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GCSE Computer Science key terms

Computational thinking, the CPU, data, networks, security, databases, programming and the law: the words and what they mean.

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Abstraction
Abstraction is removing the details that do not matter, so you can focus on the ones that do.
Decomposition
Decomposition is breaking a big problem into smaller problems that are each easier to solve.
Algorithmic thinking
Algorithmic thinking is working out the exact steps, in order, that solve a problem.
Algorithm
An algorithm is a sequence of steps that solves a problem.
Structure diagram
A structure diagram draws a decomposition: the whole problem in the top box, its subsections in a row below, and the smaller tasks of each subsection below.
Decision (flowchart symbol)
A decision is a flowchart symbol with two arrows out of it, labelled Yes and No.
Subroutine (flowchart symbol)
A subroutine box in a flowchart runs another part of the program, then comes back.
Input
Input is the data that goes into a program: typed, read from a sensor or loaded from a file.
Show all 142
Process
Process is what a program does to its input data, such as calculations, decisions or sorting.
Output
Output is what comes back from a program: data shown on screen, printed, sent or saved, or a sound.
Linear search
A linear search checks each item in turn, from the first, until it finds what it is looking for or runs out of items; it works on any list, sorted or not.
Binary search
A binary search only works on a sorted list: it compares the target with the middle item, then throws away the half that cannot hold the target.
Logic gates
Logic gates have inputs and an output that are each 1 (true, on) or 0 (false, off); every decision inside a computer is built from logic gates.
AND (logic gate)
AND is a logic gate whose output is 1 only when both inputs are 1.
OR (logic gate)
OR is a logic gate whose output is 1 when at least one input is 1.
XOR (logic gate)
XOR (exclusive OR) is a logic gate whose output is 1 when the inputs are different.
NOT (logic gate)
NOT is a logic gate with one input, whose output is the input flipped.
Expression
An expression describes a circuit in writing, where one gate's output becomes another's input.
CPU
The CPU (central processing unit) fetches and carries out instructions.
ALU
The ALU (arithmetic logic unit) does the calculations and comparisons in the CPU.
Control unit
The control unit is the part of the CPU that decodes instructions and sends signals to make the other parts act, in step with the clock.
Cache
Cache is a small amount of very fast memory on or near the CPU.
Registers
Registers are tiny, very fast stores inside the CPU, each with one job.
Von Neumann architecture
The von Neumann architecture stores program instructions and data together in the same main memory, and the CPU fetches them one at a time.
Buses
Buses connect the CPU to memory: the address bus carries addresses, the data bus carries data and the control bus carries signals such as read and write.
Clock speed
Clock speed is the speed of the CPU's clock, in cycles per second (hertz): 3.2 GHz is 3 200 000 000 cycles a second; a faster clock means more instructions a second, but more heat.
Cores
Each core is a separate processor, so several cores can work on different tasks at once.
Embedded system
An embedded system is a computer built into a larger device to do one dedicated job, such as a washing machine's controller or a thermostat.
Binary
Binary is base 2: it uses only the digits 0 and 1, each place value worth twice the one to its right; computers store everything as binary because a circuit is either off or on.
Hexadecimal
Hexadecimal is base 16, using the digits 0 to 9 and then A to F for 10 to 15.
Binary prefixes
Binary prefixes are units whose names say they use 1024 instead of 1000, because 1024 = 2^10.
Kibibyte
A kibibyte (KiB) is 1024 bytes.
Mebibyte
A mebibyte (MiB) is 1024 KiB, which is 1024 x 1024 bytes.
Gibibyte
A gibibyte (GiB) is 1024 MiB.
Tebibyte
A tebibyte (TiB) is 1024 GiB.
Overflow
Overflow is when a result needs more bits than there are, so in an 8-bit register the ninth bit is lost and the stored answer is wrong.
Left shift
A left shift moves every bit left and fills the gap on the right with 0; each place multiplies the value by 2.
Right shift
A right shift moves every bit right, filling with 0 on the left; each place divides by 2, and the result rounds down.
Database
A database is an organised, permanent store of data that can be searched, sorted and updated.
Tables
A table is made of records and fields: each row is a record and each column is a field; the data in a database sits in tables.
Record
A record is one row in a database table, such as everything stored about one film.
Record (in a program)
A record in a program groups related fields, which can have different data types.
Field
A field is one column in a database table: one piece of data stored for every record.
Primary key
A primary key is a field whose value is different for every record, so it identifies each one.
