IGCSE Computer Science (0478) – Complete Revision Notes
1. Introduction
These notes cover every high‑weight topic of the Cambridge IGCSE Computer Science syllabus, with a special focus on defining and using procedures and functions (with or without parameters). Use them to revise, practise exam questions and build a solid mental model of how hardware, software and data interact.
2. Data Representation
2.1 Number Systems
- Binary (base‑2) – each digit is a bit (0 or 1). 8‑bit byte = 2⁸ = 256 values.
- Denary (base‑10) – the system we use in everyday life.
- Hexadecimal (base‑16) – groups 4 bits; digits 0‑9 and A‑F.
- Two’s‑complement (signed integers)
- Positive numbers: normal binary.
- Negative number: invert all bits, add 1.
- Range for 8‑bit: –128 … +127.
- Logical shifts
- Left shift (<<) – moves bits towards more significant positions, inserting 0s on the right; multiplies by 2ⁿ.
- Right shift (>> ) – moves bits towards less significant positions, inserting 0s on the left; divides by 2ⁿ (floor).
Worked Example – Binary ↔ Decimal ↔ Hex
Decimal 156 → binary:
156 ÷ 2 = 78 r0
78 ÷ 2 = 39 r0
39 ÷ 2 = 19 r1
19 ÷ 2 = 9 r1
9 ÷ 2 = 4 r1
4 ÷ 2 = 2 r0
2 ÷ 2 = 1 r0
1 ÷ 2 = 0 r1
Binary = 10011100₂
Hex = 9C₁₆
2.2 Text, Sound & Images
| Domain | Key Concepts | Typical Exam Task |
|---|
| ASCII / Unicode |
ASCII = 7‑bit (0‑127). Unicode extends to 16‑bit/32‑bit, allowing many world scripts. |
Give the binary code for ‘A’ (ASCII) and for ‘Ω’ (Unicode, 16‑bit). |
| Image resolution & colour depth |
Pixels = width × height.
File size = pixels × colour‑depth ÷ 8 (bytes). |
Calculate the size of a 640 × 480 image at 24‑bit colour. |
| Sound sampling & resolution |
File size = sampling‑rate × bit‑depth × duration ÷ 8. |
Size of a 3‑minute mono audio at 44.1 kHz, 16‑bit. |
| Compression |
Lossless – original data can be perfectly restored (e.g., ZIP).
Lossy – some data discarded for smaller size (e.g., MP3, JPEG). |
Explain why a JPEG image may look fuzzy after repeated saves. |
Worked Example – Image File Size
Image: 640 × 480 pixels, colour depth = 24 bits.
pixels = 640 × 480 = 307 200
bits = 307 200 × 24 = 7 372 800
bytes = 7 372 800 ÷ 8 = 921 600 B ≈ 900 KB
Worked Example – Sound File Size
Audio: 3 min = 180 s, mono, 44.1 kHz, 16‑bit.
bits = 44 100 samples/s × 16 bits × 180 s = 127 008 000 bits
bytes = 127 008 000 ÷ 8 = 15 876 000 B ≈ 15.2 MB
3. Data Transmission
3.1 Packet Structure & Switching
- Header – source/destination address, control information.
- Payload – the actual data being transferred.
- Trailer – error‑checking bits (e.g., CRC).
- Circuit‑switched – a dedicated path is established for the whole session (e.g., old telephone networks).
- Packet‑switched – data is broken into packets; each packet may travel a different route via routers.
- Routers examine the destination address in each packet’s header and forward it toward the next hop.
- Packet loss / re‑ordering can occur on congested networks; protocols such as TCP detect loss and request retransmission.
3.2 Error‑Detection Methods
| Method | How it works | Typical use |
|---|
| Parity bit |
Even or odd count of 1‑bits; a single‑bit error flips the parity. |
Simple serial links. |
| Checksum |
Add all data bytes, keep low‑order byte; complement may be sent. |
IP, UDP. |
| CRC (Cyclic Redundancy Check) |
Polynomial division on the bit stream; remainder is appended. |
Ethernet, storage devices – detects burst errors. |
| Check digit (mod‑10) |
Sum of digits modulo 10; used for barcodes, ISBN. |
Manual data entry validation. |
| ARQ (Automatic Repeat Request) |
Receiver asks sender to resend a packet if error detected. |
TCP reliability. |
3.3 Encryption (Symmetric & Asymmetric)
- Symmetric encryption – same secret key for encryption and decryption.
