OCR GCSE Computer Science (J277) key words

OCR GCSE Computer Science (J277)

OCR GCSE Computer Science (J277) key words

OCR GCSE (9-1) Computer Science (J277) · specification version 3.1 (May 2026), for exams in 2027 and 2028

1.1 · Systems architecture

1.1.1 Architecture of the CPU

CPU: Central processing unit: the part of a computer that processes data and carries out program instructions by repeating the fetch-execute cycle.
fetch-execute cycle: The cycle the CPU repeats all the time it is on: fetch the next instruction from memory, decode it, then execute it. Also called the fetch-decode-execute cycle.
fetch: The stage where the next instruction is copied from main memory into the CPU, using the address held in the program counter.
decode: The stage where the control unit works out what the fetched instruction means and what has to happen to carry it out.
execute: The stage where the instruction is carried out, for example a calculation in the ALU or a value read from or written to memory.
instruction: One simple command the CPU can carry out, such as load a value, add two numbers or store a result. A program is a long list of them.
ALU: Arithmetic logic unit: the part of the CPU that does arithmetic, such as adding and subtracting, and logical operations and comparisons, such as checking whether one value is bigger.
control unit: CU: the part of the CPU that decodes instructions, coordinates the fetch-execute cycle and sends signals that control when parts act and which way data moves.
cache: Small, very fast memory on or next to the CPU. It keeps copies of data and instructions the CPU is likely to need again, so it waits less for slower RAM.
register: A tiny, extremely fast store inside the CPU that holds a single item, such as an address, an instruction or a result, while it is being used.
von Neumann architecture: A computer design in which instructions and data are kept together in the same main memory, and the CPU fetches and runs the instructions one after another.
stored program concept: The idea behind von Neumann architecture: a program sits in memory beside its data, so a computer can run any program without being rebuilt.
memory address: A number that identifies one location in main memory, like a house number, so the CPU can find the instruction or data stored there.
MAR: Memory address register: holds the address of the memory location that is about to be read from or written to.
MDR: Memory data register: holds the data or instruction just fetched from memory, or the data about to be written to memory.
program counter: PC: the register that holds the address of the next instruction to fetch. It goes up by one after each fetch unless a jump changes it.
accumulator: The register that holds the result of a calculation done by the ALU, such as a running total, until it is used or stored.

1.1.2 CPU performance

clock speed: How many fetch-execute cycles the CPU can carry out each second, measured in hertz. A 3.5 GHz CPU can manage about 3.5 billion cycles a second.
hertz: Hz: the unit of frequency, one cycle per second. Clock speeds are given in gigahertz (GHz): billions of cycles a second.
cache size: How much cache a CPU has. More cache means more data and instructions are close at hand, so fewer slow trips to RAM are needed.
core: A complete processing unit inside a CPU with its own control unit, ALU and registers. Each core can run instructions at the same time as the others.
multi-core processor: A CPU with two or more cores, such as dual-core or quad-core. Extra cores help most when the work can be split into parts that run at once.

1.1.3 Embedded systems

embedded system: A computer built into a larger device to do one dedicated job, such as controlling a washing machine, a pacemaker or a car's anti-lock brakes.
general-purpose computer: A computer, such as a laptop or desktop, that can run many different programs for many tasks, unlike an embedded system.
firmware: Software stored permanently in a device's ROM or flash memory to control its hardware, such as the program inside a microwave.

1.2 · Memory and storage

1.2.1 Primary storage (memory)

primary storage: Memory the CPU can reach directly, such as RAM, ROM and cache. It holds the instructions and data the computer needs right now.
RAM: Random access memory: volatile memory holding the operating system, the programs that are running and their data. Its contents can be read and changed.
ROM: Read-only memory: non-volatile memory whose contents normally cannot be changed. It holds the start-up instructions the computer needs when it is switched on.
volatile: Describes memory that loses everything it holds when the power is switched off, such as RAM.
non-volatile: Describes memory or storage that keeps its contents without power, such as ROM, flash memory and hard disks.
boot: To start up a computer: instructions in ROM check the hardware and load the operating system from secondary storage into RAM.
virtual memory: Part of secondary storage used as extra RAM when RAM is full. Data not needed at the moment is moved out to it, then moved back when needed.

