Cambridge IGCSE 0478 Computer Science – Syllabus Notes
1. Data Representation
1.1 Number Systems
- Binary (base‑2) – digits 0 and 1. Used internally by all digital devices.
- Octal (base‑8) – digits 0‑7. Often used as a shorthand for groups of three binary bits.
- Decimal (base‑10) – digits 0‑9. The system humans use.
- Hexadecimal (base‑16) – digits 0‑9 and A‑F. Convenient for representing groups of four binary bits.
| Decimal | Binary | Octal | Hexadecimal |
|---|
| 0 | 0000 | 0 | 0 |
| 13 | 1101 | 15 | D |
| 255 | 11111111 | 377 | FF |
| 1023 | 1111111111 | 1777 | 3FF |
1.2 Conversions (quick methods)
- Decimal → Binary: divide by 2, record remainders (bottom‑up).
- Binary → Hexadecimal: group bits in fours from the right, replace each group with its hex digit.
- Hexadecimal → Decimal: multiply each digit by 16ⁿ (n = position from right, starting at 0) and add.
1.3 Two’s‑Complement (signed integers)
- Write the absolute value in binary.
- Invert all bits (1 → 0, 0 → 1).
- Add 1 to the inverted result.
Example: Represent –13 in 8‑bit two’s‑complement.
13 → 00001101
invert → 11110010
add 1 → 11110011 (‑13)
1.4 Text Representation
- ASCII – 7‑bit code, 128 characters (e.g., ‘A’ = 01000001₂ = 41₁₆).
- Unicode (UTF‑8) – variable‑length, can represent all world scripts; compatible with ASCII for the first 128 characters.
1.5 Images, Sound & Video
- Image colour depth: bits per pixel (bpp). Example: 24‑bit colour = 8 bits for each of R, G, B → 16 777 216 colours.
- Resolution: number of pixels horizontally × vertically (e.g., 1920 × 1080).
- Audio sampling: Sample rate (e.g., 44.1 kHz) × bit depth (e.g., 16‑bit) × channels (mono/stereo). Data rate = rate × depth × channels.
- Compression:
- Lossless – no information lost (e.g., PNG, ZIP). Good for text and some images.
- Lossy – discards less‑noticeable data (e.g., JPEG, MP3). Higher compression but reduced quality.
2. Data Transmission
2.1 Transmission Types
2.1.1 Serial vs Parallel
- Serial – bits sent one after another on a single channel.
- Parallel – multiple bits sent simultaneously on separate channels (usually 4, 8, 16 or 32).
| Aspect | Serial | Parallel |
|---|
| Typical bit‑rate (short‑range) | USB 2.0 = 480 Mbit/s, SATA III = 6 Gbit/s | PCI‑Express x8 ≈ 8 Gbit/s (8 lanes) |
| Distance limitation | Metres to kilometres (e.g., Ethernet over fibre) | Only a few metres (cable bulk, skew) |
| Signal integrity | Low crosstalk, easier shielding | Higher crosstalk, requires matched length & shielding |
| Cost & complexity | Fewer conductors, cheaper connectors | More conductors, larger connectors, higher cost |
| Typical use | USB, Ethernet, SATA, PCIe, UART | Legacy printer ports, internal CPU‑memory buses |
2.1.2 Synchronous vs Asynchronous
- Synchronous – a shared clock (or embedded clock) coordinates sender and receiver.
- Asynchronous – each character/byte is framed by start and stop bits; no shared clock.
| Aspect | Synchronous | Asynchronous |
|---|
| Clocking | Dedicated clock line or clock recovered from data | Start/stop‑bit framing only |
| Throughput efficiency | Very high – little overhead | Lower – start/stop bits add 20‑30 % overhead |
| Hardware complexity | More complex (PLL, clock recovery) | Simple UART/USART |
| Typical use | Ethernet, USB, SPI, I²C, fibre‑optic links | RS‑232 ports, simple micro‑controller links |
2.1.3 Duplex Modes
- Simplex – one‑way only (e.g., TV broadcast).
- Half‑duplex – two‑way but not simultaneous (e.g., walkie‑talkie, CAN bus).
- Full‑duplex – simultaneous two‑way transmission (e.g., telephone, Ethernet full‑duplex).
2.2 Packet Structure & Switching
- A packet consists of a header, payload (user data) and a trailer (error‑check).
