GCSE & IGCSE Computer Science key terms

241 terms with short definitions you can learn for the exam. Type to filter the list.

Command words 39

Amend GCSE: Change code: add to it, delete from it or rearrange it. (1 to 6 marks on Paper 1)
Amend IGCSE: Change, add to, delete from or rearrange a symbolic representation such as code.
Analyse IGCSE: Examine the parts of something in detail.
Assess IGCSE: Consider all the factors, decide which matter most and make a judgement.
Calculate GCSE: Work out a numerical answer and show the working. (2 to 4)
Calculate IGCSE: Work out a numerical answer, showing working and units where relevant.
Compare and/or contrast IGCSE: Give similarities and/or differences between two or more things, referring to all of them.
Complete GCSE: Fill in a table, diagram or algorithm, in any notation. (2 to 6)
Complete IGCSE: Fill in a table, diagram or algorithm.
Construct GCSE: Create something using the subject's symbols, rules and syntax, such as an expression or a logic statement. (2 to 4)
Construct IGCSE: Create something using the subject's symbols, rules and syntax.
Convert GCSE: Change information from one representation to another, such as denary to binary. (2 to 6)
Convert IGCSE: Change information from one representation to another.
Create IGCSE: Produce an artefact using subject-specific representations, such as code.
Deduce IGCSE: Reach a conclusion from the information given.
Define GCSE: Give the meaning of a term. (1 to 2)
Describe GCSE: Give an account of something, with developed statements. No justification is needed. (2 to 4)
Describe IGCSE: Give an account of something with developed statements; no justification needed.
Devise IGCSE: Plan or invent a procedure from existing ideas.
Discuss GCSE: Identify the issue, explore all its aspects and investigate it with reasoning or argument. (6)
Discuss IGCSE: Identify the issue, explore all its aspects and investigate it with reasoning.
Draw GCSE: Produce a diagram, with a ruler or freehand. (2 to 6)
Draw IGCSE: Produce a diagram, which may need labels.
Evaluate IGCSE: Review the information, weigh up strengths and weaknesses and give a supported judgement.
Explain GCSE: Make a point and justify or exemplify it: each point needs a linked reason. (2 to 4)
Explain IGCSE: Give reasons or examples: points must be linked.
Give IGCSE: Recall one or more pieces of information.
Give a reason or reasons IGCSE: Say why something is the case.
Give, State, Name GCSE: Recall one or more pieces of information. (1)
Identify GCSE: Pick out key information from something you are given, such as code or a table. (1)
Identify IGCSE: Pick out key information from a stimulus or resource.
Name IGCSE: Recall one piece of information, for example from a diagram or algorithm.
Perform IGCSE: Carry out a task, algorithm or process.
Predict IGCSE: Give the expected result.
State IGCSE: Recall one or more pieces of technical information.
State and justify / Identify and justify IGCSE: Make a choice and give a reason for it.
State what is meant by IGCSE: Give the meaning of a term (usually 1 or 2 marks).
Write GCSE: Create or change something in the subject's notation, such as code or an algorithm. (1 to 6 on Paper 1)
Write IGCSE: Create, change or amend something in the subject's notation, such as code.