1.2.2 Secondary storage

secondary storage: Non-volatile storage that keeps programs and files long term, such as a hard disk or SSD. The CPU cannot use it directly: data is loaded into RAM first.
storage medium: The material that data is actually written on, such as a disc, tape or flash chip. The storage device is the hardware that reads and writes it.
magnetic storage: Storage that records data as magnetised areas on a surface, such as a hard disk drive or magnetic tape. High capacity and cheap, but has moving parts.
HDD: Hard disk drive: magnetic storage with spinning platters and a moving read/write head. Large and cheap per gigabyte, but slower than an SSD and easily damaged if dropped.
magnetic tape: Magnetic storage on a long reel of tape that is read in order. Very cheap for huge amounts of data, so used for backups and archives.
optical storage: Storage on a disc read and written by a laser, such as a CD, DVD or Blu-ray. Cheap and light, but low capacity and easily scratched.
solid-state storage: Storage with no moving parts that keeps data in flash memory chips, such as SSDs, USB memory sticks and SD cards. Fast and tough, but dearer per gigabyte.
flash memory: Non-volatile memory chips that store data electronically and can be rewritten many times. Solid-state storage is built from it.
SSD: Solid-state drive: solid-state storage used in place of a hard disk. Much faster to read and write, silent and shock-resistant, but costs more per gigabyte.
capacity: How much data a storage device can hold, given in units such as gigabytes or terabytes.
speed (storage): How quickly data can be read from or written to a storage device. SSDs are much faster than hard disks, which are faster than optical discs.
portability: How easily a storage device can be carried about, depending on its size and weight.
durability: How well a storage device survives knocks, drops, heat or water. Solid-state storage is durable because it has no moving parts.
reliability: How likely a storage device is to keep working and keep data safe over many uses and many years.
cost per gigabyte: The price of a storage device divided by its capacity, used to compare value. Magnetic storage is usually cheapest per gigabyte.

1.2.3 Units

binary: A number system with base 2 that uses only 0 and 1. Computers use it because their circuits are made of switches that are either off or on.
bit: Binary digit: the smallest unit of data, a single 0 or 1.
nibble: A group of 4 bits, half a byte. One nibble matches one hexadecimal digit.
byte: A group of 8 bits, enough to store one ASCII character.
kilobyte: KB: 1000 bytes. In the exam using 1024 bytes is also accepted.
megabyte: MB: 1000 kilobytes, about a million bytes. A typical photo or song is a few megabytes.
gigabyte: GB: 1000 megabytes. RAM sizes and phone storage are usually given in gigabytes.
terabyte: TB: 1000 gigabytes. Hard disks and large SSDs are measured in terabytes.
petabyte: PB: 1000 terabytes, the largest unit in the course. Data centres hold many petabytes.
file size: The amount of storage a file takes up. Calculations give an answer in bits; divide by 8 for bytes, then by 1000 for each larger unit.
sound file size: Sample rate (Hz) × duration (seconds) × bit depth, giving the size in bits.
image file size: Width (pixels) × height (pixels) × colour depth, giving the size in bits.
text file size: Bits per character × number of characters, giving the size in bits.

1.2.4 Data storage

denary: The base 10 number system we normally use, with digits 0 to 9. Also called decimal.
place value: The value of a column in a number. In 8-bit binary the columns are worth 128, 64, 32, 16, 8, 4, 2 and 1.
most significant bit: MSB: the leftmost bit of a binary number, the one with the largest place value (128 in an 8-bit number).
least significant bit: LSB: the rightmost bit of a binary number, worth 1. It shows whether the number is odd or even.
leading zero: A 0 at the left of a binary number that does not change its value, so 101 and 00000101 are both 5.
binary addition: Adding binary numbers column by column from the right: 0+0=0, 0+1=1, 1+1=0 carry 1, and 1+1+1=1 carry 1.
carry: The 1 passed into the next column to the left when a column's total is 2 or more in binary addition.
overflow: When the result of a calculation needs more bits than are available, such as an 8-bit sum above 255. The extra bit is lost, so the answer is wrong.
hexadecimal: The base 16 number system, using 0 to 9 then A to F for 10 to 15. Two hex digits show one byte, from 00 to FF (0 to 255).
binary shift: Moving every bit of a binary number a set number of places to the left or right, filling the gaps with 0s.
left shift: Moving the bits left. Each place multiplies the number by 2, so a 2-place left shift multiplies by 4. Bits pushed off the left are lost.
right shift: Moving the bits right. Each place divides the number by 2, losing any remainder, because bits pushed off the right end are lost.
character set: A defined list of characters a computer can show, each matched to a unique binary code. Both computers must use the same set to read text correctly.
character code: The number given to a character in a character set. Codes follow the alphabet, so if A is 65, B is 66.
ASCII: American Standard Code for Information Interchange: a character set using 7 bits for 128 English characters. In the exam it is shown as 8 bits per character.
extended ASCII: ASCII using 8 bits per character, which gives 256 codes and adds accented letters and extra symbols.
Unicode: A character set using more bits per character (up to 32), so it can hold characters from almost every language plus emoji. Files take more space than ASCII.
bitmap image: An image stored as a grid of pixels, each with a binary code for its colour.
pixel: Picture element: the smallest dot of colour in an image. Each pixel's colour is stored as a binary number.
colour depth: The number of bits used for each pixel's colour. n bits allow 2 to the power n colours: 1 bit gives 2, 8 bits give 256.
resolution: The number of pixels in an image, given as width × height, such as 1920 × 1080. More pixels give more detail but a larger file.
metadata: Data about data. An image's metadata includes its width, height and colour depth, so the computer can rebuild the picture from the pixel codes.
analogue: Describes data that changes smoothly and continuously, like real sound waves. It must be turned into digital data before a computer can store it.
sample: One measurement of the height (amplitude) of a sound wave at one moment, stored as a binary number.
sample rate: How many samples are taken each second, measured in hertz. A higher rate gives a closer copy of the sound and a bigger file.
bit depth: The number of bits used to store each sound sample. More bits give more possible values, so better quality and a larger file.
duration: How many seconds of audio a sound file holds. Doubling the duration doubles the file size but does not change the quality.

