Abbreviations

Computer Systems and Processor Architecture

A computer system executes instructions by moving binary data between memory and execution units inside the CPU (Central Processing Unit). Within the processor, the CU (Control Unit) manages timing and directs data flow, while the ALU (Arithmetic Logic Unit) executes arithmetic calculations and logical decisions. Modern systems often add a GPU (Graphics Processing Unit), which uses thousands of small, parallel cores originally built for graphics rendering to accelerate machine-learning workloads.

Registers and the Internal Cycle

A register is a tiny, high-speed storage location built directly inside the CPU that holds data or an address the processor is currently working on. Registers operate faster than cache, which operates faster than main RAM.

• PC – Program Counter: holds the memory address of the next instruction waiting to be fetched. Note that PC can also refer to a Personal Computer, depending entirely on question context.
• MAR – Memory Address Register: holds the physical memory address currently being read from or written to.
• MDR – Memory Data Register: temporarily holds the actual data or instruction fetched from, or waiting to be written to, main memory.
• IR (or CIR) – Instruction Register / Current Instruction Register: holds the instruction currently being decoded and executed.
• ACC – Accumulator: stores the intermediate mathematical and logical results generated by the ALU.

The fetch–decode–execute cycle repeats continuously while the machine runs:

1. Fetch: The address in the PC is copied to the MAR. The PC is immediately incremented to point to the next instruction. The CPU activates the address bus, retrieves the instruction from RAM across the data bus into the MDR, and copies it to the IR.
2. Decode: The CU splits the instruction inside the IR into its opcode and operands to determine what electrical signals to send.
3. Execute: The CU signals the relevant hardware. If arithmetic is needed, the ALU calculates the result and stores it inside the ACC.

Three physical buses connect these units: the address bus carries addresses in one direction from CPU to RAM, the data bus carries bidirectional data and instructions, and the control bus transmits coordination pulses like read/write signals.

Memory, Secondary Storage, and Peripherals

Primary memory links directly to the processor bus. It includes RAM (Random Access Memory), which is volatile and loses its contents when powered down, alongside ROM (Read Only Memory), which is non-volatile and permanently retains factory startup routines. The BIOS (Basic Input/Output System) is firmware stored inside ROM that initializes hardware checks during boot and hands control to the OS (Operating System).

Secondary storage holds files permanently. An HDD (Hard Disk Drive) uses spinning magnetic platters and mechanical read/write heads, making it susceptible to physical shock. An SSD (Solid State Drive), however, swaps out moving platters for non-volatile flash memory, which survives bumps far better and reads data almost instantly. Optical media such as CD and DVD use the spelling 'disc', whereas magnetic and flash media use 'disk'. Engineers pack millions of microscopic circuits onto a single silicon chip called an IC (Integrated Circuit), linking the CPU to outside devices through I/O (Input/Output) controllers and USB (Universal Serial Bus) sockets.

Data Representation and Encoding Standards

Every data item stored or processed in computing reduces down to binary states.

• bit – binary digit: the foundational unit of storage, representing either 0 or 1.
• nibble – a group of 4 bits (half a byte).
• byte – a collection of 8 bits, providing $2^8 = 256$ distinct binary permutations.

Storage capacities scale up by factors of 1,024 ($2^{10}$):

• 1 KB (kilobyte) = 1,024 bytes
• 1 MB (megabyte) = 1,024 KB
• 1 GB (gigabyte) = 1,024 MB
• 1 TB (terabyte) = 1,024 GB

For example, an uncompressed plain-text document holding 500 standard characters at 1 byte per character consumes 500 bytes, which equals roughly 0.49 KB.

Text Character Encodings

Computers map written text to numbers through standardised character sets:

• ASCII – American Standard Code for Information Interchange: uses 7 bits per character, accommodating 128 unique values (0 to 127) for English alphanumeric characters, punctuation, and system control codes. Extended ASCII extends this to 8 bits (256 values). Key anchor codes worth remembering are space (32), '0' (48), 'A' (65), and 'a' (97). Notice that any lowercase letter sits exactly 32 positions above its uppercase counterpart.
• Unicode – a universal character set created because 128 or 256 characters cannot represent world writing systems like Mandarin, Arabic, Greek, or emoji.
• UTF – Unicode Transformation Format: the encoding mechanism mapping Unicode code points into bit sequences. Because different alphabets need different amounts of room, UTF-8 uses a variable width of 1 to 4 bytes for each character. Its first 128 codes match ASCII bit for bit, ensuring total backwards compatibility. By comparison, UTF-32 is fixed-width, using 4 bytes for every single character regardless of simplicity.