Key terms 168

Abstraction: Removing unnecessary detail from a problem to focus on what matters.
Acceptable use policy (AUP): Rules that users of an organisation's computers agree to follow.
Access control: Accounts, passwords and permissions so users can only reach what they need.
Address bus: Carries memory addresses from the CPU to memory (one way).
Algorithm: A precise sequence of steps to solve a problem.
Algorithmic bias: Unfair decisions by an AI system, often learned from biased training data.
ALU (arithmetic logic unit): The part of the CPU that does calculations and logical comparisons.
Amplitude: The height of a sound wave at a moment, measured when sound is sampled.
Anti-malware: Software that detects and removes or quarantines malware.
Application software IGCSE: Software that lets users do tasks, such as a word processor or browser.
Arithmetic shift: A shift right that fills the gaps with copies of the sign bit, keeping the sign of a two's complement number.
Array: A data structure holding many items of the same type under one name, accessed by index (a list in Python).
ASCII: A 7-bit character set with 128 codes for English letters, digits, punctuation and control codes.
Assembler: Translates assembly language into machine code.
Assignment: Storing a value in a variable, such as total = 0.
Audit trail: A record of who accessed or changed what, and when.
Authentication: Confirming that a user is who they claim to be, for example with an ID and password.
Backup: A copy of data kept separately so it can be restored after loss or damage.
Bandwidth: The maximum amount of data that can be sent per second.
Binary: Base 2: the number system that uses only the digits 0 and 1.
Binary search: Repeatedly checks the middle item of a sorted list and discards half the list.
Bit: A single binary digit, 0 or 1.
Bit depth: The number of bits used to store each sound sample.
Bitmap: An image stored as a grid of pixels.
Boolean: A data type with only two values: True or False.
Boundary data: Valid test data at the very edge of the allowed range.
Bubble sort: Sorts by comparing neighbouring items and swapping them, in passes, until a pass makes no swaps.
Bus topology: All devices share one backbone cable with a terminator at each end.
Byte: 8 bits.
Cache IGCSE: A small amount of very fast memory close to the CPU that holds frequently used data and instructions.
Caesar cipher IGCSE: Encryption that shifts every letter the same number of places along the alphabet.
Character set: A list of characters a computer can use, each with a unique binary code.
Client-server network IGCSE: A network where a central server provides services, such as files or email, to client computers.
Clock: Sends regular pulses that keep the CPU's operations in time.
Clock speed IGCSE: The number of cycles the CPU performs per second, measured in hertz (for example 3 GHz).
Cloud storage IGCSE: Storing data on a provider's remote servers, reached over the internet.
Colour depth: The number of bits used to store the colour of each pixel.
Comment: A note in code for humans, ignored when the program runs. In Python it starts with #.
Compiler: Translates a whole high-level program into machine code before it runs, creating an executable file.
Compression: Making a file smaller, to save storage space and transmission time.
Computer Misuse Act 1990 IGCSE: UK law that makes unauthorised access to, and unauthorised modification of, computer material illegal.
Concatenation: Joining strings together, with + in Python.
Constant: A named value that does not change while the program runs.
Control bus: Carries control signals, such as read, write and clock pulses.
Control unit (CU): The part of the CPU that decodes instructions and controls the other components.
Copyright: Legal protection, automatic in the UK, for original work such as code, music and images.
Core IGCSE: A complete processing unit within a CPU; several cores can process instructions at the same time.
CSV file: A text file with one record per line and fields separated by commas.
Data bus: Carries data and instructions between the CPU and memory (two way).
Data protection law: In the UK, the Data Protection Act 2018 and UK GDPR, which control how personal data is used.
Decomposition: Breaking a problem into smaller, more manageable sub-problems.
Defragmentation: Rearranging a magnetic disk so the parts of each file are stored together, which speeds up access.
Digital divide IGCSE: The gap between people who have good access to technology and those who do not.
DNS (Domain Name System) IGCSE: Translates domain names, such as www.example.com, into IP addresses.
Embedded system: A computer built into a larger device to do one dedicated task.