- Typical header fields (simplified for IGCSE):
- Source address
- Destination address
- Length/size
- Protocol identifier (e.g., TCP, UDP)
- Checksum / CRC
- Packet switching – large messages are divided into packets; each packet may travel a different route to the destination, where they are re‑assembled.
- Contrast with circuit switching – a dedicated path is reserved for the whole session (e.g., traditional telephone networks).
2.3 Transmission Methods & Common Interfaces
| Method / Interface | Transmission type | Typical speed | Typical use |
|---|
| USB 1.1 | Serial, asynchronous, half‑duplex | 12 Mbit/s | Keyboards, mice, flash drives |
| USB 2.0 | Serial, asynchronous, full‑duplex | 480 Mbit/s | External hard disks, webcams |
| USB 3.0 (SuperSpeed) | Serial, synchronous, full‑duplex | 5 Gbit/s | High‑speed storage, video capture |
| Ethernet (10/100/1000 Mbps) | Serial, synchronous, full‑duplex | 10 Mb/s – 1 Gb/s (10 Gb/s newer) | LANs, internet back‑haul |
| Wi‑Fi (802.11ac) | Serial, asynchronous, half‑/full‑duplex (CSMA/CA) | Up to 1.3 Gb/s | Wireless LANs, mobile devices |
| Bluetooth 4.0 (LE) | Serial, asynchronous, half‑duplex | 1 Mbps (LE), 3 Mbps (Classic) | Wearables, IoT sensors |
| RS‑232 | Serial, asynchronous, simplex or half‑duplex | 115.2 kbps (typical) | Legacy equipment, industrial control |
| SPI (Serial Peripheral Interface) | Serial, synchronous, full‑duplex | Up to 50 Mbps (depends on MCU) | Microcontroller‑to‑peripheral links |
2.4 USB Packet Example (simplified)
| PID | ADDR | ENDP | CRC5 | DATA... | CRC16 |
|-----|------|------|------|---------|-------|
8b 7b 4b 5b n bytes 16b
PID = Packet Identifier (e.g., Token, Data, Handshake). The CRC fields provide error detection.
2.5 Error‑Detection, Correction & Encryption
2.5.1 Error‑Detection Methods
- Parity bit – adds one bit to make the total number of 1’s even (even parity) or odd (odd parity). Detects single‑bit errors.
- Checksum – adds the binary sum of data bytes (often 1’s‑complement). Used in TCP, UDP, IP.
- Cyclic Redundancy Check (CRC) – polynomial division generates a remainder (the CRC). Detects burst errors; common in Ethernet and storage.
- Automatic Repeat Request (ARQ) – combines detection with retransmission.
- Stop‑and‑Wait
- Go‑Back‑N
- Selective Repeat
Forms the basis of TCP reliability.
- Check digit – decimal digit calculated from other digits (e.g., ISBN, credit‑card numbers) to catch transcription errors.
2.5.2 Encryption (Data Confidentiality)
- Symmetric encryption – same key for encryption and decryption (e.g., AES, DES). Fast; suitable for bulk data.
- Asymmetric encryption – public‑key/private‑key pair (e.g., RSA, ECC). Solves key‑distribution problem; used for secure key exchange and digital signatures.
- Key points for IGCSE:
- Encryption scrambles data so only the correct key can read it.
- Symmetric is faster but requires a secure way to share the key.
- Asymmetric is slower but allows secure key exchange without a prior secret.
3. Computer Hardware (IGCSE Syllabus Block 3)
3.1 CPU Architecture & the Fetch‑Decode‑Execute Cycle
- Fetch – the CPU reads the next instruction from memory (address held in the Program Counter).
- Decode – the instruction register (IR) sends the opcode to the control unit, which determines what operation is required.
- Execute – the arithmetic‑logic unit (ALU) performs the operation; results may be stored in registers or written back to memory.
Typical registers involved: Program Counter (PC), Instruction Register (IR), Accumulator (ACC), General‑purpose registers (R0‑R7).
3.2 Core, Cache & Buses
- Core – an independent processing unit; modern CPUs have multiple cores for parallel execution.
- Cache – small, fast memory close to the CPU (L1, L2, L3). Stores frequently accessed data to reduce main‑memory accesses.
- System bus – carries data, addresses and control signals between CPU, memory and I/O devices.