1.2.5 Compression

compression: Making a file smaller, so it takes less storage and is quicker to send over a network or download.
lossy compression: Compression that permanently removes data people are unlikely to notice. Files become much smaller, but the original can never be rebuilt exactly. Used for photos, music and video.
lossless compression: Compression that stores data more efficiently without losing any, so the original file can be rebuilt exactly. Needed for text and program files.
run-length encoding: RLE: a lossless method that stores a repeated value once with a count, so AAAAA becomes 5A. Shown as an example only; carrying it out is not required.

1.3 · Computer networks, connections and protocols

1.3.1 Networks and topologies

network: Two or more computers or devices connected so they can share data, resources such as printers, and an internet connection.
stand-alone computer: A computer that is not connected to any network, so it cannot share files, printers or an internet connection.
LAN: Local area network: a network covering one small area, such as a school or a home, using hardware the owner controls.
WAN: Wide area network: a network covering a large area, such as a country or the world, often linking LANs. It usually uses hired connections such as telephone lines or satellites.
bandwidth: The amount of data that can be sent across a connection in a given time, measured in bits per second (such as Mbps).
interference: Disruption of a signal, for example by walls, distance or other wireless devices, which causes errors and data to be sent again.
client-server network: A network where clients send requests to one or more central servers, which store files, manage users and security, and run backups.
server: A computer that provides a service, such as storing files, hosting websites or handling email, to client computers that request it.
client: A computer or device that asks a server for a service or data, such as a web browser asking for a page.
peer-to-peer network: A network with no central server: each computer is equal, stores its own files and can share them directly with the others.
NIC: Network interface controller (card): the hardware that lets a device connect to a network. Each one has its own MAC address.
switch: A device that links devices on a LAN and sends each frame only to the device it is meant for, using MAC addresses.
router: A device that sends packets between different networks, such as from a LAN to the internet, using IP addresses.
WAP: Wireless access point: a device that lets wireless devices join a wired network by sending and receiving radio signals.
transmission media: What carries data between devices: copper cable, fibre optic cable or radio waves.
fibre optic cable: Cable that sends data as pulses of light through glass fibres. Very high bandwidth over long distances, with no electrical interference, but costly.
the internet: A worldwide network of networks: millions of networks joined together and able to exchange data.
DNS: Domain Name Server (the whole service is the Domain Name Service): servers that turn the domain name in a URL into the IP address of the server that holds the site.
domain name: The readable name of a website, such as bbc.co.uk, used instead of an IP address.
URL: Uniform resource locator: the full web address of a resource, such as https://www.example.com/page.html.
web server: A server that stores web pages and sends them to clients when they are requested.
file server: A server that stores files centrally so that clients on the network can save and open them.
hosting: Storing a website or files on a server that is connected to the internet so others can reach them, often run by a hosting company.
the cloud: Remote servers reached over the internet that store data, run software or provide services, so users do not need them on their own devices.
topology: The way devices on a network are arranged and connected.
star topology: A topology where every device connects to a central switch. One failed cable affects only one device, but if the switch fails the whole network stops.
mesh topology: A topology where devices connect to many or all of the others, so data can take another route if one link fails. Reliable, but needs lots of cabling or links.