Networks, Internet, and Communication Protocols

The internet began in 1969 as ARPANET (Advanced Research Projects Agency Network), which pioneered packet switching by breaking data into discrete chunks that travel independently over varied routes.

A clear distinction matters for the exam: the internet is the physical global infrastructure of interconnected computer networks, cables, and routers. The WWW (World Wide Web) is an information service operating on top of the internet, consisting of linked multimedia documents accessed via web protocols. Services like email, file transfer, and streaming run over the internet without being part of the World Wide Web.

Network Scale and Topologies

Networks are categorised by geographic range and architecture:

• PAN – Personal Area Network: operates within arm's reach of one user, such as a smartphone linked to wireless headphones.
• LAN – Local Area Network: covers a restricted site like a school or office, maintained privately by that organisation.
• WLAN – Wireless Local Area Network: a LAN operating across radio links using the IEEE 802.11 standard, commercially branded as Wi-Fi.
• WAN – Wide Area Network: spans vast geographical distances across cities or continents by leasing telecommunication lines; the internet represents the world's largest WAN.
• VPN – Virtual Private Network: sets up an encrypted software tunnel across public infrastructure, allowing remote workers to access private internal LANs securely.
• P2P – Peer-to-Peer: a decentralised network where every connected computer acts as both client and server, sharing files directly without needing any central server in the middle.

Protocol Architectures: OSI and TCP/IP

Communication models organise network functions into modular layers. The theoretical OSI (Open Systems Interconnection) reference model contains seven layers: Application, Presentation, Session, Transport, Network, Data Link, and Physical. A reliable mnemonic runs top-to-bottom: All People Seem To Need Data Processing.

In contrast, the internet physically runs on the four-layer TCP/IP suite:

• Application Layer: handles software protocols like HTTP, HTTPS, FTP, SMTP, and DNS.
• Transport Layer: manages end-to-end connections between applications. You will either use TCP (Transmission Control Protocol) to guarantee every packet arrives intact and in sequence through acknowledgements, or UDP (User Datagram Protocol) when raw speed matters more than the odd dropped packet, such as during live gaming or voice calls.
• Internet Layer: handles addressing and routing across network boundaries using IP (Internet Protocol).
• Network Access / Link Layer: deals with physical hardware media, using network interface cards and MAC address identification.

Addressing, Routing, and Email Protocols

• IP address – Internet Protocol address: a unique logical address assigned to every device on a network. Under IPv4, we write 32-bit addresses as four decimal numbers separated by dots, like 192.168.1.1. But because those 4.3 billion possible combinations ran out globally, the internet moved to IPv6, expanding each address to 128 bits written out in hexadecimal blocks.
• MAC address – Media Access Control address: a fixed 48-bit physical identifier permanently burned into a Network Interface Card during manufacturing.
• DHCP – Dynamic Host Configuration Protocol: automatically assigns dynamic IP addresses, subnet masks, and gateway values to client devices joining a network.
• DNS – Domain Name System: a distributed directory that resolves human-readable domain names (such as examinations.ie) into the numeric IP addresses needed by network routers.
• ISP – Internet Service Provider: the commercial company providing home or institutional connections to the internet backbone.
• URL – Uniform Resource Locator: the structured address specifying a resource's web location. Take a web address like https://www.examinations.ie/exams/2025.html. Here, https tells the browser what protocol to talk with, www.examinations.ie names the web host domain, /exams/ points to the directory folder on that server, and 2025.html is the exact file being loaded.
• VoIP – Voice over Internet Protocol: digitises spoken audio into packets and routes them over IP connections, replacing legacy analogue telephone lines.
• FTP – File Transfer Protocol: transfers bulk files between clients and remote file hosts.
• SMTP – Simple Mail Transfer Protocol: pushes outgoing email messages between mail servers.
• IMAP / POP3 – Internet Message Access Protocol and Post Office Protocol 3: download incoming email to local email clients; IMAP synchronises state across multiple devices, whereas POP3 typically downloads and removes mail from the server.

Web Development, Software Engineering, and Data Structures

Developing software and web applications requires combining structured languages, development tooling, and abstract data models.

Front-End Web Development

Web authoring separates documents into three distinct functional responsibilities:

• HTML – HyperText Markup Language: provides the structural foundation and semantic content of web pages through tags like <p> and <main>.
• CSS – Cascading Style Sheets: governs visual presentation, fonts, layouts, colours, and responsive breakpoints, separating design from underlying content.
• JS – JavaScript: supplies client-side scripting to produce dynamic behavior, calculate form values, and respond to user interface events.
• DOM – Document Object Model: the object-oriented hierarchical node tree constructed by the browser when parsing HTML, which JavaScript modifies to update the live screen.