3.3 Instruction Set & Machine Language
- Each CPU has a defined instruction set (e.g., ADD, SUB, LOAD, STORE, JUMP).
- Instructions are encoded as binary machine code. Assemblers translate symbolic assembly language into machine code.
3.4 Embedded Systems
- Specialised computers designed for a specific function (e.g., microwave controller, digital watch).
- Often use microcontrollers with integrated CPU, memory, I/O ports and sometimes built‑in communication interfaces (UART, SPI, I²C).
4. Software (IGCSE Syllabus Block 4)
4.1 System Software vs Application Software
- System software – manages hardware resources and provides a platform for other software (e.g., Operating System, device drivers).
- Application software – performs user‑oriented tasks (e.g., word processor, web browser, games).
4.2 Operating System Functions
- Resource management (CPU scheduling, memory allocation).
- File management (creation, deletion, organisation).
- Device control (through drivers).
- Security & user accounts.
- Providing a user interface (CLI or GUI).
4.3 Interrupts
- Hardware interrupt – generated by an external device (e.g., keyboard press) to gain immediate CPU attention.
- Software interrupt – generated by a program (e.g., system call) to request OS services.
- Interrupt handling allows multitasking and responsive I/O.
4.4 Levels of Programming Language
| Level | Typical Use | Characteristics |
|---|
| Machine language | Direct CPU control | Binary, processor‑specific |
| Assembly language | Low‑level optimisation | Mnemonic symbols, one‑to‑one with machine code |
| High‑level language | General purpose programming | English‑like syntax, portable, compiled or interpreted |
4.5 Compilers vs Interpreters
- Compiler – translates the whole source program into machine code before execution (e.g., C, Java – compiled to bytecode).
- Interpreter – reads and executes source statements one at a time (e.g., Python, BASIC).
- Compiled programs run faster; interpreted programs are easier to test and modify.
4.6 Integrated Development Environments (IDEs)
- Combine a code editor, compiler/interpreter, debugger and often a visual designer.
- Examples: Eclipse, Visual Studio, PyCharm.
5. Network Hardware & Addressing
5.1 Core Devices
| Hardware | Function | Key Identifier |
|---|
| Network Interface Card (NIC) | Provides physical and data‑link connectivity | MAC address (48‑bit) |
| Router | Forwards packets between different networks; performs IP routing | IP address (IPv4/IPv6) |
| Switch | Connects multiple devices in a LAN; forwards frames based on MAC address | MAC address table |
| Modem | Modulates/demodulates signals for transmission over telephone lines or cable | Public IP address (usually assigned by ISP) |
| Access Point (AP) | Provides wireless connectivity to a wired LAN | MAC address; SSID (network name) |
5.2 IP Addressing (IPv4)
- Four octets (e.g., 192.168.1.10). Each octet is 0‑255.
- Subnet mask – separates network and host portions (e.g., 255.255.255.0).
- From the mask you can determine:
- Network address (AND operation between IP and mask).
- Broadcast address (host bits set to 1).
- Number of usable host addresses = 2ⁿ − 2 (n = host bits).
6. Internet & Web Basics (IGCSE Syllabus Block 5)
6.1 Internet vs World Wide Web
- Internet – global network of interconnected computers and devices; provides services such as email, file transfer, remote login.
- World Wide Web (WWW) – a service that runs on the Internet; uses HTTP/HTTPS to retrieve and display hyper‑text documents (web pages).
6.2 URL (Uniform Resource Locator) Breakdown
https://www.example.com:443/path/page.html?search=term#section2
│ │ │ │ │ │ │ │
│ │ │ │ │ │ │ └─ Fragment identifier
│ │ │ │ │ │ └─ Query string
│ │ │ │ │ └─ Path to resource
│ │ │ │ └─ Port number (optional)
│ │ │ └─ Domain name (host)
│ │ └─ Sub‑domain (www)
│ └─ Protocol (https)
└─ Scheme separator (://)
6.3 HTTP & HTTPS
- HTTP – Hyper‑Text Transfer Protocol; stateless request/response model.
- HTTPS – HTTP over TLS/SSL; provides encryption, authentication and data integrity.
6.4 DNS (Domain Name System)
- Translates human‑readable domain names (e.g., www.example.com) into IP addresses.
- Hierarchical structure: root → top‑level domain (TLD) → second‑level domain → sub‑domains.
6.5 Cookies
- Small pieces of data stored by a web browser on the client side.