1.3.2 Wired and wireless networks, protocols and layers

wired connection: A network connection through a cable. Faster and more reliable and secure than wireless, but devices cannot move about freely.
Ethernet: The family of standards used for wired LAN connections.
wireless connection: A network connection using radio waves, so devices can move about without cables. Slower, with shorter range, and easier to intercept than wired.
Wi-Fi: A family of wireless standards for connecting devices to a LAN through a wireless access point.
Bluetooth: A short-range wireless standard for linking two nearby devices directly, such as headphones and a phone.
encryption: Scrambling data with a key so it cannot be understood by anyone without the matching key to decrypt it. It protects data that is intercepted.
encryption key: The value used with an encryption method to scramble and unscramble data.
IP address: Internet protocol address: a number given to a device on a network so data can be routed to it. It can change when the device joins a different network.
IPv4: IP addresses of 32 bits, written as four denary numbers from 0 to 255, such as 192.168.1.20. There are only about 4.3 billion of them.
IPv6: IP addresses of 128 bits, written as eight groups of four hex digits separated by colons. There are enough for every device for the foreseeable future.
MAC address: Media access control address: a unique 48-bit address set into each network interface when it is made, written as six pairs of hex digits. Used to find a device on a LAN.
standard: An agreed set of rules for hardware or software, so products from different makers work together.
protocol: A set of rules for how data is sent and received across a network, such as how it is formatted and how errors are handled.
packet: A small piece of data sent across a network, with a header holding information such as the sender's and receiver's addresses.
TCP/IP: Transmission Control Protocol/Internet Protocol: the protocols the internet runs on. TCP splits data into packets and checks they arrive; IP addresses and routes them.
HTTP: Hypertext Transfer Protocol: the rules a web browser and web server use to request and send web pages.
HTTPS: Hypertext Transfer Protocol Secure: HTTP with encryption, so data such as passwords and card details cannot be read if intercepted.
FTP: File Transfer Protocol: rules for sending files between computers across a network, for example uploading a website to a server.
POP: Post Office Protocol: rules for downloading emails from a mail server to one device, after which they are usually deleted from the server.
IMAP: Internet Message Access Protocol: rules for reading emails that stay on the mail server, so they stay in step across several devices.
SMTP: Simple Mail Transfer Protocol: rules for sending emails from a client to a mail server and between mail servers.
layer: One part of a group of protocols with a set job. Each layer uses the layer below and serves the layer above, so each can be changed on its own.
TCP/IP model: A four-layer model (application, transport, internet, link) used as an example of layering. Layer names and jobs are not examined.

1.4 · Network security

1.4.1 Threats to computer systems and networks

malware: Malicious software: any program written to harm a computer or its user, for example by deleting files, spying or demanding money.
virus: Malware that attaches itself to a file or program and spreads when that file is run or shared.
worm: Malware that copies itself and spreads across a network on its own, without needing to attach to a file or be opened.
trojan: Malware disguised as useful or harmless software, which tricks users into installing it.
ransomware: Malware that encrypts a victim's files and demands payment for the key to unlock them.
spyware: Malware that secretly collects information, such as passwords typed or websites visited, and sends it to an attacker.
social engineering: Tricking people, rather than breaking into computers, into giving away information or access. People are often the weakest point in a system's security.
phishing: Social engineering by fake emails, messages or websites that look genuine and ask the user to reveal personal details or click a harmful link.
brute-force attack: Trying every possible password or key, often automatically, until the right one is found. Short, simple passwords are found fastest.
DoS attack: Denial of service attack: flooding a server or network with so many requests that it cannot respond to genuine users.
DDoS attack: Distributed denial of service attack: a DoS attack sent from many devices at once, often a botnet, so it is harder to block.
botnet: A network of devices infected with malware and controlled by an attacker, for example to send spam or launch a DDoS attack.
data interception and theft: Capturing data as it travels across a network, for example on open Wi-Fi, so it can be read, stolen or changed. Encryption makes captured data useless.
packet sniffer: Software that captures and examines packets travelling across a network. Attackers use it to intercept unencrypted data.
SQL injection: Typing SQL code into a website's input box so that it runs on the database, for example to show or delete records the user should not see.
vulnerability: A weakness in a system, such as an unpatched program or a weak password, that an attacker could use.

1.4.2 Identifying and preventing vulnerabilities

penetration testing: Hiring experts to attack a system on purpose, with permission, to find weaknesses so they can be fixed before real attackers find them.
anti-malware software: Software that scans files and memory for malware, blocks it, and quarantines or removes it. It must be updated to recognise new threats.
firewall: Hardware or software that checks data coming into and leaving a network and blocks anything that does not meet its set rules.
user access levels: Settings that control which files and programs each user can see or change, so people only reach the data their job needs.
password: A secret string a user enters to prove who they are. Strong passwords are long and mix upper and lower case letters, digits and symbols.
authentication: Checking that a user really is who they say they are, for example with a password, a code sent to a phone or a fingerprint.
two-factor authentication: 2FA: logging in needs two different kinds of proof, such as a password and a code sent to the user's phone.
biometrics: Using a person's body features, such as a fingerprint or face, to identify them.
account lockout: Blocking a login after a set number of wrong password attempts, which defeats brute-force attacks.
physical security: Protecting hardware and data from being touched or taken, for example with locks, ID cards, alarms and CCTV.
input sanitisation: Cleaning user input by removing or blocking characters that could be harmful, such as SQL code, before it is used. Helps stop SQL injection.