Encryption: Scrambling data with an algorithm and a key so only someone with the key can read it.
Erroneous data: Invalid test data that should be rejected.
Ethernet: The protocol for sending data over wired local area networks.
Ethical hacking: Trying to break into a system, with permission, to find weaknesses so they can be fixed.
Fetch-decode-execute cycle: The repeated process in which the CPU fetches an instruction from memory, decodes it and carries it out.
Firewall: Hardware or software that checks network traffic and blocks anything that breaks its rules.
Flowchart: A diagram of an algorithm using standard symbols joined by arrows.
FTP: File Transfer Protocol: used to move files between computers over a network.
Function: A subprogram that returns a value.
Global variable: A variable declared in the main program that can be used anywhere in it.
Hexadecimal: Base 16, using digits 0 to 9 and A to F. One hex digit represents 4 bits.
High-level language: A language with English-like statements, such as Python, that must be translated and is portable.
HTML IGCSE: The language web pages are written in.
HTTP: Hypertext Transfer Protocol: requests and sends web pages between browsers and web servers.
HTTPS: HTTP with encryption, so intercepted data cannot be read.
IDE: Integrated development environment: software for writing, running and debugging code.
IMAP: Email protocol that keeps messages on the server and syncs them across devices.
Indentation: Spaces at the start of lines showing which code belongs to a block. Python requires it.
Initialisation: Giving a variable its first value before it is used.
Interpreter: Translates and runs a high-level program one line at a time, stopping at the first error.
IP address: A unique address for each device on a network or the internet.
IPv4 IGCSE: An IP address standard using 32 bits, written as four numbers from 0 to 255.
IPv6 IGCSE: An IP address standard using 128 bits, written in hexadecimal, giving vastly more addresses.
ISP IGCSE: Internet service provider: a company that gives people access to the internet.
Iteration: Repeating code, or visiting each item of a data structure in turn.
Kibibyte (KiB): 1024 bytes. Then MiB = 1024 KiB, GiB = 1024 MiB and TiB = 1024 GiB.
Kilobyte (kB) IGCSE: 1000 bytes, the decimal (SI) multiple. Then MB = 1000 kB and so on.
LAN: Local area network: covers a small area such as a home, school or office.
Latency: The delay before data starts to arrive.
Linear search: Checks each item in turn from the start of a list until the target is found or the list ends.
Local variable: A variable declared inside a subprogram that can only be used there.
Logic error: An error that lets a program run but gives the wrong result.
Logical shift: Moving all the bits left or right and filling the gaps with 0s.
Lossless compression: Makes a file smaller without losing any data, so the original can be rebuilt exactly.
Lossy compression: Makes a file much smaller by permanently removing some data, such as JPEG and MP3.
Low-level language: Machine code or assembly language: close to the hardware and specific to one type of processor.
Machine learning: A type of AI where a system learns patterns from data rather than being given every rule.
Malware: Malicious software, such as viruses, worms, Trojans, ransomware and key loggers.
Merge sort: Splits a list into single items, then merges pairs of lists back together in order.
Mesh topology: Devices connect to several others, giving many routes for data.
Modulus: The remainder after division, % in Python.
Nibble: 4 bits.
Normal data: Typical valid test data that should be accepted.
Open-source software: Software whose source code is available to study, modify and share under its licence.
Operating system: System software that manages files, processes, peripherals, users and the user interface.
Overflow: When a result is too large for the number of bits available to store it.
Packet: A small chunk of data sent across a network, carrying addresses and a sequence number.
PAN IGCSE: Personal area network: connects devices around one person, such as a phone and headphones by Bluetooth.
Parameter: A variable in a subprogram's definition that receives a value when the subprogram is called.
Patent: Legal protection for a new invention; it must be applied for.
Peer-to-peer network IGCSE: A network where all computers are equal and share resources directly.
Penetration testing: An authorised, simulated attack on a system to find weaknesses.
Personal data: Information about a living person who can be identified from it.
Pharming: Redirecting users to a fake website that looks genuine.
