and <body>. Content uses <h1> to <h3> for headings, <p> for a paragraph, <a href> for a link, <img src> for an image, <ol> or <ul> holding <li> items, <div>, and <form> holding <input> boxes."},{"id":"fc-1.3.4-009","topicId":"1.3.4","front":"CSS Selectors and Rules","back":"A rule is a selector then declarations in curly brackets, each written property: value; for example h1 { color: blue; } (the property is spelt color). A tag name selects every such element, a full stop prefix a class (usable on many elements) and a hash (#) prefix an id (unique to one)."},{"id":"fc-1.3.4-010","topicId":"1.3.4","front":"JavaScript in the Browser","back":"Runs client-side in response to events such as a button click. document.getElementById finds an element by its id so that its value or innerHTML can be read or changed, letting input be validated and the page updated without reloading."},{"id":"fc-1.3.4-011","topicId":"1.3.4","front":"Search Engine Indexing","back":"Programs called crawlers or spiders visit web pages, moving from page to page by following hyperlinks. For each page they record information such as its text, meta tags and the position of each word, and store it in the search engine's index."},{"id":"fc-1.3.4-012","topicId":"1.3.4","front":"PageRank Formula","back":"PR(A) = (1 - d) + d x (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn)), where T1 to Tn are the pages that link to A, C(T) is the number of outbound links on page T and d is the damping factor, a value between 0 and 1 (about 0.85)."},{"id":"fc-1.4.1-001","topicId":"1.4.1","front":"Binary","back":"A base-2 number system that uses only the digits 0 and 1, with place values that are powers of 2. Computers store all data in this form because their circuits are switches (transistors) with two states, on and off."},{"id":"fc-1.4.1-002","topicId":"1.4.1","front":"Denary","back":"The base-10 number system used in everyday counting. It has ten digits, 0 to 9, and place values that are powers of 10: units, tens, hundreds and so on."},{"id":"fc-1.4.1-003","topicId":"1.4.1","front":"Hexadecimal","back":"A base-16 number system using the digits 0 to 9 then the letters A to F for the values 10 to 15. Each digit represents four bits (a nibble), so it is a shorter way to write binary that people can read and copy with fewer mistakes."},{"id":"fc-1.4.1-004","topicId":"1.4.1","front":"Boolean","back":"A data type that can hold only one of two values, TRUE or FALSE (1 or 0). It is used for flags and for the result of a comparison or condition, such as 10 > 5."},{"id":"fc-1.4.1-005","topicId":"1.4.1","front":"Character","back":"A data type that holds a single letter, digit, punctuation mark or other symbol, such as 'j'. It is stored as that symbol's binary code from a set such as ASCII or Unicode."},{"id":"fc-1.4.1-006","topicId":"1.4.1","front":"String","back":"A data type that holds a sequence of characters treated as text, such as 'Hello World'. Digits stored this way must be converted (cast) to a numeric type before they can be used in calculations."},{"id":"fc-1.4.1-007","topicId":"1.4.1","front":"Integer","back":"A data type that holds a whole number, which can be positive, negative or zero, with no fractional part, such as -42 or 23."},{"id":"fc-1.4.1-008","topicId":"1.4.1","front":"Real/Floating Point","back":"A data type that holds a number with a fractional part, such as 3.14 or -0.5. It is stored in binary as a mantissa and an exponent."},{"id":"fc-1.4.1-009","topicId":"1.4.1","front":"ASCII","back":"A character set that maps each character to a unique binary code, using 7 bits (128 characters) or 8 bits in its extended form (256). It covers English letters, digits and common symbols but not most other alphabets."},{"id":"fc-1.4.1-010","topicId":"1.4.1","front":"Unicode","back":"A character set that can use 16 or 32 bits per character instead of 7 or 8, so it can represent far more characters, including the alphabets of other languages and emojis. The same text therefore takes more storage. Its first 128 codes match ASCII."},{"id":"fc-1.4.1-011","topicId":"1.4.1","front":"AND (∧)","back":"A logical operation whose output is 1 (TRUE) only when both of its inputs are 1; otherwise the output is 0. Used with a mask, it keeps the bits where the mask has a 1 and sets all the other bits to 0."},{"id":"fc-1.4.1-012","topicId":"1.4.1","front":"OR (∨)","back":"A logical operation whose output is 1 (TRUE) when at least one of its inputs is 1, and 0 only when every input is 0. Used with a mask, it sets to 1 the bits where the mask has a 1 and leaves the other bits unchanged."},{"id":"fc-1.4.1-013","topicId":"1.4.1","front":"XOR (⊻)","back":"A logical operation whose output is 1 (TRUE) when exactly one of its two inputs is 1, and 0 when both inputs are the same. Used with a mask, it flips the bits where the mask has a 1."},{"id":"fc-1.4.1-014","topicId":"1.4.1","front":"NOT (¬)","back":"A logical operation with a single input whose output is the opposite of that input: 1 becomes 0 and 0 becomes 1. Applied to a binary number, it inverts every bit."},{"id":"fc-1.4.1-015","topicId":"1.4.1","front":"Bitwise Manipulation","back":"Working directly on the individual bits of a binary value, by shifting them left or right or by combining the value with a mask using AND, OR or XOR, in order to extract, set, clear or flip chosen bits."},{"id":"fc-1.4.1-016","topicId":"1.4.1","front":"Shifts","back":"Moving every bit of a binary number left or right by a given number of places. Each place moved left multiplies the value by 2 and each place moved right divides it by 2, so three places right divides by 8."},{"id":"fc-1.4.1-017","topicId":"1.4.1","front":"Two's Complement","back":"A way of storing signed binary integers in which the most significant bit has a negative place value (-128 in 8 bits). It has only one representation of zero, and subtraction is done by adding a negative number, so no extra hardware is needed."},{"id":"fc-1.4.1-018","topicId":"1.4.1","front":"Sign and Magnitude","back":"A way of storing signed binary numbers in which the most significant bit shows the sign (0 for positive, 1 for negative) and the remaining bits hold the size of the number. It has two representations of zero and makes arithmetic harder."},{"id":"fc-1.4.1-019","topicId":"1.4.1","front":"Exponent","back":"The part of a floating point number that gives how many places the binary point in the mantissa must be moved: to the right if it is positive, to the left if it is negative. Giving it more bits increases the range of values that can be stored."},{"id":"fc-1.4.1-020","topicId":"1.4.1","front":"Mantissa","back":"The part of a floating point number that holds its significant digits, stored as a two's complement binary fraction with the binary point after the first bit. Giving it more bits increases the precision of the number."},{"id":"fc-1.4.1-021","topicId":"1.4.1","front":"Normalisation in Binary","back":"Adjusting a floating point number so that the first two bits of the mantissa differ, 01 for a positive number or 10 for a negative one, with the exponent changed to match. This gives the most precision for the bits available and a unique form for each value."},{"id":"fc-1.4.1-022","topicId":"1.4.1","front":"Floating Point Arithmetic","back":"Adding or subtracting numbers stored as a mantissa and an exponent: the binary point of each mantissa is moved by its exponent so that the two numbers line up, they are added or subtracted, and the result is normalised to give the new mantissa and exponent."},{"id":"fc-1.4.2-001","topicId":"1.4.2","front":"Arrays","back":"A static data structure of fixed size holding items of a single data type under one identifier. Each item is accessed directly by its index, the contents can be changed, and it can have more than one dimension, such as rows and columns."},{"id":"fc-1.4.2-002","topicId":"1.4.2","front":"Lists","back":"A data structure holding an ordered sequence of items whose size can change while the program runs (dynamic). Its items can be of different data types and can be changed, added or removed."},{"id":"fc-1.4.2-003","topicId":"1.4.2","front":"Linked Lists","back":"A dynamic data structure made of nodes, each holding a data item and a pointer to the next node. A start pointer gives the first node, the last node holds a null pointer, and items can only be reached by following the pointers in order."