Cameroon GCE O-Level Computer Science cheat sheet

CPU components: ALU performs arithmetic/logic; Control Unit fetches/decodes/executes; registers (PC, IR, MAR, MDR, ACC) hold temporary data; cache (L1/L2/L3) reduces average memory access time.

Fetch-decode-execute cycle: PC holds address of next instruction; MAR receives address; instruction fetched to MDR then IR; CU decodes; execution may involve ALU or memory; PC increments.

Memory hierarchy (fastest to slowest, smallest to largest, most to least expensive per bit): registers → cache → main memory (RAM) → secondary storage (SSD/HDD) → tertiary/offline storage.

RAM is volatile, holds current programs/data; ROM is non-volatile, holds boot firmware (BIOS/UEFI). SRAM uses flip-flops (cache); DRAM uses capacitors (main memory) and needs refresh.

Input devices: keyboard, mouse, scanner, microphone, touchscreen, sensor, barcode reader, webcam. Output devices: monitor, printer, speaker, projector, plotter, actuator.

Storage media: magnetic (HDD), optical (CD/DVD/Blu-ray), solid-state (SSD, flash). Capacity units: 1 KB = 1024 B, 1 MB = 1024 KB, 1 GB = 1024 MB, 1 TB = 1024 GB.

Printer types: impact (dot matrix, line printer) vs non-impact (laser, inkjet, thermal). Laser: faster, higher quality, higher initial cost; inkjet: cheaper, good colour, slower; dot matrix: multi-part forms, noisy.

Ports/interfaces: USB (universal, hot-swappable), HDMI (video+audio), Ethernet (RJ45), VGA (analog video). Bus: address bus width determines addressable memory; data bus width affects transfer rate.

Binary to decimal: multiply each bit by 2^position (rightmost position 0) and sum. Decimal to binary: repeated division by 2, read remainders upward.

Octal/hex to binary: replace each octal digit with 3 bits, each hex digit with 4 bits. Binary to octal/hex: group bits from right in 3s or 4s, pad left with zeros.

Unsigned 8-bit range 0–255; signed two's complement 8-bit range −128 to +127. To negate: invert bits and add 1. Overflow occurs when result exceeds range.

Character coding: ASCII uses 7 bits (128 characters); extended ASCII 8 bits (256); Unicode (e.g., UTF-8, UTF-16) supports multiple languages and symbols; EBCDIC used on mainframes.

Logic gates: AND (output 1 only if all inputs 1), OR (1 if any input 1), NOT (inverts), NAND (inverse of AND), NOR (inverse of OR), XOR (1 if inputs differ).

Truth table rows = 2^n for n inputs. Boolean algebra: A·1=A, A+0=A, A·0=0, A+1=1, A+A=A, A·A=A, De Morgan: NOT(A·B)=NOT A + NOT B; NOT(A+B)=NOT A · NOT B.

Data units: bit (0/1), nibble (4 bits), byte (8 bits), word (CPU-dependent, often 16/32/64 bits). 1 byte can represent 256 different values.

Sound/images: sampling rate and bit depth determine audio quality; resolution (pixels) and colour depth (bits per pixel) determine image file size; file size = width × height × colour depth / 8 bytes (uncompressed).

System software: operating system, utility programs, device drivers, language translators (compiler, interpreter, assembler). Application software: general-purpose (word processor, spreadsheet) and special-purpose (payroll, CAD).

OS functions: process management, memory management, file management, device management, security (access control), user interface (GUI/CLI), error handling, and providing a platform for applications.

Compiler translates whole source code to object code before execution; interpreter translates and executes line by line; assembler translates assembly language to machine code.

Utilities: antivirus, backup, disk defragmenter, disk cleanup, compression, firewall, file manager, screen saver. They maintain system performance and security.

GUI features: windows, icons, menus, pointers (WIMP). CLI requires typed commands; advantages: precise, scriptable, less resource-heavy; disadvantages: steep learning curve, syntax errors.

Multitasking: OS rapidly switches CPU between processes; multiprogramming keeps several jobs in memory; multiprocessing uses multiple CPUs/cores; real-time OS responds within strict time constraints.

