multitasking).
• Application software – programmes that perform specific tasks (word processor, web browser).
• OS functions: process management, memory management, device drivers, security, user interface.
• Interrupts – signals that temporarily halt the CPU to service an event (e.g., keyboard press).
• Programming language levels:
  • High‑level (Python, Java) – close to natural language.
  • Low‑level (Assembly) – close to machine code.
• IDE – Integrated Development Environment (e.g., Eclipse, Visual Studio) provides editor, compiler, debugger.
• Compilers vs. Interpreters:
  • Compiler translates entire source code to machine code before execution.
  • Interpreter translates line‑by‑line at run‑time.

3.5 The Internet & Digital Technologies

• Internet vs. World Wide Web – Internet is the global network; the WWW is a service that uses HTTP/HTTPS.
• URL anatomy: protocol://domain:port/path?query#fragment
• HTTP / HTTPS – request/response protocol; HTTPS adds TLS/SSL encryption.
• Browsers – render HTML, CSS, JavaScript; maintain a cache and history.
• Cookies – small data stored on the client; session vs. persistent.
• DNS – translates domain names to IP addresses.
• Digital Currency & Blockchain:
  • Blockchain = distributed ledger of blocks linked by cryptographic hashes.
  • Key properties: decentralisation, immutability, consensus (Proof‑of‑Work/Stake).
• Cyber‑security (Threats & Counter‑measures):
  • Threats: phishing, malware, DDoS, ransomware, insider attack.
  • Counter‑measures: firewalls, antivirus, strong passwords, multi‑factor authentication, SSL/TLS, regular updates.

3.6 Automated & Emerging Technologies

• Sensors – convert physical phenomena to electrical signals (temperature, light, pressure).
• Actuators – convert electrical signals into motion (motors, solenoids).
• Robotics – combination of sensors, actuators, control software; typical components: controller, power source, end‑effector.
• Artificial Intelligence (AI) – systems that mimic human reasoning (search, knowledge representation).
• Expert systems – rule‑based AI that provides advice in a specific domain.
• Machine Learning (ML) – algorithms that improve from data (supervised, unsupervised, reinforcement).
• Ethical issues – privacy, bias, job displacement, accountability.

3.7 Algorithms, Programming & Logic

• Program‑development cycle: Analyse → Design → Code → Test → Debug → Document → Maintain.
• Flowcharts – visual representation using standard symbols (process, decision, input/output, start/stop).
• Pseudocode – English‑like description of an algorithm; emphasises logic over syntax.
• Basic algorithms:
  • Linear search – O(n).
  • Binary search – O(log n) (requires sorted data).
  • Bubble sort – O(n²) simple but inefficient.
• Data types: integer, real, character, string, Boolean.
• Arrays – ordered collection of same‑type elements; zero‑based indexing in most languages.
• File handling – opening, reading, writing, closing; text vs. binary files.
• SQL basics:

  SELECT name, grade FROM results WHERE grade >= 70 ORDER BY name;

• Boolean logic – AND, OR, NOT, XOR; truth tables; use in conditional statements and circuit design.

4. Data Storage – Cloud vs Local (Unit 3.3)

4.1 Key Definitions

• Cloud storage: Remote servers managed by a third‑party provider; accessed via the Internet; provider handles hardware, physical security, and often redundancy.
• Local storage: Data kept on devices owned/controlled by the user (internal HDD/SSD, external USB, NAS, on‑premises server).
• Redundancy mechanisms: RAID (0‑5), geographic replication, snapshots.
• Encryption: AES‑256 for data‑at‑rest; TLS/SSL for data‑in‑transit.
• Service model: IaaS (Infrastructure‑as‑a‑Service) – typical for cloud storage.

4.2 Comparison Table

Aspect	Cloud Storage	Local Storage
Location of data	Remote data‑centre(s) accessed via the Internet	On‑site devices (PC, external drive, NAS, server rack)
Initial cost	Low – subscription or pay‑as‑you‑go	High – purchase of hardware, cabling, power
Scalability	Virtually unlimited; capacity added instantly	Limited by physical space; expansion needs new hardware
Accessibility	Any Internet‑connected device (mobile, laptop, tablet)	Usually limited to the local network or physically attached devices
Performance (speed)	Depends on bandwidth & latency; can be slower for large files	High read/write speeds (SSD ≈ 500 MB/s, HDD ≈ 150 MB/s)
Security control	Provider‑managed encryption & access policies; user trusts SLA	User has full control – can implement own encryption, firewalls, physical locks
Backup & redundancy	Automatic multi‑site replication, snapshots	User must configure RAID, off‑site backups, or sync to cloud manually
Reliance on Internet	Essential – no connection = no access (unless offline cache)	Not required for basic read/write; only needed for remote access
Legal / jurisdiction	Data stored where provider’s data‑centre resides; subject to that country’s laws	Data stays within the organisation’s own legal jurisdiction

4.3 Technical Details (AO2)

Bandwidth & Latency

• Bandwidth – maximum data transferred per second (e.g., 10 Mbps = 1.25 MB/s).
• Latency – delay before transfer starts, measured in milliseconds (ms). High latency (satellite links) makes even small files feel slow.
• Example calculation – Upload 2 GB over a 5 Mbps connection:
  1. 2 GB × 8 × 1024 = 16 384 Mb.
  2. Time = 16 384 Mb ÷ 5 Mbps = 3 276.8 s ≈ 54.6 minutes.