A memorable rule for this separation: HTML builds the page, CSS styles the page, and JS programs its behaviour.

Formats, Tooling, and Architecture

• IDE – Integrated Development Environment: software packaging a source-code editor, automated compiler or interpreter, and visual debugger inside a unified interface (such as Thonny or VS Code).
• API – Application Programming Interface: a set of exposed endpoints and rules permitting independent software systems to communicate, such as querying a live weather server for rainfall statistics.
• JSON – JavaScript Object Notation: a lightweight, human-readable data format storing key-value pairs and arrays, universally used for API data transfers.
• XML – eXtensible Markup Language: a flexible, tag-based data format providing custom data structures.
• CSV – Comma Separated Values: plain text storing tabular records where individual field values are split by commas.
• SQL – Structured Query Language: the standard declarative programming language we use to pull records, update entries, and run filters across tables in a relational database.

Software Life Cycles and Abstract Structures

• SDLC – Software Development Life Cycle: the phased engineering progression comprising planning, analysis, design, implementation, testing, and deployment. Common models include linear Waterfall and iterative Agile.
• OOP – Object-Oriented Programming: a design paradigm structuring software into modular classes and instantiated objects that encapsulate private attributes (data) and public methods (behaviour).
• GIGO – Garbage In, Garbage Out: an algorithmic principle stating that processing flawed or corrupt input inevitably generates flawed output, reinforcing the absolute need for robust input validation.
• ADT – Abstract Data Type: a theoretical specification for a data structure defined by the operations performed on it rather than its underlying memory implementation.
• LIFO – Last In, First Out: the operational behaviour of a stack data structure, where data is pushed onto the top and popped off the top.
• FIFO – First In, First Out: the operational behaviour of a queue data structure, where elements are inserted at the back and removed from the front.

Design, Security, Ethics, and Emerging Technologies

HCI and Universal Design

Creating usable systems involves evaluating how interfaces behave and how accessible they remain:

• UI – User Interface: the visible visual components, input fields, layouts, and interactive buttons a user sees on screen.
• GUI – Graphical User Interface: a desktop or mobile visual interface built around windows, icons, menus, and pointers.
• UX – User Experience: the subjective ease, usability, speed, and emotional satisfaction an individual feels when navigating an entire application.
• UD – Universal Design: the deliberate design of software, products, and environments to make them usable by all individuals to the greatest extent possible without needing custom adaptations. Practical examples include screen-reader compatibility, dynamic text-scaling sliders, high-contrast colour themes, and closed captions on video streams.

Security and Legislation

• SSL/TLS – Secure Sockets Layer / Transport Layer Security: cryptographic protocols securing data moving across networks. TLS is the modern, secure successor to deprecated SSL. When running over TLS, regular HTTP becomes HTTPS, authenticating web servers with digital certificates and applying encryption to protect login credentials and payment cards.
• 2FA – Two-Factor Authentication: security requiring two separate categories of evidence before granting access: something you know (a password) combined with something you possess (a mobile authenticator app push token).
• DDoS – Distributed Denial of Service: an attack where an army of infected computers, known as a botnet, swamps a target server with bogus requests so genuine users cannot get through.
• GDPR – General Data Protection Regulation: strict European Union legislation dictating exactly how companies collect, store, and process our personal information. It forces organisations to collect only what they genuinely need for lawful reasons, while giving you the legal right to inspect or wipe your records.

Artificial Intelligence and Hardware Technologies

• AI – Artificial Intelligence: building computer systems capable of tackling tasks that normally call for human thought, such as recognising faces, understanding speech, and weighing up decisions.
• ML – Machine Learning: a specialized branch of AI where statistical algorithms train models on existing datasets to make automated predictions rather than running explicit hard-coded rules.
• ANN – Artificial Neural Network: an ML architecture modelled loosely on biological neural pathways, containing input, hidden, and output layers of interconnected mathematical nodes whose connection weights update during training.
• XAI – Explainable Artificial Intelligence: AI techniques engineered to expose their reasoning, ensuring human auditors can understand and challenge algorithmic outputs.
• IoT – Internet of Things: ordinary everyday items embedded with microcontrollers, sensors, and network chips so they can broadcast data automatically, like a domestic smart thermostat reporting room temperatures.
• RFID – Radio Frequency Identification: wireless identification using microchip tags queried by nearby radio scanners.
• NFC – Near Field Communication: a ultra-short-range branch of RFID used for contactless payments and electronic transport passes.
• GPS – Global Positioning System: a satellite network providing precise geographic coordinates to receivers.
• VR – Virtual Reality: a fully simulated, computer-generated visual environment viewed through a dedicated headset.
• AR – Augmented Reality: computer-generated interactive graphics and text overlaid directly on top of the real-world view via cameras or glasses.