},{"id":"fc-1.4.2-004","topicId":"1.4.2","front":"Tuples","back":"An ordered collection of items, which can be of different data types, that is immutable: once it has been created its contents cannot be changed while the program runs."},{"id":"fc-1.4.2-005","topicId":"1.4.2","front":"Records","back":"A data structure that groups related items of data, called fields, under one identifier. The fields can be of different data types, and each one is accessed by its field name."},{"id":"fc-1.4.2-006","topicId":"1.4.2","front":"Stacks","back":"A last in, first out (LIFO) data structure. Items are pushed onto and popped from the same end, the top, which is tracked by a single pointer, so the most recently added item is the first to be removed."},{"id":"fc-1.4.2-007","topicId":"1.4.2","front":"Queues","back":"A first in, first out (FIFO) data structure. Items are added (enqueued) at the rear and removed (dequeued) from the front, so they leave in the order they arrived. A head pointer and a tail pointer mark its two ends."},{"id":"fc-1.4.2-008","topicId":"1.4.2","front":"Circular Queues","back":"A first in, first out structure held in a fixed-size array in which the head and tail pointers wrap round to index 0 after the last index (using MOD). Space freed at the front is reused, so items never have to be moved along."},{"id":"fc-1.4.2-009","topicId":"1.4.2","front":"Trees","back":"A hierarchical data structure of nodes joined by branches (edges), starting from a single root node. Each node can have child nodes, a node with no children is a leaf, and there are no cycles."},{"id":"fc-1.4.2-010","topicId":"1.4.2","front":"Binary Search Tree","back":"A tree in which each node has at most two children and the nodes are kept in order: values less than a node go to its left and greater values to its right. A search can therefore ignore one subtree at every step."},{"id":"fc-1.4.2-011","topicId":"1.4.2","front":"Graphs","back":"A data structure made of nodes (vertices) connected by edges (arcs), used to model how things are linked. Unlike a tree it has no root node and can contain cycles, and its edges can be directed or undirected and can carry weights."},{"id":"fc-1.4.2-012","topicId":"1.4.2","front":"Directed Graphs","back":"A graph in which every edge (arc) has a direction, so it can only be followed one way: an edge from A to B does not allow movement from B to A."},{"id":"fc-1.4.2-013","topicId":"1.4.2","front":"Undirected Graphs","back":"A graph in which the edges (arcs) have no direction, so every edge can be followed both ways: an edge between A and B links A to B and also B to A."},{"id":"fc-1.4.2-014","topicId":"1.4.2","front":"Breadth First Traversal","back":"Visits the start node, then all the nodes directly connected to it, then the nodes connected to those, working outwards one level at a time. It uses a queue to hold the nodes waiting to be visited."},{"id":"fc-1.4.2-015","topicId":"1.4.2","front":"Depth First Traversal","back":"Follows one branch as far as it can go, then backtracks to the most recent node with unvisited children and follows the next branch. It uses a stack to remember where to return to, and on a tree outputs the nodes in post-order, so the root is last."},{"id":"fc-1.4.2-016","topicId":"1.4.2","front":"Hash Table","back":"A data structure in which a hashing function is applied to an item's key to calculate the position where it is stored. This gives direct access without searching, so lookup time stays constant, O(1), unless two keys give the same position (a collision)."},{"id":"fc-1.4.2-017","topicId":"1.4.2","front":"Indexing","back":"Keeping an ordered list of the values in a chosen field, each with a pointer to its record, so that a database can find records by that field without checking every record. It makes searching faster but uses extra storage."},{"id":"fc-1.4.2-018","topicId":"1.4.2","front":"Dynamic Data Structures","back":"Data structures whose size is not fixed in advance: they can grow and shrink while the program runs, taking and releasing memory as needed. A linked list is one example; an array is static because its size is set when it is declared."},{"id":"fc-1.4.3-001","topicId":"1.4.3","front":"Boolean Logic","back":"A system of logic in which every value is either TRUE or FALSE (1 or 0), and values are combined using operators such as AND, OR, NOT and XOR. It is the basis of logic gates and so of all digital circuits."},{"id":"fc-1.4.3-002","topicId":"1.4.3","front":"Boolean Expressions","back":"A statement made of variables joined by logical operators such as ¬, ∧ and ∨ that evaluates to TRUE or FALSE. It can describe the output of a logic circuit, for example Q = (A ∧ B) ∨ (C ∧ D)."},{"id":"fc-1.4.3-003","topicId":"1.4.3","front":"Association Laws","back":"When the operators are all the same, moving the brackets does not change the result: (A ∧ B) ∧ C ≡ A ∧ (B ∧ C) and (A ∨ B) ∨ C ≡ A ∨ (B ∨ C)."},{"id":"fc-1.4.3-004","topicId":"1.4.3","front":"Commutation Laws","back":"The order of the operands either side of an operator does not change the result: A ∧ B ≡ B ∧ A and A ∨ B ≡ B ∨ A."},{"id":"fc-1.4.3-005","topicId":"1.4.3","front":"Distribution Laws","back":"A term outside a bracket can be combined with each term inside it, or a common term taken out as a factor: A ∧ (B ∨ C) ≡ (A ∧ B) ∨ (A ∧ C) and A ∨ (B ∧ C) ≡ (A ∨ B) ∧ (A ∨ C)."},{"id":"fc-1.4.3-006","topicId":"1.4.3","front":"De Morgan's First Law","back":"Negating an AND of two terms is the same as an OR of the two terms each negated: ¬(A ∧ B) ≡ ¬A ∨ ¬B. So the expression ¬A ∨ ¬B simplifies to ¬(A ∧ B), which is a NAND."},{"id":"fc-1.4.3-007","topicId":"1.4.3","front":"De Morgan's Second Law","back":"Negating an OR of two terms is the same as an AND of the two terms each negated: ¬(A ∨ B) ≡ ¬A ∧ ¬B. So the expression ¬A ∧ ¬B simplifies to ¬(A ∨ B), which is a NOR."},{"id":"fc-1.4.3-008","topicId":"1.4.3","front":"Double Negation Law","back":"Negating a value twice gives back the original value, so two NOTs in a row cancel each other out: ¬(¬A) ≡ A."},{"id":"fc-1.4.3-009","topicId":"1.4.3","front":"D-Type Flip Flops","back":"A logic circuit that stores one bit. It has a data input (D) and a clock input: when the clock signal goes high, the output Q changes to the value of D and holds it until the next clock pulse. A second output, NOT Q, is always the inverse of Q."},{"id":"fc-1.4.3-010","topicId":"1.4.3","front":"Half Adders","back":"A logic circuit that adds two single bits, A and B, and gives two outputs: the sum, S = A XOR B, and the carry, C = A AND B. It has no carry input, so it cannot add in a carry from a previous column."},{"id":"fc-1.4.3-011","topicId":"1.4.3","front":"Full Adders","back":"A logic circuit that adds three bits: A, B and a carry in from the previous column, giving a sum and a carry out. It is made from two half adders joined by an OR gate, and several can be chained, carry out to carry in, to add multi-bit numbers."},{"id":"fc-1.4.3-012","topicId":"1.4.3","front":"Karnaugh Maps","back":"A grid of the outputs of a logic expression for every combination of its inputs. The 1s are put into the largest possible rectangular groups of 1, 2, 4, 8 or 16, wrapping round the edges if needed, and each group gives one term of a simplified expression."},{"id":"fc-1.4.3-013","topicId":"1.4.3","front":"Logic Gate Diagrams","back":"A drawing of a circuit that uses the standard symbols for gates such as AND, OR, NOT and XOR, with lines carrying each labelled input through the gates to the output. It shows how a logical expression is built from gates."},{"id":"fc-1.5.1-001","topicId":"1.5.1","front":"Computer Misuse Act (1990)","back":"UK law that criminalises hacking. Its three offences are unauthorised access to a computer system, unauthorised access with intent to commit further offences, and unauthorised modification of computer material."},{"id":"fc-1.5.1-002","topicId":"1.5.1","front":"Unauthorised Access","back":"The basic offence under the Computer Misuse Act: knowingly gaining entry to a computer system or its data without permission, for example by using someone else's password, even if nothing is changed or stolen."},{"id":"fc-1.5.1-003","topicId":"1.5.1","front":"Cybercrimes","back":"Illegal acts carried out using computers or networks, such as hacking, spreading malware, denial-of-service attacks and online fraud. The Computer Misuse Act is the main UK law aimed at them."},{"id":"fc-1.5.1-004","topicId":"1.5.1","front":"Electronic Vandalism","back":"The Computer Misuse Act offence of unauthorised modification of computer material: deliberately changing, deleting or corrupting programs or data without permission, for example by releasing a virus or altering files."