- Used to maintain state (e.g., login sessions, shopping‑cart contents).
- Can be session cookies (deleted when the browser closes) or persistent cookies (saved for a set period).
6.6 Digital Currency & Blockchain (basic IGCSE overview)
- Digital currency – electronic money that exists only in digital form (e.g., Bitcoin).
- Blockchain – a distributed ledger where each block contains a list of transactions and a hash of the previous block, providing tamper‑evidence.
- Key ideas for the exam:
- Decentralised verification (no single authority).
- Use of cryptographic hash functions to link blocks.
7. Cyber‑Security Threats & Counter‑Measures (IGCSE 5.3)
| Threat | What it does | Typical Counter‑measure |
|---|
| Malware (virus, worm, Trojan) | Unauthorised code execution, data loss or theft | Antivirus software, regular updates, safe‑download practices |
| Phishing | Deceptive messages to obtain passwords or personal data | User education, email filtering, two‑factor authentication |
| Denial‑of‑Service (DoS) / DDoS | Overloads a service, making it unavailable | Firewalls, rate limiting, traffic‑scrubbing services |
| Man‑in‑the‑Middle (MitM) | Intercepts and possibly alters communication | HTTPS/TLS, VPNs, certificate verification |
| Unauthorised access | Intruder gains control of a system | Strong passwords, account lockout, role‑based access control |
8. Emerging & Automated Technologies (IGCSE 6.1‑6.3)
- Sensors – convert physical phenomena (temperature, light, motion) into electrical signals. Common interfaces: I²C, SPI, UART.
- Actuators – convert electrical signals into motion or force (motors, servos, solenoids).
- Robotics – integration of sensors, actuators and control algorithms to perform tasks autonomously.
- Artificial Intelligence (AI) basics – rule‑based systems and simple machine‑learning concepts (e.g., decision trees) used in smart devices.
- Internet of Things (IoT) – network of sensors/actuators communicating via low‑power wireless methods (BLE, LoRaWAN) and often using cloud services for data storage and analysis.
9. Scenario Matching – Choosing the Most Suitable Transmission Method
- Transferring a large file between two servers in the same building.
- Suitable method: Gigabit Ethernet (or 10 GbE) over Cat‑6 cable.
- Reasoning: Serial, synchronous, full‑duplex transmission gives high throughput, low latency and reliable error‑checking (CRC). Parallel cabling is unnecessary for short distances, and wireless would add interference and lower speed.
- Connecting a microcontroller to a temperature sensor on a PCB.
- Suitable method: SPI (Serial Peripheral Interface).
- Reasoning: Synchronous, full‑duplex, few pins, high data‑rate for short distances, and deterministic timing – ideal for on‑board sensor communication.
- Providing internet access to mobile laptops across a university campus.
- Suitable method: Wi‑Fi (802.11ac or newer) using multiple access points.
- Reasoning: Wireless, supports many simultaneous users, sufficient bandwidth for typical office tasks, and can cover large areas with strategically placed APs.
- Sending a short command from a wearable fitness tracker to a smartphone.
- Suitable method: Bluetooth Low Energy (BLE).
- Reasoning: Asynchronous, half‑duplex, very low power consumption, range up to 100 m, and adequate data‑rate for small packets.
- Streaming high‑definition video from a home media server to a TV over a 10‑metre link.
- Suitable method: HDMI (parallel, synchronous, full‑duplex) or HDMI‑over‑Ethernet (HDBaseT) if longer runs are needed.
- Reasoning: Parallel transmission provides very high bandwidth (up to 48 Gbps for HDMI 2.1) with minimal latency, essential for HD video. For distances beyond 5 m, HDBaseT uses Cat‑6 cable while preserving video quality.
- Remote monitoring of a weather station in a rural area with no wired infrastructure.
- Suitable method: Satellite communication (e.g., LEO broadband) or cellular (LTE) if coverage exists.
- Reasoning: Wireless, long‑range, can transmit small sensor packets. Satellite offers global coverage; LTE provides lower latency where a cell tower is reachable.
- Real‑time control of industrial machinery on a factory floor.
- Suitable method: Ethernet/IP or PROFINET over shielded twisted‑pair (STP) with full‑duplex, synchronous transmission.
- Reasoning: High reliability, deterministic timing, built‑in error detection (CRC), and immunity to EMI provided by shielding.