1.5 · Systems software

1.5.1 Operating systems

systems software: Software that runs and looks after the computer itself, rather than doing a user's tasks: the operating system and utility software.
operating system: OS: the software that manages the hardware and runs other programs. It provides the user interface and handles memory, multitasking, peripherals, users and files.
user interface: The way a user interacts with a computer, such as clicking icons, typing commands or speaking.
GUI: Graphical user interface: an interface of windows, icons, menus and a pointer, easy for beginners but needing more memory and processing.
command-line interface: CLI: an interface where the user types text commands. Quick for experts and uses few resources, but commands must be learned.
memory management: The operating system's job of deciding where programs and data go in RAM, keeping them apart, freeing space when they close and using virtual memory when needed.
multitasking: Running several programs seemingly at once. The OS gives each program a short turn on the CPU in rapid rotation and keeps their memory apart.
time slice: A short period of CPU time the operating system gives a program before switching to the next.
peripheral: A device connected to a computer but not part of its core, such as a printer, keyboard or monitor.
device driver: Software that lets the operating system talk to one particular peripheral, turning general commands into ones the device understands.
user management: The operating system's job of looking after users: setting up accounts, logins and passwords, and giving each account its access rights.
file management: The operating system's job of organising files into folders, naming, moving, copying and deleting them, and controlling who can open them.

1.5.2 Utility software

utility software: Extra programs that do housekeeping jobs the operating system may not, such as encryption, defragmentation and data compression software.
encryption software: A utility that encrypts files or whole drives so their contents cannot be read without the key, protecting them if the device is stolen.
fragmentation: When parts of a file are stored in separate, scattered places on a hard disk, because files have been added and deleted over time.
defragmentation: A utility that rearranges a hard disk so each file's parts sit together and free space is gathered, making files quicker to read. Not needed for SSDs.
compression software: A utility that makes files smaller to save storage space or speed up sending them, and decompresses them again.

1.6 · Ethical, legal, cultural and environmental impacts of digital technology

1.6.1 Ethical, legal, cultural, environmental and privacy issues

ethical issue: A question of right and wrong raised by technology, such as whether it is fair for AI to decide who gets a job.
legal issue: A question about whether something breaks the law, such as copying software or gaining access to data without permission.
cultural issue: How technology changes the way people live, work and behave, such as remote working or social media changing how people communicate.
environmental issue: How technology affects the planet, including energy use, mining for materials and e-waste, but also benefits such as fewer journeys.
privacy issue: A question about how personal data is collected, stored, shared and watched, and whether people agreed to it.
stakeholder: Anyone affected by a technology or decision, such as users, owners, employees or the public.
digital divide: The gap between people who have easy access to computers and the internet and those who do not, because of cost, location, age or skills.
automation: Using computers and robots to do tasks people used to do, which can raise output but may replace jobs.
algorithmic bias: When a computer system gives unfair results for some groups, often because it learned from biased data.
e-waste: Thrown-away electronic devices. They contain toxic materials that can harm people and the environment if not recycled safely.
personal data: Information about a living person who can be identified from it, such as name, address, date of birth or photo.
Data Protection Act 2018: The UK law controlling how organisations collect, store and use personal data, and giving people rights over their own data, such as seeing what is held about them.
data protection principles: The rules for handling personal data: use it lawfully and fairly, for a stated purpose, keep only what is needed, keep it accurate and secure, and not keep it longer than needed.
Computer Misuse Act 1990: The UK law that makes it a crime to access a computer without permission, to do so in order to commit further crime, or to change or damage data or systems without permission.
unauthorised access: Using a computer system or its data without permission, such as logging in with someone else's password.
Copyright, Designs and Patents Act 1988: The UK law that protects creators' work, such as software, music, films and images, from being copied, shared or sold without permission.
copyright: The legal right of a creator to control who copies, shares or changes their work.
software licence: A legal agreement saying how software may be used, copied, changed and shared. Buying software normally means buying a licence to use it, not the software itself.
source code: The program code as written by programmers in a high-level language, before it is translated.
open-source software: Software whose source code anyone can see, change and pass on, usually free of charge. Support may come only from a community of users.
proprietary software: Software owned by a company that keeps the source code secret. Users buy a licence, often as off-the-shelf software, but cannot change or share it; support is usually provided.

2.1 · Algorithms

2.1.1 Computational thinking

computational thinking: Approaching a problem the way a computer scientist would, using abstraction, decomposition and algorithmic thinking, so a computer could help solve it.
abstraction: Removing details that do not matter to a problem and keeping the ones that do, such as a tube map showing stops and lines but not real distances.
decomposition: Breaking a big problem into smaller parts that are easier to understand, solve and test, sometimes by different people.
algorithmic thinking: Working out the exact steps needed to solve a problem, in the right order, so they can be followed or turned into code.
algorithm: A set of precise, ordered steps that solves a problem or completes a task.