- Example: Caesar cipher (shift each letter by 3).
Plaintext “HELLO” → Ciphertext “KHOOR”.
- Asymmetric encryption – a pair of keys: public (encrypt) and private (decrypt).
- Example: “Alice publishes a public key; Bob encrypts a message with it; only Alice’s private key can decrypt.”
4. Hardware
4.1 Computer Architecture
- Von Neumann architecture – single memory for data & instructions.
- Fetch‑Decode‑Execute (FDE) cycle
- Fetch the next instruction from memory.
- Decode the instruction to determine the operation.
- Execute – ALU performs the operation or transfers data.
- CPU components
- Control Unit – directs the FDE cycle.
- ALU – arithmetic and logical operations.
- Registers – small, fast storage inside the CPU.
- Cache – fast memory close to the CPU; a cache hit reduces access time dramatically.
- Multi‑core – each core can execute its own instruction stream, improving parallel performance.
- Clock speed – measured in MHz or GHz; determines how many cycles per second.
- Instruction set – the set of operations a CPU understands (e.g., RISC vs. CISC).
4.2 Input/Output Devices & Sensors/Actuators
- Input devices – keyboard, mouse, scanner, microphone, touch screen.
- Sensors – convert a physical quantity to an electrical signal (e.g., temperature sensor, light sensor).
- Actuators – convert an electrical signal into physical action (e.g., motor, speaker, LED).
4.3 Data Storage
| Type | Characteristics |
|---|
| Primary (volatile) |
RAM – fast, loses data when power is removed. |
| Secondary (non‑volatile) |
Magnetic (HDD), Optical (CD/DVD), Flash (SSD, USB). |
| Virtual memory |
Part of secondary storage used as an extension of RAM; managed by the OS. |
| Cloud storage |
Data stored on remote servers accessed via the Internet. |
Advantages & Disadvantages of Cloud vs. Local Storage
- Advantages of cloud
- Access from any device with Internet.
- Automatic backup and scalability.
- Reduced need for local hardware.
- Disadvantages of cloud
- Requires reliable Internet connection.
- Potential security & privacy concerns.
- Ongoing subscription costs.
- Advantages of local storage
- Fast access, no Internet needed.
- Full control over data security.
- Disadvantages of local storage
- Risk of loss from hardware failure.
- Limited capacity unless upgraded.
4.4 Network Hardware
| Device | Function |
|---|
| NIC (Network Interface Card) | Provides a physical connection to a network; handles framing. |
| Hub | Repeats incoming signals to all ports; no intelligence, can cause collisions. |
| Switch | Forwards frames only to the destination port using MAC addresses; reduces collisions. |
| Router | Routes packets between different networks; uses IP addresses. |
| Firewall | Filters traffic based on rules; can be hardware or software. |
5. Software
- System software: Operating system, device drivers, utility programs.
- Application software: Word processors, browsers, games, database systems.
- Operating system functions
- Resource management (CPU, memory, I/O).
- File system control.
- Multitasking & process scheduling.
- Security & user authentication.
- Interrupts
- Hardware interrupt – generated by an external device (e.g., keyboard press).
- Software interrupt – generated by a program (e.g., system call).
- Language levels: Machine → Assembly → High‑level → Natural‑language.
- Compilers vs. Interpreters
- Compiler – translates the whole program to machine code before execution.
- Interpreter – translates and executes line‑by‑line at run‑time.
- Integrated Development Environment (IDE) – editor, debugger, auto‑complete, version‑control integration.