},{"id":"fc-1.5.1-005","topicId":"1.5.1","front":"Information Theft","back":"Entering a system without permission to obtain confidential data such as login or bank details. When the data is wanted for a further crime such as fraud, this is the Computer Misuse Act offence of access with intent to commit further offences."},{"id":"fc-1.5.1-006","topicId":"1.5.1","front":"Copyright, Designs and Patents Act (1988)","back":"UK law that gives the creator of an original work, such as software, music, video or images, automatic ownership of it. Others may not copy, use or distribute the work without permission, which can be granted, bought or licensed."},{"id":"fc-1.5.1-007","topicId":"1.5.1","front":"Intellectual Property Protection","back":"Original creative work, such as program code, music, images and writing, is treated in law as belonging to its creator. Under the Copyright, Designs and Patents Act the creator decides who may copy, adapt or sell it."},{"id":"fc-1.5.1-008","topicId":"1.5.1","front":"Copyright Duration","back":"Under the Copyright, Designs and Patents Act, protection is not permanent: for most works, including software, it lasts for the creator's lifetime plus 70 years, after which the work may be used freely."},{"id":"fc-1.5.1-009","topicId":"1.5.1","front":"Illegal Distribution","back":"Copying, sharing or selling a protected work without the owner's permission, such as pirating software or uploading films for others to download. Under the Copyright, Designs and Patents Act the offender can be fined or prosecuted."},{"id":"fc-1.5.1-010","topicId":"1.5.1","front":"Digital Copyright","back":"Under the Copyright, Designs and Patents Act, works held or sent electronically, such as software, streamed video and online images, are protected like any other. A subscription or licence only gives permission to view or use them."},{"id":"fc-1.5.1-011","topicId":"1.5.1","front":"Data Protection Act (2018 - UK GDPR)","back":"UK law governing how organisations collect, store and use personal data. Data must be processed fairly and lawfully, used only for the purpose it was collected for, kept accurate and secure, and not kept longer than necessary."},{"id":"fc-1.5.1-012","topicId":"1.5.1","front":"Regulation","back":"The Data Protection Act controls the processing of personal data: an organisation needs a lawful reason to collect it, may use it only for that stated purpose, and may hold only data that is adequate, relevant and not excessive."},{"id":"fc-1.5.1-013","topicId":"1.5.1","front":"User Rights","back":"Under the Data Protection Act, an individual may see the personal data an organisation holds about them, have it corrected if it is wrong, and in some cases have it erased or have a decision made by computer alone reviewed by a person."},{"id":"fc-1.5.1-014","topicId":"1.5.1","front":"Security Requirements","back":"The Data Protection Act principle that personal data must be kept safe from unauthorised access, loss or damage, for example by encryption and access rights, and must not be sent to countries without equivalent legal protection."},{"id":"fc-1.5.1-015","topicId":"1.5.1","front":"Accountability","back":"The organisation that holds personal data is responsible for obeying the Data Protection Act and must be able to prove that it does, for example by recording what it holds and why. It can be fined or prosecuted for a breach."},{"id":"fc-1.5.1-016","topicId":"1.5.1","front":"Regulation of Investigatory Powers Act (2000)","back":"UK law that regulates how public bodies such as the police and security services may carry out surveillance and monitor or intercept people's electronic communications, in order to detect crime and protect national security."},{"id":"fc-1.5.1-017","topicId":"1.5.1","front":"Surveillance Powers","back":"Under the Regulation of Investigatory Powers Act, authorised public bodies may watch individuals covertly, monitor their internet activity, carry out mass monitoring of communications and make ISPs fit equipment to enable it."},{"id":"fc-1.5.1-018","topicId":"1.5.1","front":"Interception of Communications","back":"Under the Regulation of Investigatory Powers Act, authorised bodies can obtain the content of private emails, text messages and phone calls, and can demand that ISPs and businesses hand over their customers' messages and records."},{"id":"fc-1.5.1-019","topicId":"1.5.1","front":"Data Access","back":"Under the Regulation of Investigatory Powers Act, authorities can demand protected data and force a person or organisation to decrypt it or hand over the encryption keys. Refusing can lead to a prison sentence."},{"id":"fc-1.5.1-020","topicId":"1.5.1","front":"Public Safety & Privacy","back":"The debate over the Regulation of Investigatory Powers Act: monitoring helps to detect and stop serious crime and terrorism, but it intrudes on innocent people's private lives and has been used by many bodies for minor offences."},{"id":"fc-1.5.2-001","topicId":"1.5.2","front":"Artificial Intelligence & Bias","back":"Systems that perform tasks normally needing human intelligence can make unfair decisions, because they inherit prejudice from their programmers or from unrepresentative training data. Thorough testing and scrutiny of data sets reduce this."},{"id":"fc-1.5.2-002","topicId":"1.5.2","front":"Automation & Job Loss","back":"Computers and robots replace people in repetitive, manual or dangerous work because they need no wages or breaks and can run 24/7, causing unemployment. New jobs appear in designing and maintaining the systems, so workers must retrain."},{"id":"fc-1.5.2-003","topicId":"1.5.2","front":"Privacy Issues","back":"Concerns that personal data, such as images, location and browsing history, is collected, stored or shared without a person's knowledge or consent. Individuals are entitled to keep such information private and to know how it is used."},{"id":"fc-1.5.2-004","topicId":"1.5.2","front":"Environmental Impact","back":"Making, powering and disposing of digital devices uses raw materials and energy and leaves toxic e-waste, often in landfill. Against this, computers cut paper use, reduce travel through home working and control heating efficiently."},{"id":"fc-1.5.2-005","topicId":"1.5.2","front":"Digital Divide","back":"The gap between people who have access to modern technology, a fast internet connection and the skills to use them and those who do not, because of cost, location or age. Those without are disadvantaged in education, work and services."},{"id":"fc-1.5.2-006","topicId":"1.5.2","front":"Monitor Behaviour","back":"Using technology to watch and record what people do, such as CCTV, tracking location or logging employees' email and web use. It can deter crime and improve safety, but it intrudes on privacy and may happen without consent."},{"id":"fc-1.5.2-007","topicId":"1.5.2","front":"Analyse Personal Information","back":"Processing large amounts of data about individuals to find patterns and build profiles, for example for targeted advertising or medical research. It can improve services, but people may not have agreed and the results can be misused."},{"id":"fc-1.5.2-008","topicId":"1.5.2","front":"Piracy and Offensive Communications","back":"Illegally copying or sharing copyrighted software, music or film, which costs creators income, together with sending abusive, threatening or hateful messages online. The internet makes both easy to do and hard to trace or police."},{"id":"fc-1.5.2-009","topicId":"1.5.2","front":"Layout, Colour Paradigms and Character Sets","back":"Cultural factors in interface design: some languages are read right to left, so the screen arrangement must change; red and green carry different meanings in different cultures; and Unicode, not ASCII, is needed to display every language."},{"id":"fc-1.5.2-010","topicId":"1.5.2","front":"Computers in the Workforce","back":"Digital technology changes how and where people work: staff can work remotely and are freed from dangerous or menial tasks, and output rises, but skills can be lost, employers can track staff closely and new technical skills are demanded."},{"id":"fc-1.5.2-011","topicId":"1.5.2","front":"Automated Decision Making","back":"A computer reaches an outcome that affects people with no human involved, such as approving a loan or steering a driverless car. It is fast and consistent but may be wrong or unfair, and people have a legal right to a human review."