💻 GCSE Computer Science

Programming Fundamentals

Variables & Data Types

• Integer: whole number (e.g. 5, -3)
• Float/Real: decimal number (e.g. 3.14)
• String: sequence of characters (e.g. "hello")
• Boolean: True or False
• Character: single character (e.g. 'A')
• Casting: converting between types — e.g. int("5") converts string to integer

Sequence, Selection & Iteration

• Sequence: instructions run one after another, top to bottom
• Selection (IF statements): executes different code based on a condition
• Iteration (loops): FOR loops (known number of times), WHILE loops (condition-based)

Subroutines

• Procedure: a named block of code that performs a task; does not return a value
• Function: like a procedure but returns a value
• Parameters: variables passed into a subroutine
• Benefits: reduces repetition, easier to test and debug, improves readability

Data Structures

Arrays (Lists)

• A collection of values stored under one variable name, accessed by index
• Most languages are zero-indexed (first item = index 0)
• 1D array: a single row (e.g. scores[0], scores[1])
• 2D array: rows and columns (e.g. grid[row][col])

Records (Structs)

• A collection of related fields, each with a name and data type
• e.g. a student record might have name (string), age (integer), grade (char)

File Handling

• Open, read, write, and close files in programs
• Text files: store data as plain text; CSV files: comma-separated values

Algorithms

Searching Algorithms

• Linear search: check each item one by one — works on unsorted lists; O(n)
• Binary search: repeatedly halve the search space — requires sorted list; O(log n) — much faster for large datasets

Sorting Algorithms

• Bubble sort: repeatedly swap adjacent items if in wrong order; simple but slow; O(n²)
• Merge sort: divide list in half repeatedly, then merge in order; efficient; O(n log n)
• Insertion sort: build sorted list one item at a time; efficient for small/nearly sorted lists

Computational Thinking

• Decomposition: breaking a problem into smaller parts
• Abstraction: removing unnecessary detail to focus on what's important
• Algorithmic thinking: creating step-by-step instructions to solve a problem
• Pattern recognition: identifying similarities between problems

Trace Tables

Used to track variable values as an algorithm runs — each column is a variable, each row is an execution step. Essential for dry-running code in exams.

Hardware & Software

CPU Components

• ALU (Arithmetic Logic Unit): performs calculations and logical operations
• CU (Control Unit): directs data flow and manages the fetch-decode-execute cycle
• Registers: tiny, fast storage locations within the CPU (e.g. Program Counter, Accumulator)
• Cache: fast memory between CPU and RAM — stores frequently used data
• Clock: controls the speed of the CPU in GHz; higher clock speed = more instructions per second

Von Neumann Architecture

• Fetch: PC holds address of next instruction; fetched from memory into MAR then MDR
• Decode: CU interprets the instruction
• Execute: ALU performs the operation

CPU Performance Factors

• Clock speed (GHz): higher = faster processing
• Number of cores: multi-core CPUs can handle multiple tasks simultaneously
• Cache size: larger cache = fewer slow RAM accesses

Types of Software

• System software: OS, utility programs, drivers
• Application software: word processors, browsers, games
• OS functions: file management, memory management, user interface, hardware communication

Data Representation

Binary & Number Systems

• Binary (base 2): uses 0 and 1; 1 bit = 0 or 1; 8 bits = 1 byte
• Denary (base 10): our everyday number system
• Hexadecimal (base 16): 0–9, A–F; 1 hex digit = 4 bits (a nibble)

• To convert denary → binary: repeatedly divide by 2, note remainders (bottom up)
• To convert binary → hex: split into groups of 4 bits, convert each group
• Adding binary: 1+1 = 10 (carry the 1); overflow occurs when the result exceeds available bits

Character Encoding

• ASCII: 7-bit encoding; 128 characters; only covers English/basic symbols
• Unicode: 16-bit or 32-bit encoding; supports over 100,000 characters worldwide

Images

• Each pixel is stored as a binary value representing its colour
• Colour depth: number of bits per pixel (e.g. 8-bit = 256 colours)
• Resolution: number of pixels — higher resolution = better quality but larger file size
• File size (bits) = image width × height × colour depth

