HKDSE ICT cheat sheet

Data are raw unprocessed facts; information is data processed into a meaningful form for decision-making; knowledge applies information with experience.

Data processing cycle: input → processing → output → storage; feedback from output can adjust later input.

Bit = binary digit (0 or 1); 8 bits = 1 byte; 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB (binary prefixes).

Character coding: ASCII uses 7 bits (128 codes); extended ASCII 8 bits (256); Unicode uses 16 or more bits to cover all writing systems.

Number bases: binary base 2, octal base 8, denary base 10, hexadecimal base 16; hex digit values 0-9 then A-F.

Binary→denary: sum place values 128,64,32,16,8,4,2,1 for each 1 bit; e.g. 1101 = 8+4+1 = 13.

Two's complement: invert all bits of the positive value and add 1; an n-bit signed range is −2^(n−1) to 2^(n−1)−1.

Data validation checks format, range, type or presence against rules; data verification checks accuracy by double entry or proofreading.

Lossless compression (e.g. ZIP, PNG) restores exact original data; lossy compression (e.g. JPEG, MP3) discards detail permanently for smaller size.

CPU components: control unit fetches and decodes instructions; ALU performs arithmetic and logical operations; registers hold current data and addresses.

Machine cycle: fetch → decode → execute → store; clock speed in Hz sets cycles per second, and one instruction may take several cycles.

Main memory is volatile RAM (working store, loses contents on power-off) or non-volatile ROM (holds bootstrap/firmware).

Cache is small fast memory between CPU and RAM; larger cache and higher hit rate reduce average memory access time.

Secondary storage: magnetic hard disk (high capacity, mechanical), SSD/flash (no moving parts, faster random access), optical (CD/DVD/Blu-ray).

System software = operating system plus utilities; application software = general-purpose (word processor) or special-purpose (payroll) packages.

OS functions: process scheduling, memory management, file management, device (I/O) management, user interface, and access control.

Von Neumann architecture stores program and data in the same memory; Harvard architecture uses separate instruction and data memories.

Input devices: keyboard, mouse, scanner, sensor, barcode reader; output devices: monitor, printer, plotter, speaker; impact vs non-impact printers.

Internet = global network of networks using TCP/IP; WWW is a service on it accessed by URL and HTTP/HTTPS.

URL structure: protocol://domain/path?query#fragment; DNS resolves a domain name to an IP address.

IPv4 addresses are 32-bit (four 0-255 octets); IPv6 is 128-bit and written in hexadecimal groups.

LAN covers a small area under one administration; WAN spans large distances; the Internet is the largest WAN.

Transmission media: guided (twisted pair, coaxial, fibre optic) and unguided (Wi-Fi radio, microwave, satellite); fibre offers highest bandwidth.

Bandwidth is capacity in bits per second; latency is delay; packet switching splits messages into packets routed independently and reassembled.

Protocols: HTTP/HTTPS (web), SMTP (send mail), POP3/IMAP (receive mail), FTP (file transfer), TCP (reliable, connection-oriented), UDP (fast, connectionless).

Search techniques: use quotation marks for exact phrase, minus sign to exclude a term, site: to restrict to a domain, filetype: to restrict file format.

E-commerce models: B2C, B2B, C2C; security relies on HTTPS, digital certificates and encryption of payment data.

Three control structures: sequence, selection (if/else, switch) and iteration (for, while, do-while).

Variable = named storage location whose value can change; constant is fixed after declaration; identifier naming rules forbid spaces and leading digits.

Data types: integer, real/float, character, string, Boolean; a type mismatch (e.g. adding a string to an integer) causes a runtime or compile error.

Arithmetic operators: + − * / with precedence, integer division and MOD for remainder; brackets override precedence.

Relational operators (=, <>, <, >, <=, >=) and logical operators (AND, OR, NOT) build conditions; AND is true only if both operands are true.

A for loop is used when the number of iterations is known; a while loop tests the condition before the body and may run zero times.

A procedure performs actions without returning a value; a function returns a value via a return statement.

Parameters pass data into a subroutine; pass by value copies the argument, pass by reference lets the subroutine modify the caller's variable.

Syntax error violates language grammar (detected at compile time); logic error produces wrong output with valid syntax (found by testing/tracing).

Digital divide = gap between those with and without reliable access to devices, connectivity and digital skills; factors include income, age, location and education.

Personal Data (Privacy) Protection Ordinance in Hong Kong governs collection, accuracy, retention, security and access to personal data.

Copyright protects original works automatically on creation; unauthorized copying, downloading or distribution can be an infringement.

Computer crime categories include unauthorized access (hacking), data theft, malware distribution, online fraud and denial-of-service attacks.

Malware types: virus (attaches to files), worm (self-replicates over networks), Trojan (disguised as useful software), ransomware (encrypts data for ransom), spyware (collects data secretly).

Phishing tricks users into revealing credentials via fake messages or sites; countermeasures include verifying sender, checking URLs and two-factor authentication.

Health and social issues: repetitive strain injury, eye strain, sedentary lifestyle, internet addiction, cyberbullying and reduced face-to-face interaction.

Green computing: power management, device recycling, duplex printing, virtualization and cloud consolidation to reduce energy and e-waste.

Intellectual property and licensing: proprietary software restricts copying and modification; open-source and freeware licences grant different rights.

Artificial intelligence systems learn patterns from data; machine learning needs large labelled datasets and can inherit bias present in that data.

Generative AI produces text, images or code from prompts; outputs may be inaccurate (hallucination) and require human verification.

Internet of Things = network of sensor-equipped physical devices exchanging data; applications include smart homes, wearables and smart city sensors.

Cloud computing delivers computing as a service: IaaS (infrastructure), PaaS (platform), SaaS (software); pay-per-use and scalable.

5G offers high bandwidth and low latency, enabling real-time applications such as remote surgery support and autonomous vehicles.

Big data is characterized by volume, velocity, variety and veracity; analytics extract patterns that support decisions.

Automation and robotics raise productivity but displace routine jobs, shifting demand toward design, maintenance and data skills.

Blockchain is a distributed, tamper-resistant ledger; cryptocurrency is one application, with risks of volatility and fraud.

Emerging issues: algorithmic bias, surveillance and privacy loss, deepfakes, e-waste from rapid device replacement, and the digital divide widening as technology advances.