},{"id":"fc-1.5.2-012","topicId":"1.5.2","front":"Censorship and the Internet","back":"Governments, ISPs or other organisations deliberately block or remove online content. This can protect people from illegal or harmful material, but it restricts freedom of speech and access to information and can be abused for political control."},{"id":"fc-2.1.1-001","topicId":"2.1.1","front":"Abstraction","back":"The removal of unnecessary detail from a problem so that only the features essential to solving it remain. This simplifies a complex problem and lets the programmer focus on its core aspects."},{"id":"fc-2.1.1-002","topicId":"2.1.1","front":"Data Abstraction","back":"Hiding how data is actually stored so that it is used only through its logical operations, for example a stack used through push and pop without knowing whether it is built from an array or a linked list."},{"id":"fc-2.1.1-003","topicId":"2.1.1","front":"Graph Theory","back":"Modelling a problem as a set of nodes joined by edges and leaving out all other detail, for example towns as nodes and roads as weighted edges, so that standard algorithms can find solutions such as the shortest path."},{"id":"fc-2.1.1-004","topicId":"2.1.1","front":"Abstraction vs Reality","back":"The real world contains every detail, much of it irrelevant to a program, whereas a computer model deliberately leaves features out or replaces them with symbols. The model is therefore simpler to build but less realistic."},{"id":"fc-2.1.1-005","topicId":"2.1.1","front":"Representational Abstraction","back":"Producing a simplified picture of something real in which irrelevant details are left out and what remains is shown with symbols or shapes, as an Underground map shows stations and connections but not true distances or geography."},{"id":"fc-2.1.1-006","topicId":"2.1.1","front":"Abstraction by Generalisation","back":"Grouping things by their shared characteristics so that one category, class or solution covers them all, as when cars, vans and lorries are all treated as vehicles. Recognising such common patterns reduces the complexity of the code."},{"id":"fc-2.1.1-007","topicId":"2.1.1","front":"Procedural Abstraction","back":"Using a subroutine through its name, parameters and result without needing to know how it works inside, as when calling sort(). The detail of its implementation is hidden from the programmer who calls it."},{"id":"fc-2.1.1-008","topicId":"2.1.1","front":"Need for Abstraction","back":"Reduces the complexity of design and code, so development time is shorter; makes the problem easier to understand and solve; and produces a program that needs less memory and processing power than one modelling every real detail."},{"id":"fc-2.1.1-009","topicId":"2.1.1","front":"Abstract Model","back":"A simplified representation of a real system, held as data and rules that a program can work with, such as a road network stored as junctions and distances. It includes only the features that affect the solution."},{"id":"fc-2.1.1-010","topicId":"2.1.1","front":"Devising an Abstract Model","back":"Deciding what the solution is for, identifying the real-world features and data that affect it, discarding everything else, and choosing how to represent what is kept, for example as symbols, variables or a data structure."},{"id":"fc-2.1.1-011","topicId":"2.1.1","front":"Layers of Abstraction","back":"A complex system is split into levels, each hiding the detail of the level below and offering a simple interface to the one above, as in the TCP/IP stack or a high-level language above assembly and machine code."},{"id":"fc-2.1.1-012","topicId":"2.1.1","front":"Example: Chess Program","back":"The board is modelled as an 8 by 8 two-dimensional array and each piece as an object with a type, colour and position. The material and size of the real set, the players' names and the surroundings are left out as irrelevant."},{"id":"fc-2.1.2-001","topicId":"2.1.2","front":"Caching","back":"Storing data or instructions that have already been used in a location that can be accessed quickly, such as RAM, in case they are needed again, so that later retrieval is faster."},{"id":"fc-2.1.2-002","topicId":"2.1.2","front":"Inputs","back":"Data that a system is given to process, such as a username typed by a user, a key pressed to steer a vehicle or a sensor reading. It is the data itself in the context of the problem, not the device used to enter it."},{"id":"fc-2.1.2-003","topicId":"2.1.2","front":"Outputs","back":"The results a system produces from its processing and passes to the user or another system, such as a message that a login succeeded or an updated score on screen. It is the information itself, not the device that presents it."},{"id":"fc-2.1.2-004","topicId":"2.1.2","front":"Preconditions","back":"Requirements that must be met before an algorithm or subroutine runs for it to work correctly, for example the data must be sorted before a binary search. Stating them tells whoever uses the code what it assumes."},{"id":"fc-2.1.2-005","topicId":"2.1.2","front":"Reusable Program Components","back":"Self-contained pieces of code, such as subroutines, classes and library routines, that are written once and can then be called many times or used in other programs without being rewritten."},{"id":"fc-2.1.2-006","topicId":"2.1.2","front":"Identifying Inputs and Outputs","back":"Working out, before designing a solution, exactly what data it must be given and what results it must produce in the context of the problem. This is where recognising the problem starts, and it fixes what the processing must achieve."},{"id":"fc-2.1.2-007","topicId":"2.1.2","front":"Benefits of Caching","back":"Data needed again is read from fast memory instead of being fetched again from secondary storage or over a network, so access is faster, the program performs better and less bandwidth and processor time is used."},{"id":"fc-2.1.2-008","topicId":"2.1.2","front":"Drawbacks of Caching","back":"Keeping copies of used data only helps if the same data is requested again. The copies take up memory and may become out of date, and the technique is more complicated to design and implement, adding development time."},{"id":"fc-2.1.2-009","topicId":"2.1.2","front":"Web Caching Example","back":"A browser keeps copies of a site's images, style sheets and scripts on the local device, so that on a return visit they load from local storage instead of being downloaded again: the page appears sooner and less bandwidth is used."},{"id":"fc-2.1.2-010","topicId":"2.1.2","front":"Reusable Components: Benefits","back":"The code does not have to be written again, which saves development time; it has already been tested, so less testing is needed and it is more reliable; and it can be kept in a library for use in other programs."},{"id":"fc-2.1.2-011","topicId":"2.1.2","front":"Reusable Components: Drawbacks","back":"Code written for general use takes longer to plan and design, may not exactly fit a new program so needs adapting, and when it comes from a third party it may be less efficient and depends on others to maintain it."},{"id":"fc-2.1.2-012","topicId":"2.1.2","front":"Preconditions in Practice","back":"Requirements on a subroutine's arguments are either documented, so the caller must guarantee them, or tested in code before the main work begins, as when a function that pops a stack first checks that the stack is not empty."},{"id":"fc-2.1.3-001","topicId":"2.1.3","front":"Procedural Abstraction","back":"Hiding the steps of a computation inside a named subroutine, so that the rest of the program needs to know only what it does, what it is passed and what it returns, not how it is carried out."},{"id":"fc-2.1.3-002","topicId":"2.1.3","front":"Components of a Problem","back":"The distinct sub-problems found when a task is analysed, each a single job that must be dealt with before the whole is solved, such as rolling dice, moving a counter and checking for a winner in a board game."},{"id":"fc-2.1.3-003","topicId":"2.1.3","front":"Decomposition","back":"Breaking a problem down into smaller sub-problems that can each be solved independently. It is repeated until every sub-problem is simple enough to solve, and each can then be written as a subroutine or module."},{"id":"fc-2.1.3-004","topicId":"2.1.3","front":"Sub-procedure","back":"A named block of code that carries out one specific task within a larger program, such as checking a password against the stored one. It is called by name when needed and can be written, tested and reused separately."