2.1.2 Designing, creating and refining algorithms

input: Data that goes into a program or system, for example typed by a user, read from a file or measured by a sensor.
process: What a program does to its inputs, such as calculating, comparing or sorting, to produce the outputs.
output: Information a program gives back, for example shown on screen, printed or saved to a file.
structure diagram: A diagram that shows a problem broken into smaller sub-tasks as a tree, with the main task at the top. It shows decomposition.
pseudocode: A way of writing an algorithm in structured, code-like statements that are not tied to any one programming language.
Exam Reference Language: ERL: OCR's own code-like language, used for every piece of code in the J277 exam papers. Answers may be written in ERL or a high-level language.
flowchart: A diagram that shows an algorithm as symbols joined by arrows, showing the order of the steps and the decisions.
terminal symbol: The rounded flowchart symbol for Start or Stop.
process symbol: The rectangle flowchart symbol for a step that does something, such as a calculation or assignment.
input/output symbol: The parallelogram flowchart symbol for taking data in or giving data out.
decision symbol: The diamond flowchart symbol for a yes/no question, with one arrow out for each answer.
sub program symbol: The rectangle with double lines at each side, used in a flowchart to call a sub program that is described elsewhere.
line: The arrow joining flowchart symbols, showing which step comes next.
trace table: A table used to follow an algorithm by hand, with a column for each variable and the output, and a new row each time a value changes.
dry run: Working through an algorithm by hand, step by step, without a computer, to find what it does or to find errors.
off-by-one error: A common error where a loop runs one time too many or too few, often from using < instead of <=, or from Python's range stopping before its end value.

2.1.3 Searching and sorting algorithms

linear search: Checking each item in a list in turn, from the start, until the value is found or the list ends. Works on unsorted lists but is slow on long ones.
binary search: Searching a sorted list by checking the middle item and discarding the half that cannot hold the value, repeating until it is found or none are left. Much faster on long lists.
bubble sort: A sort that compares each pair of neighbouring items and swaps them if they are in the wrong order, making passes until a pass needs no swaps.
pass: One trip through a list during a sort. In a bubble sort, the largest unsorted item reaches its place after each pass.
insertion sort: A sort that takes each item in turn and inserts it into the correct place among the already sorted items to its left.
merge sort: A sort that splits a list in half again and again until each list has one item, then merges the lists back together in order. Fast on large lists but uses more memory.
efficiency: How quickly an algorithm runs or how little memory it uses, especially as the amount of data grows.

2.2 · Programming fundamentals

2.2.1 Programming fundamentals

variable: A named location in memory that holds a value which can change while the program runs, such as score = 0.
constant: A named value that cannot change while the program runs, such as const VAT = 0.2 in ERL. It makes code easier to read and update.
identifier: The name given to a variable, constant, array or sub program.
assignment: Giving a variable a value with =, such as total = total + 5. The right side is worked out first, then stored.
print(): Outputs a value to the screen, in both ERL and Python, such as print("Hello " + name).
input(): Shows a prompt and waits for the user to type something, which is returned as a string, such as name = input("Your name?").
sequence: Running instructions one after another in the order they are written.
selection: Choosing which code to run depending on whether a condition is true or false, using if or switch.
iteration: Repeating a block of code, using a count-controlled or condition-controlled loop.
condition: An expression that is either True or False, such as age >= 18, used in selection and loops.
if … then … endif: ERL selection: the block runs only if the condition is true. elseif adds more conditions and else gives the code for when none are true.
switch … endswitch: ERL selection that compares one variable with several values, using case lines for each value and default for any other. Python uses if … elif or match.
nesting: Putting one structure inside another, such as an if inside a loop or a loop inside a loop.
count-controlled loop: A loop that repeats a set number of times. ERL: for k = 1 to 5 … next k runs 5 times, including both 1 and 5.
step: Sets how much a for loop counter changes each time, such as for k = 10 to 0 step -2 in ERL. Python's range takes the step as its third value.
range(): Python's way of making a for loop's values. range(1, 6) gives 1 to 5: it stops before the end value, unlike ERL's for … to.
condition-controlled loop: A loop that repeats until or while a condition is met, so the number of repeats is not known in advance.
while … endwhile: ERL loop that checks its condition before each run, so the body may never run at all. Python uses while too.
do … until: ERL loop that checks its condition after each run, so the body always runs at least once. Python has no direct equivalent.
arithmetic operator: An operator that does maths: + add, - subtract, * multiply, / divide, ^ power, MOD and DIV.
MOD: Modulo: gives the remainder of a division. 17 MOD 5 is 2. Python uses %.
DIV: Quotient: gives the whole number part of a division. 17 DIV 5 is 3. Python uses //.
^: Exponentiation (to the power of) in ERL: 2^3 is 8. Python uses **.
comparison operator: An operator that compares two values and gives True or False: == equal to, != not equal to, <, <=, > and >=. A single = is assignment, not comparison.
AND: Boolean operator that is True only when both conditions are true, such as age >= 13 AND age <= 19.
OR: Boolean operator that is True when at least one of the conditions is true.
NOT: Boolean operator that reverses a condition: True becomes False and False becomes True.

2.2.2 Data types

data type: The kind of data a value is, which sets what can be done with it and how it is stored: integer, real, Boolean, character or string.
integer: A whole number, positive, negative or zero, such as 42 or -7.
real: A number with a decimal part, such as 3.75. Python calls this type float.
Boolean: A data type with only two values, True or False.
char: Character data type: a single letter, digit or symbol, such as 'A' or '?'.
string: A sequence of characters, such as "Hello" or a phone number, written in quotation marks.
casting: Changing a value from one data type to another, such as int("25") to turn a string into an integer so it can be used in a sum.
str(), int(), float(), real(), bool(): The casting functions: str() to string, int() to integer, float() or real() to real, bool() to Boolean. Python has no real().