Phishing: Fake messages that look genuine, used to steal details or spread malware.
Pixel: The smallest element of a bitmap image: one dot of colour.
POP3: Email protocol that downloads messages from the server to one device.
Presence check: Validation that checks data has been entered.
Procedure: A subprogram that carries out a task but does not return a value.
Proprietary software: Software whose source code is kept secret, usually used under a paid licence.
Protocol: An agreed set of rules for sending and receiving data.
Qubit IGCSE: The unit of a quantum computer, which can be in a combination of 0 and 1 at the same time.
RAM: Volatile main memory holding the programs and data in use.
Range check: Validation that checks a value is between limits.
Ransomware: Malware that encrypts files and demands payment to unlock them.
Record: A data structure grouping related fields that can be of different types.
Register: A tiny, very fast store inside the CPU.
Resolution: The number of pixels in an image (width × height).
Ring topology IGCSE: Each device connects to exactly two others in a loop; data travels round in one direction.
ROM: Non-volatile memory holding instructions such as those used to start the computer.
Router: Forwards packets between networks towards their destination.
Run-length encoding (RLE) IGCSE: Lossless compression that stores each run of repeated data as a count and a value.
Runtime error: An error that makes a program crash while it is running, such as dividing by zero.
Sample interval: The time between sound samples: 1 ÷ sample rate.
Sample rate: The number of sound samples taken per second, in hertz.
Secondary storage: Non-volatile storage for files and programs: magnetic, optical or solid state.
Selection: Choosing which code runs using a condition, such as if, elif and else.
Sequence: Instructions carried out one after another in order.
Shoulder surfing: Watching someone enter a password or PIN.
Sign and magnitude IGCSE: Signed binary where the left bit is the sign (1 = negative) and the rest is the size.
Simulation IGCSE: Software that models a real system using abstraction, to test ideas safely.
SMTP: Email protocol used to send email to and between mail servers.
Social engineering: Tricking people into giving away information or access.
Solid state storage: Storage with no moving parts, such as an SSD or USB stick.
Star topology: Every device connects to a central switch.
Stored program concept: Program instructions and data are stored together in main memory.
String: A sequence of characters.
Subprogram: A named block of code that can be called: a function or a procedure.
Switch: Connects devices in a LAN and sends data only to the device it is meant for.
Syntax error: Code that breaks the rules of the language, so it cannot be translated or run.
System software: Software that runs and maintains the computer: the operating system and utilities.
TCP/IP model: Four layers for network communication: application, transport, internet and link.
Trace table: A table that tracks the value of each variable as an algorithm runs.
Trademark: Legal protection for a brand name, logo or slogan.
Trojan: Malware disguised as useful software.
Truth table: A table showing the output of a logic statement for every combination of inputs.
Two's complement: Signed binary where the left bit has a negative place value (-128 in 8 bits).
Unicode IGCSE: A character set that uses more bits than ASCII so it can represent characters from most of the world's languages.
URL IGCSE: A web address: protocol, domain name and path.
Utility software: Software that maintains the computer, such as backup, compression, defragmentation and anti-malware.
Validation: Checking that input data is sensible and allowed before it is used.
Variable: A named store whose value can change while the program runs.
Vigenère cipher IGCSE: Encryption that uses a keyword to give a different shift for each letter.
Virtual memory: Part of secondary storage used as extra memory when RAM is full; it is much slower.
Virus: Malware that attaches to a file and spreads when that file is run or shared.
Volatile: Loses its contents when the power is off, like RAM.
Von Neumann architecture: A computer design where instructions and data share the same main memory.
WAN: Wide area network: covers a large area and connects LANs, such as the internet.
Web server IGCSE: A computer that stores websites and sends pages to browsers.
Wi-Fi: The protocol for sending data over wireless local area networks.
Wireless access point (WAP) IGCSE: Lets wireless devices join a network.
World Wide Web IGCSE: The collection of web pages accessed over the internet.
Worm: Malware that copies itself across networks without any user action.