},{"id":"fc-2.1.3-005","topicId":"2.1.3","front":"Components of a Solution","back":"The subroutines, data structures and other building blocks chosen to solve each sub-problem and then combined into the finished program, for example separate routines to load questions, check an answer and update a score."},{"id":"fc-2.1.3-006","topicId":"2.1.3","front":"Order of Steps","back":"Deciding the sequence in which the sub-tasks of a solution must be carried out, since some need the results of others, as a password can be checked only after it is entered. Tasks that do not depend on each other could run concurrently."},{"id":"fc-2.1.3-007","topicId":"2.1.3","front":"Top-down Design","back":"A design approach that begins with the whole system as a single task and works downwards one level at a time, postponing detail until the lowest-level modules are simple enough to code. Also known as stepwise refinement."},{"id":"fc-2.1.3-008","topicId":"2.1.3","front":"Structure (Hierarchy) Chart","back":"A diagram showing a problem broken down level by level: the overall task is in a box at the top, and beneath each box are the smaller sub-tasks it is split into, joined by lines to show which module each belongs to."},{"id":"fc-2.1.3-009","topicId":"2.1.3","front":"Benefits of Thinking Procedurally","back":"Each sub-problem is smaller, so it is easier to solve, test and debug on its own; the work can be divided between programmers and done at the same time; and subroutines can be reused instead of being written again."},{"id":"fc-2.1.3-010","topicId":"2.1.3","front":"Interface of a Sub-procedure","back":"The name of a subroutine together with the parameters it must be passed and the value it returns. Agreeing this in advance tells each programmer the requirements for their part and lets modules be called without seeing their code."},{"id":"fc-2.1.3-011","topicId":"2.1.3","front":"Example: Checkout Sub-procedures","back":"A supermarket till program split into separate routines: one to scan an item, one to apply discounts, one to calculate the total, one to take payment and one to print the receipt, each called in turn by the main program."},{"id":"fc-2.1.3-012","topicId":"2.1.3","front":"Prerequisite Step","back":"A task that must be finished before another can begin because the later one needs its result, as data must be validated before it is saved. Finding these dependencies fixes the sequence in which a solution is carried out."},{"id":"fc-2.1.4-001","topicId":"2.1.4","front":"Flowcharts","back":"Diagrams that show the steps of an algorithm as standard symbols joined by arrows: rectangles for processes, parallelograms for input and output, and diamonds where a condition sends the flow down one of two paths."},{"id":"fc-2.1.4-002","topicId":"2.1.4","front":"Logical Conditions","back":"Tests that evaluate to either true or false, such as whether a stack is full or a timer has reached zero; a program checks them to decide which action to carry out next."},{"id":"fc-2.1.4-003","topicId":"2.1.4","front":"Decision Points in a Solution","back":"The places in an algorithm where a question with a true or false answer has to be asked, such as whether a chosen space is already taken, with one action following if it is true and a different one if it is false."},{"id":"fc-2.1.4-004","topicId":"2.1.4","front":"Selection (Branching)","back":"A programming construct in which the result of a condition decides which block of code is run, so only one of the possible paths is executed, and only once. Written with if, elseif and else, or with switch and case."},{"id":"fc-2.1.4-005","topicId":"2.1.4","front":"Boolean Operators in Conditions","back":"AND, OR and NOT, used to join or reverse the tests inside one condition: AND is true only when both parts are true, OR is true when at least one part is true, and NOT turns true into false and false into true."},{"id":"fc-2.1.4-006","topicId":"2.1.4","front":"Nested Decisions","back":"One if statement placed inside a branch of another, so the inner condition is tested only when the outer condition has already given a particular result."},{"id":"fc-2.1.4-007","topicId":"2.1.4","front":"Decisions and Program Flow","back":"The way the outcome of each decision changes the order in which instructions are executed: one set is followed when the condition is true and another when it is false, so different inputs take different routes through the code."},{"id":"fc-2.1.4-008","topicId":"2.1.4","front":"Loop Conditions","back":"The tests that control repetition. A while loop checks its test at the start (pre-condition), so its body may never run; a do-until loop checks at the end (post-condition), so its body always runs at least once."},{"id":"fc-2.1.4-009","topicId":"2.1.4","front":"Trace Table","back":"A grid with a column for each variable and output, filled in one row at a time as an algorithm is followed by hand, recording every change of value so that its logic can be checked and errors found."},{"id":"fc-2.1.4-010","topicId":"2.1.4","front":"Validation as a Decision","back":"A test that input data is sensible and obeys set rules, such as a range, type, length, presence or format check, made before the data is used; data that fails is rejected and the user is asked to enter it again."},{"id":"fc-2.1.4-011","topicId":"2.1.4","front":"Comparison Operators","back":"Symbols that compare two values and give a result of true or false: == equal to, != not equal to, < less than, <= less than or equal to, > greater than, >= greater than or equal to. Also called relational operators."},{"id":"fc-2.1.4-012","topicId":"2.1.4","front":"Flowchart Decision Symbol","back":"A diamond containing a condition, with one arrow entering and two labelled arrows leaving it, Yes and No or True and False, showing the two paths that the algorithm can take from that point."},{"id":"fc-2.1.5-001","topicId":"2.1.5","front":"Concurrent Processing","back":"Several processes are in progress at the same time or at overlapping times, so one can start before another has finished. Each is given a slice of processor time, or different processes are run by different cores."},{"id":"fc-2.1.5-002","topicId":"2.1.5","front":"Concurrent vs Parallel Processing","back":"In the first, processes overlap in time, which a single core can achieve by giving each a slice of processor time; in the second, instructions are executed at the very same instant, which needs more than one core or processor."},{"id":"fc-2.1.5-003","topicId":"2.1.5","front":"Thread","back":"A single sequence of instructions within a program that runs independently and has its own lifetime. A concurrent program consists of several of these, which may all need access to the same data."},{"id":"fc-2.1.5-004","topicId":"2.1.5","front":"Identifying Parts That Can Run Concurrently","back":"Breaking a problem into components and working out which of them do not rely on each other's results, so they can be tackled at the same time; a part that needs another's output has to wait for it."},{"id":"fc-2.1.5-005","topicId":"2.1.5","front":"Dependency Between Tasks","back":"Where one process cannot start or continue until another has produced its result, so the two must be carried out in order. This limits how much of a problem can be worked on at the same time."},{"id":"fc-2.1.5-006","topicId":"2.1.5","front":"Benefits of Concurrent Processing","back":"More efficient use of the processor, with less idle time and greater throughput; long-running tasks do not delay short ones, and the user can carry on interacting with the computer while other tasks are running."},{"id":"fc-2.1.5-007","topicId":"2.1.5","front":"Trade-offs of Concurrent Processing","back":"Programs are more complex to write and test, because only one process at a time can be allowed to change shared data, which makes the others wait; and the speed gained is less than the number of processors suggests."},{"id":"fc-2.1.5-008","topicId":"2.1.5","front":"Race Condition","back":"A fault in which two processes read and change the same data at overlapping times, so the result depends on which happens to run first and one update can overwrite the other, such as the same seat being sold twice."},{"id":"fc-2.1.5-009","topicId":"2.1.5","front":"Deadlock","back":"A situation in which two or more processes are each waiting for a resource that another of them is holding, so none of them can ever continue. It is a risk whenever processes are given exclusive access to shared records."