Relational database
A relational database keeps data in separate tables linked by keys.
Foreign key
A foreign key is a field in one table that holds the primary key of another table.
Data redundancy
Data redundancy is the same data stored more than once.
Data inconsistency
Data inconsistency is when the same data is stored more than once and one copy is changed but another is missed.
2D array
A 2D array holds a table in a program: a table of records can be held in a 2D array, with one row per record and one column per field.
SQL
SQL (structured query language) is a language that asks questions of a database.
SELECT
SELECT names the fields to show in an SQL query; SELECT * shows every field.
FROM
FROM is the part of an SQL query that names the table.
WHERE
WHERE keeps only the records that meet a condition in an SQL query.
ORDER BY
ORDER BY sorts the result of an SQL query, in ASC (the default) or DESC order.
ASC
ASC sorts smallest, earliest or A first, and is the default for ORDER BY.
DESC
DESC sorts largest, latest or Z first in ORDER BY.
% (SQL wildcard)
The % wildcard in SQL stands for any number of characters, even none, when LIKE matches a pattern.
_ (SQL wildcard)
The _ wildcard in SQL stands for exactly one character when LIKE matches a pattern.
Stakeholders
Stakeholders are the different people who feel the benefits and costs of technology.
Privacy
Privacy is about data people may not want shared, collected by location tracking, smart speakers, face recognition and monitoring at work.
Fairness
Fairness means important decisions should be explainable and open to challenge, since an AI trained on biased data repeats the bias.
Jobs (impact of technology)
Automation removes some jobs and creates others with different skills.
Wellbeing (impact of technology)
Wellbeing is the impact of technology on people, through screen time, cyberbullying and designs that keep people scrolling or spending.
Culture and access
Culture and access is about the digital divide and accessibility features such as screen readers and captions.
Digital divide
The digital divide is the gap between people with good access and skills and those without.
Data Protection Act 2018
The Data Protection Act 2018 (the UK's version of the GDPR) controls how organisations use personal data.
Personal data
Personal data is data about an identifiable living person.
Data subject
The data subject is the living person the data is about, who can see their data and have it corrected or deleted.
Computer Misuse Act 1990
The Computer Misuse Act 1990 is the law that made computer misuse, such as hacking and malware, an offence.
Copyright, Designs and Patents Act 1988
The Copyright, Designs and Patents Act 1988 protects original work (code, music, images, text) as soon as it is made; copying it without permission is illegal.
Patent
A patent protects a registered invention.
Licences
Licences say how software, or any creative work, may be studied, changed, shared or reused.
Open source
Open source software publishes its source code, which anyone may study, change and share under the licence; it is usually free.
Proprietary
Proprietary software keeps its code private and charges for use, but usually includes support and updates.
Machine code
Machine code is binary instructions the CPU carries out directly; each processor family has its own.
Assembly language
Assembly language is machine code instructions written as short mnemonics (LDA, ADD, STA) with addresses, one line per instruction.
High-level languages
High-level languages (Python, C#, Java) are close to English, so quicker to write, read and debug, and run on different processors once translated.
Mnemonics
Mnemonics are the short written instructions of assembly language, such as LDA, ADD and STA; one assembly line is one machine code instruction.
Source code
Source code is code that must be translated into machine code, because the CPU only understands machine code.
Assembler
An assembler translates assembly language into machine code, one instruction for one instruction.
Network
A network is two or more devices connected so they can share data and resources.
LAN
A LAN (local area network) is a network over a small area such as one building, where the organisation owns the hardware.
WAN
A WAN (wide area network) links LANs across a large area over lines rented from other companies.
Client-server
Client-server means a central server stores files, accounts and services, and clients request them.
Peer-to-peer
Peer-to-peer means every computer is both client and server, with no central backup or security.
Transmission media
Transmission media means copper cable (cheap, short distances), fibre optic (light, very fast over long distances) or radio waves (Wi-Fi).
Performance (network)
Network performance depends on bandwidth, the number of users sharing it, the media, distance, interference and latency.
Star (topology)
In a star topology, every device has its own cable to a central switch; a failed switch stops everything.
Mesh (topology)
In a mesh topology, devices are linked to several others, so data can take another route if a link fails.
Bus (topology)
In a bus topology, every device has a short drop cable to one shared main cable, the backbone, with a terminator at each end.
Backbone
The backbone is the shared main cable of a bus network, and every device has a short drop cable to the backbone.
