Write algorithms using pseudocode or flowcharts

Cambridge IGCSE Computer Science (0478) – Complete Revision Notes

This single‑page resource covers all syllabus sections (1‑6 Computer Systems and 7‑10 Algorithms, Programming & Logic). It is designed for quick exam revision – clear headings, bullet points, tables, and exam‑style examples.

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1 – 6 Computer Systems

1.1 Data Representation

• Number systems – binary, octal, decimal, hexadecimal.
  • Conversion example: 45₁₀ = 101101₂ = 55₈ = 2D₁₆
• Signed numbers – two’s complement (invert bits + 1). Example: −13 (8‑bit) = 11110011₂
• Character encoding
  • ASCII – 7‑bit, e.g. A = 65₁₀ = 01000001₂
  • Unicode – 16‑bit (UTF‑16) or variable‑length (UTF‑8)
• Image representation
  • Pixel = colour depth × number of pixels.
  • Example: 640 × 480 image, 24‑bit colour → 640×480×24 = 7 372 800 bits ≈ 900 KB
• Sound representation
  • Sample rate × bit depth × channels × time.
  • Example: 44.1 kHz, 16‑bit stereo, 10 s → 44 100×16×2×10 = 14 112 000 bits ≈ 1.68 MB
• Colour models – RGB (8‑bit per channel), hexadecimal colour code (e.g. #FF00AA).
• Data compression
  • Lossless – e.g. ZIP, PNG.
  • Lossy – e.g. MP3, JPEG.
• Error‑detection & correction
  • Parity bit (even/odd).
  • Checksum, CRC.
• Simple encryption – Caesar shift, substitution, symmetric key (e.g. DES), public‑key basics.

1.2 Computer Hardware

• CPU – ALU, control unit, registers, cache (L1/L2), instruction set, clock speed.
• Memory hierarchy
  • Registers (nanoseconds), cache, primary RAM, secondary storage (HDD/SSD), tertiary (magnetic tape, cloud).
• Embedded systems – microcontrollers, real‑time control, sensors & actuators.
• I/O devices – input (keyboard, mouse, scanner), output (monitor, printer, speaker), storage (USB, CD‑ROM).
• Network hardware – NIC, router, switch, hub, modem, access point.
• Storage media characteristics
  • Volatile vs. non‑volatile, magnetic vs. solid‑state, capacity units (bit, byte, KiB, MiB, GiB).

1.3 Software

• System software – Operating System (OS) functions: process management, memory management, file system, device drivers.
• Application software – word processors, browsers, games, specialised apps.
• Utility programmes – antivirus, backup, compression, disk defragmenter.
• Programming‑language categories
  • Low‑level (machine, assembly) vs. high‑level (Python, Java).
  • Compiled (C, Pascal) vs. interpreted (BASIC, Python).
• Interrupts – hardware & software signals that temporarily suspend the CPU’s current task.

1.4 The Internet & Data Transmission

• Client/Server model – request/response paradigm.
• Addressing
  • MAC address (hardware, 48‑bit).
  • IP address – IPv4 (dotted decimal) e.g. 192.168.0.1, IPv6 (hex groups).
• Domain Name System (DNS) – translates URLs to IP addresses.
• Packet structure – header (source/destination IP, protocol, length, checksum) + data + footer (optional).
• Key protocols
  • HTTP / HTTPS (web), TCP (reliable), UDP (unreliable), FTP (file transfer), SMTP (email).
• Data transmission concepts
  • Bandwidth, latency, packet loss, error‑control (ARQ).

1.5 Cyber‑Security

• Common threats – virus, worm, Trojan, ransomware, phishing, DDoS, spyware.
• Vulnerabilities – weak passwords, unpatched software, social engineering.
• Security measures
  • Firewalls, antivirus/anti‑malware, encryption (AES, RSA), authentication (passwords, biometrics, 2‑FA), hashing (SHA‑256), digital signatures, PKI.
• Safe Internet use – strong passwords, regular updates, backup, avoid suspicious links, use HTTPS.

1.6 Emerging Technologies

• Artificial Intelligence & Machine Learning – supervised vs. unsupervised learning, neural networks (basic concept).
• Robotics – sensors, actuators, control algorithms (feedback loops).
• Internet of Things (IoT) – networked devices, data collection, cloud integration.
• Big Data – 3 Vs (volume, velocity, variety), simple analysis (sorting, counting).
• Blockchain & Digital Currency – distributed ledger, hashing, decentralised verification.
• Cloud Computing – SaaS, PaaS, IaaS, virtualisation.