},{"id":"fc-2.1.5-010","topicId":"2.1.5","front":"Lock (Mutual Exclusion)","back":"Allows only one user or process at a time to access or modify a shared record or resource; all others must wait until it is released. This stops updates overwriting each other but can cause delays."},{"id":"fc-2.1.5-011","topicId":"2.1.5","front":"Concurrency in a Multi-user System","back":"A server may receive a large number of requests at once and must respond to each in reasonable time, so requests are handled as separate overlapping processes, with access to shared data restricted so users cannot overwrite each other's changes."},{"id":"fc-2.1.5-012","topicId":"2.1.5","front":"When Concurrency Is Not Worth It","back":"When the steps of a task rely on each other and cannot be split up, as in a binary search, or when a bottleneck such as access to a single storage device stops extra processors from reducing the time taken."},{"id":"fc-2.2.1-001","topicId":"2.2.1","front":"Branching","back":"A construct in which a condition decides which block of code is run next, so program control passes down one of several possible paths and that code is run once rather than repeatedly. Also called selection."},{"id":"fc-2.2.1-002","topicId":"2.2.1","front":"Functions","back":"Named subroutines that always return a value to the code that called them, so a call can be used in an expression or assigned to a variable. Simply outputting a result is not the same as returning one."},{"id":"fc-2.2.1-003","topicId":"2.2.1","front":"Global Variables","back":"Variables that can be accessed from anywhere in the program, so any subroutine can use or change them without them being passed to it. They stay in memory for the whole run-time, and a change in one place can have unwanted side effects elsewhere."},{"id":"fc-2.2.1-004","topicId":"2.2.1","front":"Integrated Development Environment (IDE)","back":"Software that combines the tools for writing and testing code: an editor with features such as auto-complete and syntax highlighting, a translator and run-time environment, and debugging tools such as breakpoints, stepping and a variable watch window."},{"id":"fc-2.2.1-005","topicId":"2.2.1","front":"Iterations","back":"A construct in which the same block of code is run repeatedly, either a fixed number of times (count-controlled, as in a for loop) or while or until a condition is met (condition-controlled, as in a while or do-until loop)."},{"id":"fc-2.2.1-006","topicId":"2.2.1","front":"Local Variables","back":"Variables that exist only within the subroutine in which they are declared and cannot be accessed from outside it. They are removed from memory when the subroutine ends, so data has to be passed into the subroutine and returned from it."},{"id":"fc-2.2.1-007","topicId":"2.2.1","front":"Modularity","back":"Splitting a program into self-contained modules that each carry out one task, so that each can be written by a different programmer, tested on its own, and reused in this or other programs."},{"id":"fc-2.2.1-008","topicId":"2.2.1","front":"Parameter Passing by Reference","back":"The subroutine is given the memory address of the original variable rather than a copy, so any change made inside the subroutine alters the original and is kept after the subroutine ends; less memory is used, as nothing is copied."},{"id":"fc-2.2.1-009","topicId":"2.2.1","front":"Parameter Passing by Value","back":"The subroutine is given a copy of the variable's contents, so any changes are made to the copy and do not overwrite the original variable; the copy is discarded when the subroutine ends."},{"id":"fc-2.2.1-010","topicId":"2.2.1","front":"Parameters","back":"Items of data passed into a subroutine when it is called, named in the brackets after the subroutine's identifier, which allow values to be sent to it. Inside the subroutine each behaves as a variable that exists only there."},{"id":"fc-2.2.1-011","topicId":"2.2.1","front":"Procedures","back":"Named subroutines that carry out a task when called but do not return a value to the calling code, although they can still produce output, such as printing to the screen."},{"id":"fc-2.2.1-012","topicId":"2.2.1","front":"Recursion","back":"Where a subroutine calls itself, with a base case that stops the calls. Each call puts a new copy of its variables on the stack, so it uses more memory than a loop and can run out of stack space."},{"id":"fc-2.2.1-013","topicId":"2.2.1","front":"Sequences","back":"A construct in which instructions are executed one after another, each exactly once, in the order in which they are written, with nothing skipped or repeated."},{"id":"fc-2.2.1-014","topicId":"2.2.1","front":"Subroutines","back":"Named, self-contained blocks of code that carry out one task and can be called from many places in a program, so the code is written and tested once and then reused; some return a value and some do not."},{"id":"fc-2.2.2-001","topicId":"2.2.2","front":"Backtracking","back":"Following one path towards a solution until it can go no further, then returning to the most recent point that still has an unexplored option and trying that instead, as a depth-first traversal does on reaching a leaf node."},{"id":"fc-2.2.2-002","topicId":"2.2.2","front":"Computable Problems","back":"Problems for which an algorithm exists that produces the correct answer in a finite number of steps. To be worth solving by computer they must also be tractable, finishing in reasonable time and memory as the input grows."},{"id":"fc-2.2.2-003","topicId":"2.2.2","front":"Computational Methods","back":"Techniques for analysing a problem and designing a solution that a computer can carry out, such as problem recognition, decomposition, abstraction, heuristics, data mining and visualisation; programming constructs such as iteration do not count."},{"id":"fc-2.2.2-004","topicId":"2.2.2","front":"Data Mining","back":"Analysing very large quantities of data to turn them into useful information, by finding patterns, trends, anomalies and relationships that are not otherwise obvious and that can be used to make predictions and inform decisions."},{"id":"fc-2.2.2-005","topicId":"2.2.2","front":"Divide and Conquer","back":"Repeatedly breaking a problem down into smaller sub-problems of the same kind, finding a solution to each one separately, then combining those solutions to give the solution to the whole problem, as merge sort and quick sort do."},{"id":"fc-2.2.2-006","topicId":"2.2.2","front":"Heuristics","back":"Rules of thumb or educated guesses, such as an estimate of the distance from a node to the goal in pathfinding, used to reach a good-enough solution more quickly, although not guaranteed to be the best one."},{"id":"fc-2.2.2-007","topicId":"2.2.2","front":"Performance Modelling","back":"Simulating or testing the behaviour of a system before it is used, particularly under heavy load such as very large numbers of users or data items, to predict how well it scales when testing the real system would be too expensive or unsafe."},{"id":"fc-2.2.2-008","topicId":"2.2.2","front":"Problem Decomposition","back":"Splitting a problem into smaller sub-problems, and splitting those again, until each is manageable enough to be solved independently; this reveals the subroutines needed and lets the work be shared between programmers."},{"id":"fc-2.2.2-009","topicId":"2.2.2","front":"Problem Recognition","back":"Identifying that there is a problem to be solved and working out exactly what it is from the scenario, including its inputs and required outputs, and whether it can be solved by a computer at all."},{"id":"fc-2.2.2-010","topicId":"2.2.2","front":"Visualisation","back":"Presenting a problem or data in graphical form, using symbols or objects such as the nodes and edges of a graph, so that it is simplified and easier to understand; it is for the benefit of people rather than the computer."},{"id":"fc-2.2.2-011","topicId":"2.2.2","front":"Pipelining","back":"Arranging a task as a series of stages in which the output of one process is the input of the next, so that different stages can overlap; in a processor, one instruction is fetched while another is decoded and a third is executed."},{"id":"fc-2.2.2-012","topicId":"2.2.2","front":"Abstraction in Computational Methods","back":"Removing unnecessary detail from a problem so that only the features needed to solve it remain, for example leaving the scenery out of a simulation; this simplifies the problem and reduces the memory and processing a solution needs."