Cost Models

• Typical cloud pricing: £0.02 per GB per month + £0.01 per GB data‑out.
• Local storage example: £45 for a 2 TB external HDD (one‑off).
• 5‑year cost comparison (500 GB usage):
  • Cloud: 500 GB × £0.02 × 60 months = £600 + transfer fees ≈ £650.
  • Local: £45 (hardware) + £20/year maintenance = £145.

  Long‑term static storage often favours local devices; variable or collaborative data favours the cloud.

Redundancy & Reliability

• RAID 1 (mirroring): 2 × identical drives → 100 % redundancy, 2 × cost.
• RAID 5 (striping with parity): Minimum 3 drives; survives loss of one drive, good capacity‑to‑safety ratio.
• Cloud providers typically use geographic replication (e.g., three data‑centres on different continents) delivering “five‑nine” availability (99.999 %).

Encryption & Legal Issues

• At‑rest: AES‑256 encryption on disks.
• In‑transit: TLS/SSL (HTTPS) protects data travelling over the Internet.
• Regulations (GDPR, Data Protection Act) require data to be stored in compliant jurisdictions; users must verify provider locations.

4.4 Advantages & Disadvantages

Cloud Storage – Advantages

• Low upfront cost – pay only for what you use.
• Instant scalability – add capacity via a web portal.
• Automatic multi‑site replication → strong disaster recovery.
• Access from any Internet‑connected device – ideal for remote learning and group projects.
• Provider handles hardware upgrades and physical security.
• Built‑in versioning and easy sharing features.

Cloud Storage – Disadvantages

• Requires reliable, high‑speed Internet; performance drops with low bandwidth or high latency.
• Ongoing subscription fees can become expensive over many years.
• Data privacy depends on provider policies and the legal jurisdiction of their data‑centres.
• Service outages are beyond the user’s control.
• Large uploads/downloads consume data allowances and may take considerable time.

Local Storage – Advantages

• Fast read/write speeds, especially with SSDs.
• Full control over security – hardware encryption, firewalls, physical locks.
• One‑time purchase – no recurring fees.
• Works without Internet – useful in remote or low‑connectivity settings.
• Data remains within the organisation’s legal jurisdiction, simplifying compliance.

Local Storage – Disadvantages

• High initial capital outlay for hardware and accessories.
• Limited scalability – expanding capacity requires new drives or servers.
• All backup, redundancy and maintenance responsibilities lie with the user.
• Risk of loss from hardware failure, fire, theft, or accidental deletion if not properly backed up.
• Access usually confined to the local network or physically attached devices.

4.5 Evaluation Checklist (AO3)

When asked to recommend a storage solution, consider the following factors and justify each choice.

1. Budget: Upfront vs. recurring costs over the expected period.
2. Data volume & growth rate: Large, rapidly expanding datasets → cloud elasticity.
3. Access requirements: Frequent remote access → cloud; high‑performance local processing → local.
4. Internet reliability: Unstable connection → local or hybrid approach.
5. Security & legal compliance: Sensitive personal data may need on‑premises control or a provider with relevant certifications (ISO 27001, GDPR‑compliant).
6. Disaster recovery: Cloud’s built‑in replication vs. local RAID + off‑site backup.
7. Environmental impact: Shared cloud infrastructure can be more energy‑efficient than many isolated local servers.

4.6 Mini‑Case Study (Practice AO3)

Scenario: A secondary school must store 1 TB of student exam results, project files, and multimedia resources. The school has a reliable broadband connection (20 Mbps download, 5 Mbps upload) but a limited IT budget. Data must be retained for at least 5 years and be accessible to teachers for remote grading.

Task: Write a short answer (150‑200 words) recommending either cloud storage, local storage, or a hybrid solution. Use the checklist above to justify your choice, mention at least two advantages and two disadvantages of the selected option, and include a simple cost estimate.

5. Summary (AO1‑AO3)

• Understand binary, hexadecimal, two’s‑complement and how they affect storage and processing.
• Know how data is transmitted, protected and error‑checked across networks.
• Recognise the roles of hardware components, OS functions and software development tools.
• Explain Internet fundamentals, digital currency basics and key cyber‑security concepts.
• Identify emerging technologies and their ethical implications.
• Design simple algorithms, draw flowcharts, write pseudocode, and use basic SQL.
• Compare cloud and local storage, calculate bandwidth, latency and cost, and evaluate which solution best fits a given scenario.

6. Suggested Diagrams for Revision

• Venn diagram – Cloud‑only benefits, Local‑only benefits, Overlap (security, cost‑effectiveness when managed well).
• Layered Internet model – Application, Transport, Network, Data Link, Physical.
• RAID configuration diagrams (RAID 1, RAID 5).
• Flowchart of a simple “search for a student record” algorithm.
• Block diagram of a sensor‑actuator‑controller loop in a robot.