ICT/CS — Core Revision Notes

1. Computer Systems & Hardware

A computer system consists of hardware (physical components) and software (programs/instructions). The CPU (Central Processing Unit) is the 'brain' — it executes instructions. The CPU contains the ALU (Arithmetic Logic Unit, which performs calculations and logical operations), the Control Unit (CU, which fetches, decodes, and executes instructions), and registers (tiny, ultra-fast storage locations: PC = Program Counter, MAR = Memory Address Register, MDR = Memory Data Register, ACC = Accumulator). The Fetch-Decode-Execute (FDE) cycle: (1) Fetch: PC contents sent to MAR; instruction fetched from RAM to MDR, then to CIR; PC incremented. (2) Decode: CU decodes the instruction. (3) Execute: ALU or memory operation carried out. CPU performance depends on: clock speed (GHz — cycles per second), number of cores (parallel execution), cache size (small, fast memory between CPU and RAM).

Primary and secondary storage

• RAM (Random Access Memory): volatile, fast, temporary — holds data and programs currently in use
• ROM (Read-Only Memory): non-volatile, permanent — stores BIOS/firmware needed at startup; cannot be overwritten by user
• Cache: faster than RAM, smaller, stores frequently used data close to the CPU; L1 cache is fastest/smallest
• Hard Disk Drive (HDD): magnetic storage, large capacity, slow, cheap per GB, non-volatile — secondary storage
• Solid State Drive (SSD): flash (NAND) memory, no moving parts, faster than HDD, more expensive per GB, non-volatile
• Optical discs (CD/DVD/Blu-ray): laser reads/writes data, portable, relatively slow
• Flash memory / USB: non-volatile, portable, shock-resistant; used in USB sticks, SD cards, SSDs
• Cloud storage: remote servers accessed via internet — accessible anywhere, requires internet, raises privacy concerns

2. Data Representation & Binary

Computers use binary (base 2) — only 0s and 1s (representing off/on states). A bit is a single binary digit. A byte = 8 bits. A nibble = 4 bits. Prefixes: 1 Kibibyte (KiB) = 1024 bytes; 1 Mebibyte (MiB) = 1024 KiB; 1 Gibibyte (GiB) = 1024 MiB. Binary to denary: use column headings (128, 64, 32, 16, 8, 4, 2, 1) and add the values where bit = 1. Denary to binary: repeatedly divide by 2 and record remainders bottom-up. Binary addition: 0+0=0; 1+0=1; 1+1=10 (carry 1); 1+1+1=11 (carry 1). Overflow occurs when the result is too large for the available number of bits. Left shift multiplies by 2ⁿ; right shift divides by 2ⁿ (logical shift fills with 0s). Hexadecimal (base 16) uses digits 0–9, A–F; one hex digit = 4 bits (a nibble). Used in memory addresses, colour codes (#FF5733), MAC addresses.

Character encoding and data types

• ASCII: 7-bit code (128 characters); extended ASCII is 8-bit (256 chars); represents English letters/numbers/symbols
• Unicode: 16-bit or more; covers all world languages and symbols; UTF-8 is the most common encoding (variable width, backward-compatible with ASCII)
• Pixel: smallest element of a digital image; colour depth = bits per pixel (1-bit = B&W, 8-bit = 256 colours, 24-bit = 16.7M colours)
• Image file size = width × height × colour depth (bits) ÷ 8 (to get bytes)
• Sound: sampling rate (how often sound amplitude is measured, in Hz) × sample resolution (bits per sample) × duration × channels = file size
• Higher sampling rate and resolution = better quality sound but larger file size
• Compression: lossy (removes data permanently, smaller files — JPEG, MP3); lossless (no quality loss — PNG, FLAC, ZIP)

3. Networking

A network connects two or more devices to share resources and communicate. LAN (Local Area Network): small area, same building, usually wired or Wi-Fi. WAN (Wide Area Network): large geographic area, uses public infrastructure (Internet is the global WAN). Network topologies: Bus (all connected to one cable — cheap but single point of failure), Ring (each device connected to two others in a loop — failure of one device breaks the ring), Star (all devices connect to a central switch — most common, reliable, easy to manage but depends on switch), Mesh (every device connects to every other — most resilient, expensive). Protocols: rules for communication. TCP/IP is the core internet protocol. HTTP/HTTPS (web browsing), FTP (file transfer), SMTP/POP3/IMAP (email), DNS (domain name to IP address lookup).