},{"id":"fc-2.3.1-001","topicId":"2.3.1","front":"A* Algorithm","back":"Finds the shortest path between two nodes by always visiting next the node with the lowest total of distance travelled so far plus a heuristic estimate of the distance remaining to the goal, so fewer nodes need to be explored."},{"id":"fc-2.3.1-002","topicId":"2.3.1","front":"Big O Notation","back":"A way of expressing how the number of steps an algorithm takes, or the memory it uses, grows as the amount of data increases. It shows the order of growth rather than an actual time, and is written in a form such as O(n log n)."},{"id":"fc-2.3.1-003","topicId":"2.3.1","front":"Binary Search","back":"Needs sorted data. Compares the middle item with the target and, if they differ, discards the half that cannot contain it by moving the lower or upper bound, repeating until the target is found or no items remain. Worst-case time O(log n)."},{"id":"fc-2.3.1-004","topicId":"2.3.1","front":"Bubble Sort","back":"Compares each pair of adjacent items in turn and swaps them if they are in the wrong order. At the end of the list it starts another pass, stopping when a flag shows that a pass made no swaps. Worst-case time O(n²); space O(1)."},{"id":"fc-2.3.1-005","topicId":"2.3.1","front":"Dijkstra's Shortest Path Algorithm","back":"Finds the shortest path from a start node to the other nodes of a weighted graph by repeatedly visiting the unvisited node with the smallest distance from the start and updating its neighbours whenever a shorter route is found. Uses no heuristic."},{"id":"fc-2.3.1-006","topicId":"2.3.1","front":"Insertion Sort","back":"Treats the first item as a sorted list, then takes each item in turn from the unsorted part and places it at its correct position in the sorted part. Time is O(n²) at worst and O(n) if already in order; space is O(1)."},{"id":"fc-2.3.1-007","topicId":"2.3.1","front":"Linear Search","back":"Compares the first item with the target and, if they are not equal, moves on to the next item, repeating until the target is found or the end of the list is reached. The data does not need to be sorted. Worst-case time O(n)."},{"id":"fc-2.3.1-008","topicId":"2.3.1","front":"Merge Sort","back":"Repeatedly splits the list in half until each sublist holds one item, then combines pairs of sublists by comparing their first items and writing the smaller to a new list, until one sorted list remains. Time O(n log n); space O(n)."},{"id":"fc-2.3.1-009","topicId":"2.3.1","front":"O(1) (Constant Time/Space)","back":"The time taken, or the memory needed, stays the same however many items of data there are, as when an array element is accessed by its index or a sort works in place and needs no extra storage."},{"id":"fc-2.3.1-010","topicId":"2.3.1","front":"O(2ⁿ) (Exponential Time/Space)","back":"The time or memory multiplies by a fixed factor, for example doubling, with every extra item of data, so it grows at an ever-increasing rate and soon becomes unmanageable; not the same as growth in proportion to n squared."},{"id":"fc-2.3.1-011","topicId":"2.3.1","front":"O(log(n)) (Logarithmic Time/Space)","back":"The time or memory grows at a decreasing rate as the number of items increases, so doubling the amount of data adds only one more step; typical of algorithms that halve the data at each stage."},{"id":"fc-2.3.1-012","topicId":"2.3.1","front":"O(n) (Linear Time/Space)","back":"The time or memory increases in direct proportion to the number of items, so doubling the data doubles the number of steps; typical of a single loop through the data. Proportional to n does not mean equal to n."},{"id":"fc-2.3.1-013","topicId":"2.3.1","front":"O(nᵏ) (Polynomial Time/Space)","back":"The time or memory is proportional to the number of items raised to a fixed power, such as n squared when one loop through the data is nested inside another, as in the worst case of the simpler sorting algorithms."},{"id":"fc-2.3.1-014","topicId":"2.3.1","front":"Quick Sort","back":"Selects a pivot, compares every other item with it and places smaller items to its left and larger items to its right, then repeats the process on each sublist until all items are in place. Average time O(n log n); worst case O(n²)."},{"id":"fc-2.3.1-015","topicId":"2.3.1","front":"Space Complexity","back":"How the amount of memory an algorithm needs, over and above the data itself, grows as the number of items increases. A sort that builds new lists has a higher one than a sort that works in place."},{"id":"fc-2.3.1-016","topicId":"2.3.1","front":"Time Complexity","back":"How the number of steps an algorithm takes grows as the number of items to process increases. 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Its results are placed in the accumulator."},{"id":"fc-1.1.1-004","topicId":"1.1.1","front":"Buses","back":"Sets of parallel wires that transfer data, addresses and control signals between the processor, memory and input/output devices. They only carry signals; they do not store, process or control anything."},{"id":"fc-1.1.1-005","topicId":"1.1.1","front":"Contemporary Processor Architecture","back":"A modern processor design that keeps the Von Neumann stored-program model but adds features to improve performance, such as pipelining, cache memory, multiple cores and Harvard-style separate memory for instructions and data."},{"id":"fc-1.1.1-006","topicId":"1.1.1","front":"Control Bus","back":"Carries control signals, such as memory read, memory write, clock and interrupt signals, between the control unit and the other components to coordinate their operation. It is bidirectional."},{"id":"fc-1.1.1-007","topicId":"1.1.1","front":"Control Unit (CU)","back":"Decodes instructions and coordinates the processor by sending control signals, such as memory read and write, along the control bus, directing the movement of data between registers, the ALU, memory and input/output devices."},{"id":"fc-1.1.1-008","topicId":"1.1.1","front":"Current Instruction Register (CIR)","back":"Holds the instruction that has just been fetched, copied from the MDR, while it is decoded and executed. The instruction is split into an opcode (the operation) and an operand (the data or address it uses)."},{"id":"fc-1.1.1-009","topicId":"1.1.1","front":"Data Bus","back":"Carries data and instructions between the processor and memory, and to and from input/output devices. It is bidirectional, since data is both read into the processor and written back out."},{"id":"fc-1.1.1-010","topicId":"1.1.1","front":"Fetch-Decode-Execute Cycle","back":"The sequence the processor repeats continuously to run a stored program: the next instruction is copied from memory into the processor, interpreted by the control unit and then carried out, one instruction after another."},{"id":"fc-1.1.1-011","topicId":"1.1.1","front":"Harvard Architecture","back":"A design in which instructions and data are stored in separate memory units, each with its own set of buses. An instruction and data can therefore be fetched at the same time, rather than one after the other."},{"id":"fc-1.1.1-012","topicId":"1.1.1","front":"Memory Address Register (MAR)","back":"Holds the address of the memory location that is about to be read from or written to, whether it contains an instruction or data. This address is then sent to memory along the address bus."},{"id":"fc-1.1.1-013","topicId":"1.1.1","front":"Memory Data Register (MDR)","back":"Temporarily holds the instruction or data that has just been read from memory, or the data that is about to be written to memory. Its contents travel to and from memory along the data bus."},{"id":"fc-1.1.1-014","topicId":"1.1.1","front":"Pipelining","back":"One instruction is fetched while the previous one is decoded and the one before that is executed, so stages of the cycle overlap. This increases throughput: more instructions are completed in a given time and no part of the processor sits idle."},{"id":"fc-1.1.1-015","topicId":"1.1.1","front":"Program Counter (PC)","back":"Holds the address of the next instruction to be fetched. Its contents are copied to the MAR at the start of each cycle and it is then incremented; a branch instruction overwrites it with a new address."},{"id":"fc-1.1.1-016","topicId":"1.1.1","front":"Registers","back":"Very fast temporary storage locations inside the processor, each used for a single specific purpose, holding one item of data, an instruction or an address. They are quicker to access than RAM or secondary storage."