1.7 Quick‑Reference Table – Symbols & Units

Symbol / Term	Meaning	Unit / Example
bit	binary digit (0 or 1)	1 bit = 0.125 bytes
byte	8 bits	ASCII character = 1 byte
KiB, MiB, GiB	2¹⁰, 2²⁰, 2³⁰ bytes	1 MiB = 1 048 576 bytes
MAC	Media Access Control address	48‑bit hardware identifier
IP	Internet Protocol address	IPv4: 4 × 8‑bit octets
TTL	Time‑to‑Live (packet hops)	Typical default = 64
CRC	Cyclic Redundancy Check	Used in Ethernet frames
ASCII	American Standard Code for Information Interchange	0‑127 decimal codes
Unicode	Universal character set	U+1F600 = 😀

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7 – 10 Algorithms, Programming & Logic

2.1 The Complete Problem‑Solving Cycle

1. Understand the problem – list inputs, required processing, expected outputs.
2. Analyse – decompose into sub‑tasks, choose efficient structures (loops, decisions, arrays).
3. Design – sketch a flowchart or write a pseudocode outline.
4. Write – produce the full algorithm using syllabus‑specific conventions.
5. Validate & Verify – ensure inputs are acceptable and the algorithm logically solves the problem.
6. Test – run with typical, edge‑case, and invalid data; record results in a trace table.
7. Refine – improve efficiency, readability, add comments.
8. Document – title, START … END, assumptions, and any limitations.

2.2 Pseudocode – Syllabus‑Specific Conventions

2.2.1 General Rules

• Plain English – no programming‑language syntax (no semicolons, braces, etc.).
• One statement per line.
• Indentation shows structure (4 spaces per block).
• All keywords in UPPER CASE.
• Optional line numbers: 1: INPUT n.
• Comments start with // and occupy the remainder of the line.

2.2.2 Mandatory Keywords

Keyword	Purpose
START / END	Marks the entry and exit points.
INPUT / OUTPUT	Read from or write to the user.
IF … THEN … ELSE … END IF	Two‑way decision.
CASE … OF … END CASE	Multiple‑choice decision (optional).
FOR … TO … STEP … END FOR	Count‑controlled loop.
WHILE … DO … END WHILE	Pre‑test condition loop.
REPEAT … UNTIL … END REPEAT	Post‑test condition loop.

2.2.3 Allowed Operators

Category	Operators
Arithmetic	+  −  *  /  DIV  MOD
Relational	=  ≠  <  ≤  >  ≥
Boolean	AND  OR  NOT

2.2.4 Variable‑Naming Conventions

• Meaningful, PascalCase (e.g., TotalScore).
• First character a letter; subsequent characters letters or digits.
• No underscores; case‑insensitive.
• Implicit declaration on first use.

2.2.5 Pseudocode Checklist (Exam Quick‑Review)

Item	✔
START and END present
All keywords in UPPER CASE
One statement per line
Correct indentation (4‑space blocks)
Only allowed operators used
Variable names follow conventions
Comments start with // (optional)
Loops and decisions closed correctly (END IF, END FOR, …)

2.3 Flowcharts – Standard Symbols & Layout Rules

Symbol	Name	Purpose
⧈ (rounded rectangle)	Start / End	Entry and exit points.
▭ (rectangle)	Process	Assignment, calculation, or any non‑decision operation.
▱ (parallelogram)	Input / Output	Read data or display results.
◇ (diamond)	Decision	Test a condition – two arrows labelled Yes / No.
⬥ (pentagon) – optional	Connector	Links separate parts of a large chart (labelled A, B, …).

Layout Guidelines

• Flow direction: top‑to‑bottom, left‑to‑right.
• One operation per process symbol.
• Arrows must be clear; avoid crossing lines where possible.
• Label decision arrows (Yes / No or True / False).
• Title above the chart (e.g., “Average of Positive Numbers”).

2.4 Validation, Verification & Testing

2.4.1 Validation (checking input)

• Range check – IF n < 1 THEN …
• Length check – for strings or lists.
• Type check – ensure numeric input.
• Format check – e.g., date DD/MM/YYYY.
• Checksum / check‑digit – e.g., ISBN validation.

2.4.2 Verification (does it do the right thing?)

• Use a trace table to record variable values after each step.
• Compare algorithm output with expected results for known inputs.