Software licensing: proprietary (commercial, closed source), freeware (free but closed), open source (source code available, may be free), shareware (trial then pay).

Algorithm: finite sequence of unambiguous steps to solve a problem. Must have input, output, definiteness, finiteness, effectiveness. Pseudocode uses structured English; flowchart uses standard symbols.

Flowchart symbols: oval = start/end; parallelogram = input/output; rectangle = process; diamond = decision; arrow = flow; circle = connector; hexagon = preparation (loop).

Three basic control structures: sequence (steps in order), selection (IF…THEN…ELSE, CASE), iteration (FOR…NEXT, WHILE…ENDWHILE, REPEAT…UNTIL).

Pseudocode example for sum 1 to N: total ← 0; FOR i ← 1 TO N DO total ← total + i; ENDFOR; OUTPUT total. Use ← for assignment, = for comparison.

Dry run (trace table): manually execute algorithm step by step recording variable values in columns to verify logic and find errors.

Validation vs verification: validation checks data is reasonable (range, type, presence, length, format, check digit); verification checks data is copied/entered correctly (double entry, visual check).

Searching: linear search checks each item in turn (works on unsorted, O(n)); binary search repeatedly halves a sorted list (O(log n)). Sorting: bubble, insertion, selection, merge.

Structured programming: avoid GOTO; use top-down design, modularity, and stepwise refinement. A procedure performs actions; a function returns a value.

Variable: named memory location whose value can change; constant: fixed value. Data types: integer, real/float, string, character, Boolean, date. Declaration reserves memory; initialisation assigns first value.

Selection: IF condition THEN statements [ELSE statements] ENDIF; CASE/switch for multiple choices. Condition uses relational operators (=, <>, <, >, <=, >=) and logical operators (AND, OR, NOT).

Iteration: definite (FOR i ← start TO end [STEP n] … NEXT i) when number of repetitions known; indefinite (WHILE condition … ENDWHILE, REPEAT … UNTIL condition) when repetitions depend on condition.

Subprogram: procedure (no return value) or function (returns one value). Parameters pass data; pass by value copies, pass by reference passes address (changes affect original). Local variables exist only inside subprogram; global variables accessible throughout.

File handling: OPEN file FOR READ/WRITE/APPEND; READ/WRITE records; CLOSE file. Sequential access reads from start; random access uses record number. Always close files to flush buffers.

String operations: LENGTH, SUBSTRING, LEFT, RIGHT, CONCATENATION (+), UCASE, LCASE, ASCII conversion. Array: collection of same-type elements accessed by index; 1D and 2D arrays.

Arithmetic operators: + - * / ^; MOD gives remainder; DIV gives integer quotient. Operator precedence: brackets, exponentiation, multiplication/division/MOD/DIV, addition/subtraction; use parentheses to clarify.

Debugging: syntax errors (grammar), logic errors (wrong result), runtime errors (e.g., division by zero). Use test data: normal, boundary, erroneous.

File organisation: serial (records stored in order of arrival), sequential (ordered by key field), random/direct (calculated address), indexed sequential (index + sequential data).

Database hierarchy: field (attribute) → record (tuple) → file/table (relation) → database. Key field uniquely identifies a record; primary key is unique and not null; foreign key links to primary key in another table.

Database models: flat file (single table, redundancy), relational (tables linked by keys, uses SQL), hierarchical (tree), network (graph). Relational reduces data redundancy and inconsistency.

SQL basics: SELECT fields FROM table WHERE condition; INSERT INTO table VALUES (…); UPDATE table SET field=value WHERE condition; DELETE FROM table WHERE condition; CREATE TABLE; ORDER BY; GROUP BY.

Data integrity: entity integrity (primary key not null), referential integrity (foreign key matches existing primary key), domain integrity (valid values). Validation rules enforce integrity.

Normalisation: 1NF removes repeating groups; 2NF removes partial dependencies; 3NF removes transitive dependencies. Aim: eliminate redundancy and update anomalies.

File access methods: sequential (read one after another), direct/random (jump to record). Backup: full, incremental, differential; restore procedures.

Data protection: encryption, access rights, passwords, audit trails, backup, physical security. Data privacy laws govern collection and use of personal data.