2.2.3 Additional programming techniques

concatenation: Joining strings end to end with +, such as "Sam" + "my" giving "Sammy".
.length: ERL: gives the number of characters in a string, such as word.length. Python uses len(word).
.substring(start, count): ERL: takes count characters starting at position start (the first character is 0). word.substring(2, 3) matches Python's word[2:5].
.left(n) and .right(n): ERL: give the first n or last n characters of a string. Python uses word[:n] and word[-n:].
.upper and .lower: ERL: give a copy of a string in capitals or small letters. Python uses word.upper() and word.lower().
ASC(): ERL: gives the character code of a character, so ASC("A") is 65. Python uses ord().
CHR(): ERL: gives the character for a character code, so CHR(66) is "B". Python uses chr().
slicing: Taking part of a string (or array) by giving start and end positions, such as Python's word[1:4]. ERL uses word.substring(1, 3).
file handling: Reading data from, or writing data to, a file in storage, so it is kept after the program ends.
open(): Opens a file so a program can use it. In ERL the file must be held in a variable, such as f = open("log.txt").
newFile(): ERL: creates a new, empty file with the name given, such as newFile("scores.txt").
readLine(): ERL: reads the next line from an open file, such as line = f.readLine().
writeLine(): ERL: writes a line to the end of an open file, such as f.writeLine("Sam,12").
endOfFile(): ERL: gives True when there are no more lines left to read, often used to control a while loop.
close(): Closes an open file so the data is saved and other programs can use it.
record: A data structure that holds related items about one thing, which can be of different data types, such as a pupil's name, age and form.
field: One item of data in a record, or one column in a database table, such as surname.
database: An organised collection of data, usually stored in tables, that can be searched and updated.
SQL: Structured Query Language: a language for searching and managing data in a database.
SELECT: SQL keyword naming the fields to show, such as SELECT name, age.
FROM: SQL keyword naming the table to search, such as FROM Pupils.
WHERE: SQL keyword giving the condition records must meet, such as WHERE age > 14 AND form = '10B'.
*: In SQL, stands for all fields, so SELECT * FROM Pupils shows every field.
LIKE: SQL operator used in WHERE to match a pattern, such as WHERE name LIKE 'S%'.
%: SQL wildcard that matches any run of characters, so 'S%' finds every value starting with S. In Python, % is the remainder operator.
array: A fixed-size (static) data structure holding items of one data type under one name, each reached by its index. ERL: array marks[5].
element: One item stored in an array or list.
index: The position number of an element in an array or a character in a string. Indexes start at 0.
one-dimensional array: An array that stores items in a single row, such as array days = ["Mon", "Tue"], reached with one index: days[1].
two-dimensional array: An array of rows and columns, reached with two indexes: grid[0,2] in ERL, grid[0][2] in Python. It can act like a database table of records and fields.
list: Python's version of an array. Unlike an ERL array it can grow, shrink and hold different data types.
sub program: A named block of code that does one task and can be called from anywhere in the program. It is either a procedure or a function.
procedure: A sub program that carries out a task but does not return a value. ERL: procedure greet(name) … endprocedure.
function: A sub program that returns a value to the code that called it. ERL: function area(w, h) … return w * h … endfunction.
parameter: A variable named in a sub program's definition that receives a value when it is called, such as w and h in area(w, h).
argument: The actual value passed into a sub program when it is called, such as 3 and 4 in area(3, 4).
return value: The value a function sends back to the code that called it, using return.
local variable: A variable declared inside a sub program. It only exists while that sub program runs and cannot be used outside it.
global variable: A variable that can be used anywhere in the program. ERL: global score = 0. Too many make code harder to debug.
scope: The part of a program where a variable can be used: local scope inside one sub program, or global scope across the whole program.
random number: A number picked by chance, used in games and simulations. ERL: random(1, 6) gives a whole number from 1 to 6; Python uses random.randint(1, 6).

2.3 · Producing robust programs

2.3.1 Defensive design

robust program: A program that keeps working correctly even when it is given unexpected input or used in ways the programmer did not plan.
defensive design: Designing a program to cope with misuse and errors: planning for what users might do, checking input, controlling access and keeping code easy to maintain.
anticipating misuse: Thinking ahead about the ways users could cause problems, by accident or on purpose, and planning code to stop them.
input validation: Checking that input is sensible and meets set rules before the program uses it. It cannot check that the data is actually correct.
range check: Validation that a value lies between a lower and upper limit, such as a mark from 0 to 100.
type check: Validation that input is the right data type, such as a whole number for an age.
length check: Validation that input has an allowed number of characters, such as a password of at least 8.
presence check: Validation that something has been entered and the field is not left empty.
format check: Validation that input follows a set pattern, such as a date written dd/mm/yyyy or a postcode.
maintainability: How easy a program is for other programmers to read, understand, fix and update later.
comment: A note in code that explains what it does and is ignored when the program runs. ERL and Python comments start with // and # respectively.
indentation: Spacing at the start of lines to show which code is inside a loop, selection or sub program. Python requires it.
naming conventions: Rules for choosing clear, meaningful identifiers, such as totalScore rather than x, so code is easier to follow.