Match‑the‑Definition
| Term | Definition |
|---|
| Driver | Software that allows the OS to communicate with hardware. |
| Multitasking | Running more than one process at the same time. |
| Garbage collection | Automatic reclamation of memory that is no longer referenced. |
6. Internet & Its Uses
- Internet vs. World Wide Web – Internet = global network of networks; WWW = collection of web pages accessed via HTTP/HTTPS.
- URL structure –
protocol://domain[:port]/path?query#fragment.
- HTTP/HTTPS – request‑response protocol; HTTPS adds TLS encryption.
- Cookies – small pieces of data stored on the client to maintain state.
- Digital currency (e.g., Bitcoin) – uses cryptographic hashing and a public ledger (blockchain).
- Cyber‑security basics
- Phishing – deceptive e‑mail or website.
- Malware – viruses, worms, ransomware.
- Firewalls – filter incoming/outgoing traffic.
Web‑Request Flow (simplified)
Client → HTTP GET → Router → Server
Server processes request → HTTP RESPONSE → Router → Client → Browser renders page
7. Automated & Emerging Technologies
- Sensors & Actuators – convert physical quantities to digital signals and vice‑versa.
- Robotics – integration of sensors, actuators, control software; often driven by a microcontroller.
- Artificial Intelligence
- Expert system – rule‑based decision making.
- Machine learning – algorithms improve from data (e.g., linear regression, decision trees).
Rule‑Based AI Example (Pseudocode)
PROCEDURE DiagnosePlant(temp, wilting)
IF temp > 30 AND wilting = TRUE THEN
RETURN "Water stress"
ELSE IF temp < 10 THEN
RETURN "Cold stress"
ELSE
RETURN "Healthy"
END IF
END PROCEDURE
8. Algorithm Design & Problem Solving
- Problem‑solving cycle (PDLC): Analyse → Design → Implement → Test → Evaluate.
- Decomposition – break a problem into smaller sub‑problems.
- Flowcharts – standard symbols: process, decision, input/output, loop.
- Validation & Verification
- Validation – does the program meet user needs?
- Verification – does the code do what the design specifies?
- Trace tables – track variable values step‑by‑step.
Linear Search – Flowchart to Pseudocode
PROCEDURE LinearSearch(list, key) // list = array, key = value to find
index ← 0
WHILE index < LENGTH(list) DO
IF list[index] = key THEN
RETURN index // found
END IF
index ← index + 1
END WHILE
RETURN -1 // not found
END PROCEDURE
9. Core Programming Concepts
| Concept | What you must know |
|---|
| Variables & Data Types |
Integer, Real, Boolean, Character, String, Array. Scope (local vs. global). |
| Input / Output |
READ/INPUT, PRINT/OUTPUT statements; formatting of numbers and strings. |
| Selection |
If‑Then‑Else, nested if, (optional) switch‑case. |
| Iteration |
FOR, WHILE, REPEAT‑UNTIL loops; loop counters and termination conditions. |
| Arrays |
One‑dimensional; indexing starts at 0 or 1 (specify). Bounds checking. |
| File handling |
OPEN, READ, WRITE, CLOSE; text vs. binary files; sequential access. |
Mini‑Project – Read, Sort & Write Scores
PROCEDURE Main()
OPEN "scores.txt" FOR READ AS inFile
scores ← []
WHILE NOT EOF(inFile) DO
line ← READLINE(inFile)
scores ← scores + [INTEGER(line)]
END WHILE
CLOSE inFile
CALL BubbleSort(scores) // procedure – no return value
OPEN "sorted.txt" FOR WRITE AS outFile
FOR i FROM 0 TO LENGTH(scores)-1 DO
WRITELINE(outFile, STRING(scores[i]))
END FOR
CLOSE outFile
END PROCEDURE
PROCEDURE BubbleSort(arr)
n ← LENGTH(arr)
REPEAT
swapped ← FALSE
FOR i FROM 0 TO n-2 DO
IF arr[i] > arr[i+1] THEN
temp ← arr[i]
arr[i] ← arr[i+1]
arr[i+1] ← temp
swapped ← TRUE
END IF
END FOR
n ← n - 1
UNTIL NOT swapped
END PROCEDURE
CALL Main()
10. Procedures and Functions (with or without Parameters)
10.1 Definitions
- Procedure – a named block of code that performs an action. May accept parameters but does not return a value. Called with
CALL (or simply the name in some languages).