Network topologies: bus (single backbone, cheap, collision-prone), star (central switch/hub, easy fault isolation, single point of failure), ring (token passing, unidirectional), mesh (redundant, expensive).

LAN vs WAN: LAN covers small area (building), high speed, owned; WAN covers large area (countries), uses public/leased lines, slower, more expensive. WLAN uses Wi-Fi (IEEE 802.11).

Transmission media: guided (twisted pair, coaxial, fibre optic) and unguided (radio, microwave, infrared, satellite). Fibre: high bandwidth, immune to EMI, expensive; UTP: cheap, susceptible to interference.

Protocols: TCP/IP (Internet), HTTP/HTTPS (web), FTP (file transfer), SMTP/POP3/IMAP (email), DNS (domain name to IP), DHCP (automatic IP configuration), Ethernet (LAN).

IP addressing: IPv4 32-bit (e.g., 192.168.1.1), IPv6 128-bit. Public vs private IP; subnet mask; default gateway; MAC address is hardware address (48-bit).

Network devices: hub (broadcasts to all ports), switch (forwards by MAC address), router (forwards between networks by IP), modem, access point, gateway, repeater, bridge, NIC.

Internet services: WWW, email, instant messaging, VoIP, streaming, cloud storage, e-commerce. ISP provides access. Bandwidth measured in bits per second (bps).

Security basics: firewall filters traffic; antivirus; encryption (SSL/TLS); strong passwords; two-factor authentication; phishing, malware (virus, worm, Trojan, spyware, ransomware), denial-of-service attacks; VPN for secure remote access.

ICT applications: education (e-learning, LMS), business (e-commerce, ERP, remote work), health (telemedicine, electronic records), government (e-government), banking (ATM, online banking), entertainment, transport (GPS, ticketing).

Ethical issues: privacy, intellectual property/copyright, software piracy, plagiarism, digital divide, acceptable use policies, netiquette, whistleblowing.

Health and safety: repetitive strain injury (RSI) from keyboard/mouse; eye strain from monitors; back pain from poor posture; use ergonomic chairs, wrist rests, anti-glare screens, regular breaks, correct lighting.

Safety hazards: electrical shock, fire from overloaded sockets, tripping over cables, heavy equipment handling. Use surge protectors, cable management, and safe working environment.

Computer crime: hacking, identity theft, fraud, phishing, spam, cyberbullying, malware distribution. Legislation: data protection acts, computer misuse acts, copyright laws.

Digital divide: inequality in access to ICT due to cost, infrastructure, skills, geography. Bridging measures: community centres, subsidised devices, broadband rollout, digital literacy training.

Emerging technologies: artificial intelligence (machine learning, expert systems), robotics, Internet of Things (IoT), cloud computing (IaaS, PaaS, SaaS), big data, blockchain, virtual/augmented reality, 5G.

Environmental impact: e-waste disposal, energy consumption of data centres, recycling, green computing, power management settings.

SDLC stages: analysis (feasibility, requirements), design, development/coding, testing, implementation, maintenance, review. Iterative models: waterfall (sequential), agile (incremental, iterative), spiral (risk-driven), prototyping.

Feasibility study: technical (can it be built?), economic (cost-benefit), legal (compliance), operational (will it be used?), schedule (time).

Requirements analysis: interviews, questionnaires, observation, document analysis. Produce requirements specification (functional and non-functional).

Design tools: data flow diagrams (DFD), entity-relationship diagrams (ERD), structure charts, flowcharts, pseudocode, UML (use case, class diagrams).

Testing types: unit (individual modules), integration (modules together), system (whole system), acceptance (user). Black-box (functionality) vs white-box (internal logic). Test data: normal, boundary, invalid/erroneous.

Implementation/changeover methods: direct (old stops, new starts), parallel (both run), phased (gradual by module), pilot (one department first). Parallel safest but costly; direct riskiest but cheapest.

Documentation: user documentation (user manual, tutorial, FAQ, online help) for end users; technical documentation (system specification, design, code comments, maintenance guide) for developers.

Maintenance types: corrective (fix errors), adaptive (new environment), perfective (improve performance), preventive (reduce future problems).