2.3.2 Testing

testing: Running a program with chosen data to check it works as intended, finding errors and checking it meets its requirements.
iterative testing: Testing each part of a program as it is developed, fixing problems and testing again, over and over.
final testing: Testing the whole program once it is finished, to check all its parts work together and it meets the requirements. Also called terminal testing.
syntax error: An error where code breaks the rules of the language, such as a missing bracket or misspelt keyword, so it cannot be translated or run.
logic error: An error where the program runs but gives the wrong result or behaves wrongly, such as using > instead of >=.
test data: Values chosen to test a program, covering normal, boundary, invalid and erroneous cases.
normal test data: Typical, valid data the program should accept and process correctly, such as 50 for a mark out of 100.
boundary test data: Data of the correct type that sits right on the edge of being valid, such as 0 and 100 for a mark out of 100.
invalid test data: Data of the right type but outside the allowed range, such as 101 for a mark out of 100, which should be rejected.
erroneous test data: Data of the wrong type, such as "ten" where a number is expected, which should be rejected.
test plan: A table listing each test: what is tested, the test data, its type, the expected result, and later the actual result.
expected result: What a program should do or output for a given test, written before the test is run.
debugging: Finding and fixing errors in a program.
refining: Improving an algorithm or program, for example fixing errors, making it more efficient or making it more robust.

2.4 · Boolean logic

2.4.1 Boolean logic

Boolean logic: Logic where every value is either true (1) or false (0), used to build the circuits inside a computer.
logic gate: An electronic part that takes one or more binary inputs and gives one binary output by following a rule such as AND, OR or NOT.
AND gate: A logic gate whose output is 1 only when both inputs are 1 (conjunction). Drawn with a flat back and a curved front.
OR gate: A logic gate whose output is 1 when either input, or both, is 1 (disjunction). Drawn with a curved back and a pointed front.
NOT gate: A logic gate with one input that gives the opposite as output, 0 to 1 and 1 to 0 (negation). Drawn as a triangle with a small circle.
truth table: A table listing every possible combination of inputs to a logic circuit and the output for each. Two inputs need 4 rows, three inputs need 8.
logic diagram: A drawing of logic gates joined together, showing how the inputs pass through them to the output.
logic circuit: Two or more logic gates connected so that the output of one becomes the input of another.
Boolean expression: A logic statement written with AND, OR and NOT, such as Q = (A AND B) OR NOT C, which matches a logic diagram.

2.5 · Programming languages and Integrated Development Environments

2.5.1 Languages

high-level language: A programming language that uses English-like words and maths symbols, such as Python. Easy to read and write, works on many types of CPU, but must be translated.
low-level language: A programming language close to what the CPU understands: machine code or assembly language. Gives direct control of the hardware but is hard to write.
machine code: Instructions in binary that the CPU can run directly. Each type of CPU has its own machine code.
assembly language: A low-level language using short words, such as ADD and LDA, for machine code instructions. One assembly instruction matches one machine code instruction.
translator: A program that turns source code into machine code the CPU can run. The course covers two kinds: compilers and interpreters. An IDE includes one.
compiler: A translator that turns the whole program into machine code in one go, making an executable file. It runs fast and the code is hidden, but errors are listed only after translating everything.
interpreter: A translator that translates and runs a program one line at a time, stopping at the first error. Good for testing, but slower and needs the interpreter every run.
executable file: A file of machine code made by a compiler that can be run on its own, without the source code or a translator.

2.5.2 The Integrated Development Environment (IDE)

IDE: Integrated development environment: one program that brings together an editor, error diagnostics, a run-time environment and a translator, such as IDLE, Thonny or VS Code.
editor: The part of an IDE where code is typed and changed, with features such as line numbers, syntax highlighting, auto-indent and autocomplete.
syntax highlighting: Showing keywords, strings, comments and so on in different colours, so code is easier to read and mistakes stand out.
error diagnostics: IDE tools that find errors and report them with a message and line number, such as underlining a syntax error.
run-time environment: Lets a program be run and tested inside the IDE, showing its output and any errors while it runs.
debugger: An IDE tool for finding errors by running code a step at a time and watching how values change.
breakpoint: A marker that pauses a program at a chosen line, so the programmer can inspect the values of variables.
variable watch: A debugger window that shows the current value of chosen variables as the program runs.

KS3 year plan

IGCSE year plan

GCSE year plan

IB year plan

A Level year plan

Get free resources