- Function – a named block that performs a calculation and returns exactly one value. May also accept parameters. The returned value is assigned with the assignment operator (
← or =).
10.2 Key Differences
| Aspect | Procedure | Function |
|---|
| Purpose | Carry out a task (e.g., display, modify data) | Compute and return a value |
| Return value | None (uses CALL) | Single value (assigned to a variable) |
| Typical use | I/O, updating structures, calling other sub‑programs | Mathematical calculations, data conversion |
| Invocation syntax | CALL ProcName(arg1, arg2) | result ← FuncName(arg1, arg2) |
| Scope of parameters | Passed by value (original unchanged) or by reference (original can be changed). | Usually passed by value; the function cannot alter the caller’s variables unless reference is allowed. |
10.3 Parameter Types
- No parameters – useful for actions that always behave the same (e.g., display a welcome message).
- Input parameters – values supplied by the caller; read‑only inside the sub‑program.
- Output parameters – variables whose values are set by the sub‑program and returned to the caller (common in some languages).
- Input‑output parameters – both read and modified by the sub‑program.
10.4 Example – Procedure without Parameters
PROCEDURE ShowHeader()
PRINT "=== Student Scores ==="
END PROCEDURE
CALL ShowHeader()
10.5 Example – Procedure with Parameters (by value)
PROCEDURE Increment(val)
val ← val + 1 // local copy only
END PROCEDURE
x ← 5
CALL Increment(x)
PRINT x // still 5
10.6 Example – Procedure with Parameters (by reference)
PROCEDURE IncrementRef(REF val)
val ← val + 1 // modifies caller's variable
END PROCEDURE
x ← 5
CALL IncrementRef(x)
PRINT x // now 6
10.7 Example – Function without Parameters
FUNCTION GetRandomNumber()
RETURN 42 // placeholder for a real RNG
END FUNCTION
num ← GetRandomNumber()
PRINT num
10.8 Example – Function with Input Parameters
FUNCTION AreaRectangle(width, height)
RETURN width * height
END FUNCTION
area ← AreaRectangle(7, 3)
PRINT "Area =", area
10.9 Example – Function Returning a Value Used Directly
PRINT "The maximum is", MAX(12, 27) // MAX is a built‑in function
10.10 Recursion (Function calling itself)
FUNCTION Factorial(n)
IF n = 0 THEN
RETURN 1
ELSE
RETURN n * Factorial(n-1)
END IF
END FUNCTION
PRINT Factorial(5) // outputs 120
10.11 Common Pitfalls for the Exam
- Confusing procedure with function – remember only functions return a value.
- Omitting parentheses when a function has no parameters (e.g.,
NOW()).
- Mixing up call syntax:
CALL for procedures, assignment for functions.
- For parameters passed by reference, the original variable is changed; this is often tested.
- Recursive functions must have a clear base case to avoid infinite recursion.
10.12 Quick Revision Checklist
- Can you write the syntax for a procedure and a function?
- Do you know the difference between input, output and input‑output parameters?
- Can you explain why a function must return exactly one value?
- Are you comfortable tracing a call where a procedure modifies a variable by reference?
- Can you recognise a recursive function and identify its base case?
11. Summary of High‑Weight Topics for the Exam
- Number systems, logical shift, two’s‑complement.
- ASCII vs. Unicode, image & sound file‑size calculations, compression.
- Packet structure, routers, packet loss, CRC, ARQ.
- Symmetric & asymmetric encryption (simple examples).
- Von Neumann architecture, FDE cycle, cache, multi‑core, instruction set.
- Sensors & actuators, cloud vs. local storage.
- Network devices – NIC, hub, switch, router, firewall.
- Procedures vs. functions, parameter passing, recursion.
- Algorithm design – flowcharts, trace tables, validation/verification.
Use these notes to practise past‑paper questions, write your own pseudocode, and test yourself with the quick‑check questions at the end of each section. Good luck!