Conversion methods and file size formulas 15

ASCII codes: Codes run in order: A = 65, a = 97, 0 = 48, space = 32. If D = 68, then G = 71.
Binary addition: 0 + 0 = 0, 0 + 1 = 1, 1 + 1 = 0 carry 1, 1 + 1 + 1 = 1 carry 1. A carry out of the leftmost bit means overflow.
Binary place values (8 bits): 128, 64, 32, 16, 8, 4, 2, 1. Binary to denary: add the place values under each 1. Example: 10110110 = 128 + 32 + 16 + 4 + 2 = 182.
Construct an expression GCSE: Write the calculation with the right values and operators, such as 800 * 600 * 24 / 8 / 1024. You do not have to work it out unless the question asks. Paper 1 has no calculator.
Denary to binary: Work from 128 down: write 1 and subtract if the place value fits, otherwise write 0. Example: 203 = 11001011.
Hex to denary: Left digit × 16 + right digit. Example: B4 = 11 × 16 + 4 = 180. FF = 255.
Image file size: bits = width × height × colour depth. Divide by 8 for bytes. Example: 800 × 600 × 24 ÷ 8 = 1,440,000 bytes.
Number of states: n bits give 2^{n} different values. 8 bits give 256; 7-bit ASCII gives 128 characters.
Ranges in 8 bits GCSE: Unsigned: 0 to 255. Two's complement: -128 to +127.
Ranges in 8 bits IGCSE: Unsigned: 0 to 255. Two's complement: -128 to +127. Sign and magnitude: -127 to +127 (with two zeros).
Shifts: Logical shift left one place multiplies by 2; logical shift right one place divides by 2 (losing any remainder); both fill with 0. Arithmetic shift right fills with copies of the sign bit. Example: 00010110 (22) left 2 = 01011000 (88).
Sound file size: bits = sample rate × bit depth × seconds (for one channel). Example: 8000 × 16 × 10 ÷ 8 = 160,000 bytes.
Transmission time: time (s) = file size (bits) ÷ transmission rate (bits per second). Convert bytes to bits by multiplying by 8. Example: 2,000,000 bytes at 8 megabits per second = 16,000,000 ÷ 8,000,000 = 2 s.
Two's complement: make a negative: Write the positive number in binary, flip every bit, add 1. Example: -5: 00000101, flip 11111010, add 1 = 11111011.
Two's complement: read a number: The left bit is worth -128. Example: 10000001 = -128 + 1 = -127.

Pearson pseudocode and Python 19

& IGCSE: Joins strings. Python: +
= and <> IGCSE: Equal to and not equal to in conditions. Python: == and !=
^ IGCSE: To the power of. Python: ∗∗ (two asterisks)
CONST VAT_RATE = 0.2 IGCSE: A constant, set once. Python: VAT_RATE = 0.2 (capitals by convention)
FOR EACH item FROM list DO ... END FOREACH IGCSE: Visits every item. Python: for item in list:
FOR i FROM 1 TO 10 DO ... END FOR IGCSE: Count-controlled loop including both ends. Python: for i in range(1, 11):
FUNCTION ... RETURN ... END FUNCTION IGCSE: A subprogram that returns a value. Python: def with return
IF ... THEN ... ELSE ... END IF IGCSE: Selection. Python: if ...: then else:
LENGTH(name) IGCSE: Length of a string or array. Python: len(name)
MOD and DIV IGCSE: Remainder and integer division. Python: % and //
PROCEDURE ... END PROCEDURE IGCSE: A subprogram that does not return a value. Python: def name(parameters):
RANDOM(6) IGCSE: A random whole number from 0 to 6. Python: random.randint(0, 6)
READ file record / WRITE file record IGCSE: File handling. Python: open() with "r", "w" or "a", then read or write
RECEIVE name FROM (STRING) KEYBOARD IGCSE: Input with its data type. Python: name = input()
REPEAT ... UNTIL ... IGCSE: Post-condition loop: always runs at least once, stops when the condition is True. Python has no direct equivalent; use while with a condition set to run once.
REPEAT 5 TIMES ... END REPEAT IGCSE: Count-controlled loop. Python: for count in range(5):
SEND total TO DISPLAY IGCSE: Output. Python: print(total)
SET total TO 0 IGCSE: Assignment. Python: total = 0
WHILE ... DO ... END WHILE IGCSE: Pre-condition loop: may run zero times. Python: while ...:

Practise Computer Science free Revision checklist