The TCP/IP model layers

• Application layer: provides network services to applications — HTTP, HTTPS, FTP, SMTP, DNS
• Transport layer: TCP (reliable, connection-oriented, error checking, re-orders packets) or UDP (fast, no error checking — used for streaming/gaming)
• Internet layer: IP addressing, routing — IP protocol assigns source and destination IP addresses to packets
• Link layer (Network Access): physical transmission — Ethernet, Wi-Fi; deals with MAC addresses
• Packets: data broken into small units for transmission; each has a header (source IP, destination IP, packet number) and payload (data)
• Router: forwards packets between networks using IP addresses; maintains routing tables
• Switch: forwards packets within a LAN using MAC addresses
• Firewall: monitors and filters incoming/outgoing packets based on rules — blocks unauthorised access

4. Programming & Algorithms

An algorithm is a precise, finite set of instructions for solving a problem. Key constructs: Sequence (instructions executed in order), Selection (if/else statements to choose a path), Iteration (loops: for loop runs a set number of times; while loop runs until a condition is false). Variables store data values that can change. Constants store fixed values. Data types: integer (whole number), real/float (decimal), Boolean (True/False), string (text), character. Arrays/lists store multiple values of the same type accessed by index (index usually starts at 0). Pseudocode uses structured English to express algorithms without worrying about exact syntax. Flowcharts use shapes: oval (start/end), rectangle (process), diamond (decision with Yes/No), parallelogram (input/output), arrow (flow direction). Trace tables are used to dry-run code: one column per variable, one row per iteration.

Searching and sorting algorithms

• Linear search: check each item in sequence; works on unsorted data; O(n) time complexity
• Binary search: only works on SORTED data; check middle item, discard half, repeat; O(log n) — much faster for large datasets
• Bubble sort: compare adjacent pairs, swap if needed, repeat; simple but slow for large datasets; O(n²)
• Merge sort: recursively split list in half, sort each half, merge back together; O(n log n) — efficient but uses more memory
• Insertion sort: build sorted list one item at a time; efficient for nearly-sorted data; O(n²) worst case
• Always state WHY you chose an algorithm — binary search requires sorted data; bubble sort is easy to implement; merge sort is best for large datasets

5. Cybersecurity & Ethics

Cybersecurity threats: Malware types — virus (attaches to files, spreads when file is shared), worm (self-replicating over a network without a host file), trojan (disguises itself as legitimate software), ransomware (encrypts user files and demands payment), spyware (monitors activity and steals data). Social engineering attacks trick users rather than exploiting technical weaknesses: phishing (fake emails/websites to steal credentials), smishing (via SMS), vishing (voice call), pharming (redirects web traffic to fake sites), shouldering/shoulder surfing (observing someone entering a password). Brute force attack: systematically tries all possible passwords. SQL injection: malicious SQL code inserted via input fields to access databases. DDoS (Distributed Denial of Service): floods a server with requests from many machines to take it offline.

Defences against cybersecurity threats

• Firewall: hardware or software; filters incoming/outgoing traffic by rules; blocks suspicious packets
• Antivirus/antimalware: scans for known malware signatures and suspicious behaviour; must be kept updated
• Strong passwords: at least 12 characters; mix of upper/lowercase, digits, symbols; never reuse across sites
• Two-factor authentication (2FA): something you know (password) + something you have (phone/token) — prevents access even if password is stolen
• Encryption: data is transformed using a key so only authorised parties can read it; HTTPS uses TLS encryption
• Regular updates/patches: fix known vulnerabilities that attackers exploit
• User access levels: least-privilege principle — users only access what they need; prevents insider threats
• Physical security: locked server rooms, cable locks, CCTV, biometric access — prevents physical theft of data

6. Ethical, Legal & Environmental Issues

The Data Protection Act (or GDPR in the EU) governs how personal data is collected, stored, and used. Principles include: data must be collected for a specific purpose, be accurate, not kept longer than necessary, and be kept secure. The Computer Misuse Act makes unauthorised access to computer systems, unauthorised modification of data, and hacking illegal. Copyright law protects software and digital content from unauthorised copying/distribution. Open-source software has publicly available source code that anyone can modify and distribute (e.g. Linux, Firefox). Proprietary software is owned by a company; users pay a licence and cannot access source code (e.g. Microsoft Office). Environmental impacts of IT: energy consumption of data centres, e-waste from discarded devices, carbon footprint of manufacturing; mitigated by energy-efficient hardware, cloud computing optimisation, recycling programmes. Ethical considerations: AI bias, surveillance and privacy, digital divide (unequal access to technology between countries and groups).