NSSCO Computer Studies (6134) cheat sheet

Von Neumann architecture: CPU (control unit, ALU, registers), memory, and I/O share one bus; instructions and data occupy the same address space, so both travel the same path.

Fetch-decode-execute cycle: PC holds the next address, MAR carries it to memory, MDR holds the fetched word, CIR/IR holds the current instruction, then decode and execute; the accumulator holds ALU operands.

Memory hierarchy by speed and cost: registers > cache (L1/L2/L3) > RAM > SSD/flash > magnetic disk > optical/tape; capacity and access latency grow in the opposite direction to cost per bit.

RAM is volatile and holds programs and data in use; ROM is non-volatile and holds firmware such as the bootstrap loader; cache exploits locality of reference.

Input devices convert physical quantities to digital signals (keyboard, scanner, sensor, microphone); output devices convert digital data to human-usable form (monitor, printer, actuator, speaker).

System software: operating system (process scheduling, memory management, file system, device drivers, security), utility programs, and language translators (compiler, interpreter, assembler).

Application software is task-oriented (word processor, spreadsheet, browser); general-purpose vs special-purpose and off-the-shelf vs bespoke are standard classifications.

Compiler translates the whole source program before execution and produces an object file; an interpreter translates and runs line by line, which aids debugging but slows execution.

Primary storage is directly addressable by the CPU (RAM, ROM, cache); secondary storage is non-volatile and accessed via I/O controllers (HDD, SSD, optical, magnetic tape).

Binary place values double leftwards: 128 64 32 16 8 4 2 1; convert binary to denary by summing the place values of the 1 bits, and denary to binary by repeated division by 2 reading remainders upwards.

Hexadecimal uses base 16 with digits 0-9 and A-F; each hex digit maps to exactly 4 bits, so 1 byte = 2 hex digits (e.g. 1010 1111 = AF = 175).

Unsigned n-bit range is 0 to 2^n - 1; two's complement n-bit range is -2^(n-1) to 2^(n-1) - 1; negate by inverting all bits and adding 1.

Binary addition rules: 0+0=0, 0+1=1, 1+1=0 carry 1, 1+1+1=1 carry 1; overflow occurs when a carry out of the sign bit disagrees with the carry into it.

Logical shifts left multiply by 2 and right divide by 2; arithmetic right shift preserves the sign bit; a shift that pushes a 1 out of the register loses data.

Character sets: ASCII uses 7 bits (128 codes), extended ASCII 8 bits (256 codes), Unicode uses variable encodings (UTF-8, UTF-16) to cover all writing systems.

File size in bits = number of samples x bits per sample; for images, resolution width x height x colour depth; for sound, sample rate x bit depth x duration x channels.

Lossy compression (JPEG, MP3) discards data permanently for high ratios; lossless compression (PNG, FLAC, ZIP) reconstructs the original exactly using repeated-pattern encoding.

Bit, nibble (4 bits), byte (8 bits), kibibyte (1024 bytes), mebibyte (1024 KiB), gibibyte (1024 MiB); decimal SI prefixes use powers of 1000.

LAN covers a small site under one administration; WAN spans large distances and typically uses leased lines or public infrastructure; the Internet is a global network of networks.

Topologies: star uses a central switch (single point of failure but easy to isolate faults); bus shares one backbone; ring passes data node to node; mesh offers redundant paths at high cabling cost.

TCP is connection-oriented, orders and retransmits segments, and suits file transfer and web traffic; UDP is connectionless with no delivery guarantee and suits streaming, VoIP and DNS queries.

IPv4 addresses are 32 bits in dotted decimal with a subnet mask separating network and host portions; IPv6 uses 128 bits in hexadecimal groups to relieve address exhaustion.

DNS resolves domain names to IP addresses; DHCP assigns IP configuration automatically; NAT maps private addresses to a public address; a router forwards packets between networks.

The OSI model has seven layers: Physical, Data Link, Network, Transport, Session, Presentation, Application; TCP/IP merges these into Network Access, Internet, Transport and Application.

Transmission media: twisted pair and coaxial are copper; fibre optic carries light with high bandwidth and immunity to electromagnetic interference; wireless uses radio, microwave or infrared.