2.4.3 Sample Trace‑Table Template

Step	Variable(s)	Values after step
Initialisation	sum	0
Loop 1 (i=1)	i, sum	1, 0
Loop 2 (i=2)	i, sum	2, 2
…	…	…
After loop	sum	12 (for n=6)

2.4.4 Test‑Data Recommendations

• Typical case – e.g., n = 10.
• Edge cases – n = 1, n = 0, very large n.
• Invalid input – negative numbers, non‑numeric characters, out‑of‑range dates.

2.5 Core Programming Concepts (IGCSE Level)

• Variables & Data Types – integer, real, Boolean, character, string.
• Operators – arithmetic, relational, Boolean (as listed above).
• Control structures – IF…THEN, CASE, FOR, WHILE, REPEAT.
• Arrays
  • One‑dimensional only (e.g., scores[10]).
  • Index starts at 0 or 1 – be consistent.
  • Example: sum of array elements.
• File handling (basic) – OPEN, READ, WRITE, CLOSE (exam may ask to describe only).
• Databases (basic) – tables, records, fields, primary key, simple SELECT query description.
• Boolean logic & logic gates – AND, OR, NOT, NAND, NOR, XOR; truth tables useful for decision making.

Array Example – Find Maximum Value

// Find the maximum number in an array of 5 integers
START
    FOR i ← 1 TO 5 DO
        INPUT number
        numbers[i] ← number
    END FOR

    max ← numbers[1]
    FOR i ← 2 TO 5 DO
        IF numbers[i] > max THEN
            max ← numbers[i]
        END IF
    END FOR
    OUTPUT "Maximum =", max
END

File‑Handling Example (description only)

Open a text file “scores.txt” for reading, read each line into a variable score, add to a total, close the file, then output the average.

Database Example (description)

Table Students with fields StudentID, Name, Mark. To list students with marks ≥ 70: SELECT Name, Mark FROM Students WHERE Mark ≥ 70;

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Worked Example – Sum of All Even Numbers from 1 to n

Problem Statement

Read a **positive integer** n and output the sum of all even numbers between 1 and n inclusive.

Pseudocode Solution

// Sum of even numbers
START
    INPUT n
    // Validation
    IF n < 1 THEN
        OUTPUT "Error: n must be positive"
        END
    END IF

    sum ← 0
    FOR i ← 1 TO n DO
        IF i MOD 2 = 0 THEN
            sum ← sum + i
        END IF
    END FOR
    OUTPUT sum
END

Flowchart (text description – draw using symbols above)

1. Start (⧈)
2. Input n (▱)
3. Decision n < 1 ? (◇)
   • Yes → Output error (▱) → End.
   • No → Continue.
4. Process sum ← 0 (▭)
5. Loop (FOR) i = 1 … n (▭ with loop arrows)
   • Decision i MOD 2 = 0 ? (◇)
     • Yes → Process sum ← sum + i (▭)
     • No → Back to loop.
6. Output sum (▱)
7. End (⧈)

Sample Trace Table (n = 6)

i	i MOD 2	sum (after step)
1	1	0
2	0	2
3	1	2
4	0	6
5	1	6
6	0	12

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Quick‑Reference Summary (All Topics)

Area	Key Points
Data Representation	Binary/hex conversion, two’s complement, ASCII/Unicode, image & sound size formulas, compression, error‑detect, simple encryption.
Computer Hardware	CPU components, registers, cache, memory hierarchy, embedded systems, I/O devices, network hardware.
Software	OS functions, utility programmes, language categories, interrupts.
Internet & Transmission	Client/Server, MAC & IP, DNS, packet header fields, HTTP/HTTPS, TCP/UDP, FTP, SMTP.
Cyber‑Security	Threats (virus, ransomware, phishing, DDoS), safeguards (firewall, antivirus, encryption, hashing, 2‑FA).
Emerging Tech	AI/ML basics, robotics control loop, IoT data flow, big‑data 3 Vs, blockchain hashing, cloud service models.
Algorithms	Problem‑solving cycle, pseudocode conventions, flowchart symbols, validation, verification, testing, trace tables.
Programming Concepts	Variables, data types, operators, IF/CASE, loops, arrays (1‑D), file handling description, simple database query, Boolean logic.

Use this sheet to tick each syllabus requirement, practice the examples, and test yourself with the provided trace tables. Good luck with your IGCSE Computer Science exam!