Essential Confusion Pairs

Short-answer exam questions routinely ask candidates to distinguish between pairs of related acronyms. Learn these direct contrasts:

• HTTP vs HTTPS: Regular HTTP sends your web traffic in unencrypted plain text across port 80, while HTTPS wraps that data in TLS encryption over port 443 so nobody snooping on the network can read passwords or bank details.
• TCP vs UDP: TCP establishes a verified three-way handshake and guarantees ordered, error-checked packet arrival, whereas UDP sends packets without handshakes or delivery confirmation to maximise speed.
• RAM vs ROM: RAM provides volatile, fast read/write workspace for executing software, whereas ROM contains non-volatile, permanent, factory-burned startup instructions.
• HDD vs SSD: A traditional HDD relies on magnetic platters spinning under a mechanical read head, whereas an SSD stores everything directly into flash memory chips, giving you vastly faster load times and zero moving parts to break.
• LAN vs WAN: A LAN covers a single localized building or school managed internally, whereas a WAN links geographically dispersed networks across public infrastructure.
• UI vs UX: UI comprises the visual elements, colours, and controls presented on screen, whereas UX represents the overall efficiency, accessibility, and satisfaction experienced by the user.
• ASCII vs Unicode: ASCII is a 7-bit character set supporting 128 English characters, whereas Unicode is a global system supporting over a million characters across all world writing systems, commonly encoded via variable-width UTF-8.
• Internet vs WWW: The internet is the global physical infrastructure of hardware, computers, and cables, whereas the World Wide Web is an information-sharing service of hypertext web pages running on top of that infrastructure.
• MAR vs MDR: The MAR holds the address pointing to a memory cell, whereas the MDR holds the actual data value moving into or out of that location.
• IPv4 vs IPv6: We write 32-bit IPv4 addresses in dotted decimal notation, which tops out at about 4.3 billion unique devices; to stop the world running out of space, IPv6 gives us 128 bits written in hexadecimal notation, creating virtually limitless addresses.
• bit vs byte: A bit is the smallest possible slice of computing data, holding a single 0 or 1, while a byte bundles 8 bits together to hold one of 256 distinct values, enough for a single ASCII character.

Key terms

ALU

Arithmetic Logic Unit; the processor sub-unit performing mathematical calculations and logical comparisons.

CU

Control Unit; the CPU component directing data flow and managing instruction cycles.

PC

Program Counter; CPU register holding the memory address of the next instruction to be fetched.

MAR

Memory Address Register; CPU register storing the memory address currently targeted for reading or writing.

MDR

Memory Data Register; CPU register holding the data or instruction retrieved from or written to RAM.

IR

Instruction Register; register holding the instruction currently being decoded and executed.

ACC

Accumulator; internal register holding intermediate results produced by the ALU.

DNS

Domain Name System; network service resolving human-readable domain names into numeric IP addresses.

IP

Internet Protocol; fundamental protocol handling logical device addressing and packet routing.

TCP

Transmission Control Protocol; connection-oriented protocol providing reliable, sequenced packet transmission.

HTTPS

HyperText Transfer Protocol Secure; encrypted web transfer protocol running over TLS.

HTML

HyperText Markup Language; standard markup language defining web page content and structure.

CSS

Cascading Style Sheets; style sheet language governing visual presentation and layout of web documents.

DOM

Document Object Model; hierarchical object tree representing HTML elements in memory for scripting.

API

Application Programming Interface; set of definitions and protocols enabling distinct software programs to interact.

JSON

JavaScript Object Notation; text-based, human-readable data format used to store and exchange structured data.

SDLC

Software Development Life Cycle; phased framework structuring software engineering from planning to maintenance.

UD

Universal Design; designing environments and software so they remain accessible to everyone without modification.

GDPR

General Data Protection Regulation; EU privacy law governing personal data collection, processing, and user rights.

ASCII

American Standard Code for Information Interchange; 7-bit character encoding system providing 128 character codes.

UTF

Unicode Transformation Format; variable-width encoding scheme converting Unicode code points into binary bytes.

LIFO

Last In, First Out; data structure access principle governing stacks, where the last item pushed is popped first.

FIFO

First In, First Out; data structure access principle governing queues, where the first item queued is processed first.

XAI

Explainable Artificial Intelligence; machine-learning models designed so their internal reasoning can be audited by humans.