Bandwidth is capacity in bits per second; latency is delay in milliseconds; a switch forwards frames by MAC address within a LAN while a hub broadcasts to all ports.

Security controls: firewall filters traffic by rules, encryption protects confidentiality in transit, authentication verifies identity, and WPA2/WPA3 secures wireless links.

A database is an organised collection of related data; a DBMS is the software that creates, queries, updates and controls access to it.

Relational model: tables (relations) of rows (records) and columns (fields); the primary key uniquely identifies a row; a foreign key references a primary key in another table to enforce referential integrity.

Entity relationship modelling: one-to-one, one-to-many and many-to-many relationships; a many-to-many relationship is resolved into two one-to-many links via a junction table.

Data types constrain fields: text/character, integer, decimal/currency, date/time, boolean, and autonumber; validation checks include range, type, presence, length and format.

SQL core statements: SELECT columns FROM table WHERE condition; INSERT INTO, UPDATE ... SET, DELETE FROM; ORDER BY sorts, GROUP BY aggregates with COUNT, SUM, AVG, MIN, MAX.

Normalisation removes redundancy and update anomalies: 1NF eliminates repeating groups, 2NF removes partial dependencies on part of a composite key, 3NF removes transitive dependencies.

Data validation checks input against rules; data verification confirms accuracy by double entry, parity checks or visual comparison against a source document.

A management information system summarises transaction data into routine reports for operational decisions; a decision support system adds modelling and what-if analysis.

Backup strategies: full, incremental (changes since last backup) and differential (changes since last full backup); a grandfather-father-son rotation retains generations of copies.

Algorithm properties: finite (terminates), definite (unambiguous steps), has input and output, and is effective (each step is executable).

Program structure: sequence, selection (IF/ELSE, CASE) and iteration (count-controlled FOR, condition-controlled WHILE/REPEAT); every loop must have a reachable exit condition.

Standard algorithms to know: linear search, binary search (requires sorted data, halves the interval), bubble sort, insertion sort, and merge sort (divide and conquer).

Pseudocode conventions: INPUT/OUTPUT for I/O, assignment with <-, counters and accumulators initialised before loops, and indentation to show block structure.

Trace tables record variable values after each statement; dry runs expose off-by-one errors, uninitialised variables and incorrect loop conditions.

Modular design: procedures perform an action, functions return a value; parameters pass data in, and local variables are scoped to the subprogram.

Testing types: normal, boundary and erroneous data; a syntax error breaks translation, a logic error runs but gives wrong output, a runtime error aborts during execution.

Flowchart symbols: oval for start/stop, parallelogram for input/output, rectangle for process, diamond for decision, arrow for flow direction.

Boolean logic: AND is true only if both inputs are true, OR if at least one is true, NOT inverts; truth tables enumerate all input combinations (2^n rows for n inputs).

ICT applications span education (e-learning), health (telemedicine, records), commerce (e-commerce, EFTPOS), banking (ATM, online banking), government (e-government) and industry (CAD/CAM, robotics).

Computer misuse offences include unauthorised access, unauthorised modification and unauthorised access with intent to commit further offences; copyright law protects software and digital content.

Malware categories: virus (attaches to files), worm (self-replicating over networks), trojan (disguised as legitimate), spyware (collects data), ransomware (encrypts and demands payment).

Phishing, pharming, social engineering and keylogging target users rather than hardware; safeguards include strong passwords, two-factor authentication, anti-malware and user education.

Encryption protects confidentiality, digital signatures and checksums support integrity and authenticity, and access rights (read, write, execute) enforce authorisation.

Digital divide: unequal access to devices, connectivity and skills across income, geography, age and disability; ergonomics and health issues include RSI, eye strain and sedentary risk.

Privacy principles: collect only necessary data, state the purpose, allow access and correction, secure storage, and limit retention; data protection laws enforce these duties.

Emerging technologies: artificial intelligence and machine learning, cloud computing (IaaS, PaaS, SaaS), Internet of Things, blockchain, virtual and augmented reality, and 5G connectivity.

Automation and AI raise employment, accountability and bias concerns; green computing addresses energy use, e-waste recycling and disposal of obsolete hardware.