Sound

• Analogue sound converted to digital via sampling
• Sample rate: number of samples per second (Hz) — higher = better quality
• Bit depth: bits per sample — higher = more accurate amplitude values
• File size (bits) = sample rate × bit depth × duration (seconds)

Compression

• Lossy: permanently removes data; smaller file; quality loss (e.g. MP3, JPEG)
• Lossless: no data lost; original can be fully restored; slightly larger (e.g. PNG, FLAC, ZIP)
• Run Length Encoding (RLE): a lossless method — stores repeated values as a count+value pair

Storage & Memory

• RAM (Random Access Memory): primary storage; fast; volatile (loses data when power off)
• ROM (Read-Only Memory): stores BIOS/firmware; non-volatile; cannot be changed
• Secondary storage: permanent; slower than RAM. Types: HDD, SSD, optical (CD/DVD), USB
• HDD: uses magnetic platters; cheap, high capacity, slower, mechanical
• SSD: uses flash memory; faster, lighter, no moving parts, more expensive per GB
• Virtual memory: section of secondary storage used as extra RAM when RAM is full — slower

Network Fundamentals

Types of Network

• LAN (Local Area Network): covers a small area (e.g. a school or office)
• WAN (Wide Area Network): covers large geographical areas (e.g. the internet)
• Client-server: dedicated servers manage resources; easier to manage and back up
• Peer-to-peer: all computers share resources equally; cheaper, less secure

Hardware

• Router: connects different networks and routes data packets between them
• Switch: connects devices within a LAN; sends data to specific device (not broadcast)
• NIC (Network Interface Card): allows a device to connect to a network
• WAP (Wireless Access Point): allows wireless connection to a LAN

Transmission Media

• Copper cable (Ethernet): cheap, reliable, susceptible to interference
• Fibre optic: uses light; very fast, no interference, expensive
• Wi-Fi: wireless; convenient; less secure, slower, signal degrades with distance/walls

Protocols & Layers

• Protocol: a set of rules for communication between devices
• TCP/IP: core internet protocol suite
• HTTP/HTTPS: web browsing (HTTPS is encrypted with SSL/TLS)
• FTP: file transfer
• SMTP, IMAP, POP3: email protocols
• DNS: translates domain names to IP addresses

The Internet

• IP address: unique numerical label for each device on a network (e.g. 192.168.1.1)
• IPv4: 32-bit addresses (about 4 billion); IPv6: 128-bit addresses (virtually unlimited)
• Packet switching: data split into packets, each routed independently, reassembled at destination
• Each packet has: source IP, destination IP, payload (data), packet number
• Web servers host websites; browsers request pages via HTTP; DNS resolves domain to IP
• HTML: structure; CSS: style; JavaScript: interactivity
• Cloud computing: storage and processing via remote servers on the internet

Cyber Security

Common Threats

• Malware: malicious software including viruses, worms, trojans, ransomware, spyware
• Phishing: fake emails/websites trick users into revealing credentials
• Brute force attack: trying all possible passwords until one works
• SQL injection: inserting malicious SQL code into input fields to access databases
• Man-in-the-middle: attacker intercepts communication between two parties
• DoS (Denial of Service): flooding a server with requests to make it unavailable
• Social engineering: manipulating people to reveal confidential information

Defences

• Strong passwords and multi-factor authentication (MFA)
• Firewalls: monitor and filter incoming/outgoing network traffic
• Encryption: scrambles data so only authorised parties can read it
• Anti-malware software: detects and removes malicious software
• Regular software updates: patch known vulnerabilities
• HTTPS and SSL/TLS certificates for secure web communication
• User access levels: give users only the permissions they need (principle of least privilege)

Ethical, Legal & Environmental Issues

Legislation

• Data Protection Act 2018 / GDPR: regulates how personal data is stored and used
• Computer Misuse Act 1990: makes unauthorised access and hacking illegal (three tiers)
• Copyright, Designs and Patents Act 1988: protects creative works from copying
• Freedom of Information Act 2000: right to access public authority data

Ethical Issues

• Privacy: surveillance, data harvesting by tech companies
• Artificial intelligence: bias in algorithms, job automation, decision-making
• Digital divide: unequal access to technology between different groups/countries
• Environmental impact: e-waste, energy consumption of data centres