Terminator
A terminator is the part at each end of the backbone in a bus network that absorbs the signal.
Internet
The internet is a worldwide network of networks.
World Wide Web
The World Wide Web is the collection of pages reached over the internet.
Web hosting
Web hosting stores a website on a server connected to the internet.
The cloud
The cloud is remote servers that store data and run services, reachable anywhere with a connection.
IPv4
An IPv4 address is 32 bits, written as four denary numbers 0 to 255, and is used to route packets between networks.
IPv6
An IPv6 address is 128 bits, written as eight groups of hex digits, because IPv4 ran out of addresses.
MAC address
A MAC address is the address fixed into a device's NIC when it is made, used on the local network: 48 bits, written as six pairs of hex digits.
Subroutine
A subroutine is a named block of code.
Function
A function is a subroutine that returns a value with return.
Procedure
A procedure is a subroutine that does a job, such as printing, and returns nothing.
Parameters
Parameters are the names in brackets on the def line of a subroutine.
Arguments
Arguments are the values passed in when a subroutine is called.
Main program
The main program is the code outside the subroutines: it reads the input, calls the subroutines and prints what they return.
Syntax error
A syntax error breaks the rules of the language, such as a missing colon or bracket, so the program will not run at all.
Runtime error
A runtime error happens while the program runs: the program starts, then stops part way with an error message, such as dividing by zero.
Logic error
A logic error lets the program run to the end, but the result is wrong, such as a total that is one item short.
Data type
A data type is the kind of value something is, which decides what can be done with the value; every value in a program has one.
Casting
Casting changes a value's type: str(7) gives the text "7", and int("42") gives the number 42.
String
A string is a data type for text; joining two strings with + sticks them together, so "4" + "5" is "45".
Variable
A variable is a named place in memory whose value can change.
Constant
A constant keeps the same value for the whole run, such as const VAT_RATE = 0.2, so the value cannot be changed by mistake.
DIV
DIV gives the whole number part of a division: 17 DIV 5 = 3.
MOD
MOD gives the remainder of a division: 17 MOD 5 = 2.
AND (Boolean operator)
AND is a Boolean operator that is true only if both sides are true.
OR (Boolean operator)
OR is a Boolean operator that is true if either side is true, or both.
NOT (Boolean operator)
NOT is a Boolean operator that flips true and false.
If (statement)
An if statement chooses which code runs: the conditions are checked from the top and the first true one runs.
Count-controlled (loop)
A count-controlled loop, such as a for loop, runs a set number of times.
For (loop)
A for loop is count-controlled: for i = 1 to 5 runs with i = 1, 2, 3, 4, 5, both ends included.
Condition-controlled, checked first
A condition-controlled loop checked first, such as a while loop, checks its condition before each pass, so it might not run at all.
While (loop)
A while loop is condition-controlled and checks its condition before each pass, so a while loop might not run at all.
Condition-controlled, checked last
Condition-controlled, checked last, means the body runs and then the condition is checked, so the body always runs at least once.
Repeat until (loop)
A repeat until loop runs its body, then checks, so it always runs at least once; Python has no repeat loop.
Malware
Malware is software written to cause harm or to steal.
Social engineering
Social engineering tricks people into giving away information or access.
Phishing
Phishing is fake emails or texts that look genuine and push you to click a link or give details.
Blagging
Blagging (pretexting) is inventing a story, often by phone, to get information.
Shouldering
Shouldering is watching someone type a PIN or password.
Brute force (attack)
Brute force is trying every possible password, or a list of common ones, until one works.
Denial of service (DoS)
Denial of service (DoS) is flooding a server with requests so real users cannot get through.
Distributed (DoS)
A distributed DoS floods a server with requests from many computers at once, often a botnet.
SQL injection
SQL injection is typing SQL into an input box on a site that puts input straight into its database queries, so the extra code runs.
Data interception
Data interception is capturing data as it travels, for example with a packet sniffer on open Wi-Fi.
Validation
Validation checks that input is sensible before a program uses it, but cannot check that the data is true.
Range check
A range check is a validation check that a number is between two limits, such as a month from 1 to 12.
Length check
A length check is a validation check that text has an allowed number of characters.
Presence check
A presence check is a validation check that something has been entered at all.
Format check
A format check is a validation check that text follows a pattern, such as two letters then three digits.
Type check
A type check is a validation check that the input is the right kind of data, such as a whole number.
Authentication
Authentication checks that a user is who they say they are, usually with a username and a password.