},{"id":"fc-1.1.1-017","topicId":"1.1.1","front":"Von Neumann Architecture","back":"A design in which instructions and data are stored together in the same memory, in the same format, and share a single set of buses. A single control unit works through the instructions in sequence, fetching, decoding and executing each in turn."},{"id":"fc-1.1.1-018","topicId":"1.1.1","front":"Fetch","back":"The address in the PC is copied to the MAR and sent along the address bus, while the control unit sends a read signal along the control bus. The instruction returns along the data bus into the MDR and is copied to the CIR, and the PC is incremented."},{"id":"fc-1.1.1-019","topicId":"1.1.1","front":"Decode","back":"The control unit interprets the instruction held in the CIR. It is split into an opcode, which identifies the operation to carry out, and an operand, which is the data or address to use, and the control signals needed are prepared."},{"id":"fc-1.1.1-020","topicId":"1.1.1","front":"Execute","back":"The operation given by the opcode is carried out: the ALU may perform a calculation and place the result in the accumulator, data may be read from or written to memory using the MAR and MDR, or a branch may change the address in the PC."},{"id":"fc-1.1.1-021","topicId":"1.1.1","front":"Clock Speed","back":"The number of cycles the processor's clock completes each second, measured in hertz (usually GHz). An instruction takes one or more cycles, so a higher rate means more instructions are executed per second."},{"id":"fc-1.1.1-022","topicId":"1.1.1","front":"Number of Cores","back":"How many independent processing units a processor contains, each able to fetch, decode and execute its own instructions. More of them lets several instructions be processed at the same time, if the software is written to use them."},{"id":"fc-1.1.1-023","topicId":"1.1.1","front":"Cache","back":"A small amount of very fast memory in or next to the processor that holds frequently used instructions and data. It is quicker to access than RAM, so the more there is, the less time the processor spends waiting for main memory."},{"id":"fc-1.1.2-001","topicId":"1.1.2","front":"Complex Instruction Set Computer (CISC)","back":"A processor with a large instruction set, including complex instructions that each do a multi-step task and take several clock cycles. It has more addressing modes and more complex circuitry, uses more power and is hard to pipeline, but needs less RAM."},{"id":"fc-1.1.2-002","topicId":"1.1.2","front":"Graphics Processing Unit (GPU)","back":"A processor with many cores designed to perform the same simple operation on large sets of data in parallel. It renders graphics and is also used for other calculations on matrices and vectors, such as modelling and machine learning."},{"id":"fc-1.1.2-003","topicId":"1.1.2","front":"Multicore Systems","back":"A single processor that contains more than one processing unit, each of which can fetch, decode and execute instructions independently and at the same time as the others."},{"id":"fc-1.1.2-004","topicId":"1.1.2","front":"Parallel Processing System","back":"A system that executes more than one instruction at the same moment, by dividing a task between several processors or cores so that different parts are worked on simultaneously. Only tasks that can be split up in this way benefit."},{"id":"fc-1.1.2-005","topicId":"1.1.2","front":"Reduced Instruction Set Computer (RISC)","back":"A processor with a small set of simple instructions that each take one clock cycle, making it easy to pipeline. It has fewer addressing modes and simpler circuitry with fewer transistors, so it uses less power and needs little cooling, but needs more RAM."},{"id":"fc-1.1.2-006","topicId":"1.1.2","front":"SISD","back":"One processor executes one stream of instructions on one stream of data, one instruction at a time, with no parallelism, as in a traditional single-core Von Neumann computer."},{"id":"fc-1.1.2-007","topicId":"1.1.2","front":"SIMD","back":"The same instruction is applied to many items of data at the same time, as when a graphics processor performs one operation on every pixel of an image or every element of a vector or matrix."},{"id":"fc-1.1.2-008","topicId":"1.1.2","front":"MISD","back":"Several processors each carry out a different instruction on the same data at the same time. It is rare in practice, used mainly in fault-tolerant systems where the results are compared to detect errors."},{"id":"fc-1.1.2-009","topicId":"1.1.2","front":"MIMD","back":"Several processors or cores each carry out different instructions on different data at the same time, as in a multicore processor, a supercomputer or a distributed system."},{"id":"fc-1.1.2-010","topicId":"1.1.2","front":"Why RISC suits mobile devices","back":"Its simple instructions need fewer transistors and less complex circuitry, so it uses less power and produces less heat. The device lasts longer on a smaller battery and needs little or no cooling."},{"id":"fc-1.1.2-011","topicId":"1.1.2","front":"Why CISC suits desktops and servers","back":"Mains power and space for cooling mean its higher energy use and heat matter less. Single instructions carry out complex tasks, so programs are shorter and need less RAM, and existing desktop software runs without being rewritten."},{"id":"fc-1.1.2-012","topicId":"1.1.2","front":"Pipelining and RISC","back":"Each instruction is simple, similar in format and takes one clock cycle, so the stages of successive instructions overlap easily. Complex instructions that take varying numbers of cycles are difficult to overlap in this way."},{"id":"fc-1.1.2-013","topicId":"1.1.2","front":"Co-processor","back":"An extra processor that works alongside the main processor and takes over one specialised type of task, such as floating-point arithmetic or graphics, which it performs more efficiently, leaving the main processor free for other work."},{"id":"fc-1.1.2-014","topicId":"1.1.2","front":"Benefit of multicore processors","back":"More than one instruction or thread can be processed at the same time, one on each processing unit. This enables parallel processing and improves multitasking, so suitably written programs finish sooner."},{"id":"fc-1.1.2-015","topicId":"1.1.2","front":"Limitation of multicore processors","back":"Extra processing units only help if the program or operating system is written to use them. Instructions that depend on the results of others cannot run in parallel, and speed is still limited by clock speed, cache and bottlenecks."},{"id":"fc-1.1.2-016","topicId":"1.1.2","front":"GPU versus CPU design","back":"A general-purpose processor has a few powerful cores suited to complex operations on smaller sets of data. A graphics processor has many simpler cores suited to performing simple operations in parallel on large sets of data."},{"id":"fc-1.1.3-001","topicId":"1.1.3","front":"Input Devices","back":"Hardware that captures data or commands from a user or the surroundings and passes them into the computer as digital data for processing, such as a keyboard, a microphone or a sensor."},{"id":"fc-1.1.3-002","topicId":"1.1.3","front":"Output Devices","back":"Hardware that receives processed data from the computer and presents it to the user or acts on the surroundings, as images, sound, print or movement; for example a monitor, speakers or LEDs."},{"id":"fc-1.1.3-003","topicId":"1.1.3","front":"Storage Device","back":"Hardware that retains programs and data when the power is off (secondary storage), ready to be loaded into main memory when needed. The main types are magnetic, optical and solid state (flash)."},{"id":"fc-1.1.3-004","topicId":"1.1.3","front":"Random Access Memory (RAM)","back":"Volatile primary memory that holds the programs, data and parts of the operating system currently in use; its contents are lost when the power is off. Having more of it reduces the need for slower virtual memory."},{"id":"fc-1.1.3-005","topicId":"1.1.3","front":"Read-Only Memory (ROM)","back":"Non-volatile memory whose contents are kept without power and cannot normally be overwritten. It holds the start-up instructions (BIOS or bootstrap loader) and, in embedded systems, firmware or the operating system."},{"id":"fc-1.1.3-006","topicId":"1.1.3","front":"Flash Storage","back":"The solid state technology used in memory cards, USB sticks and drives. Data is held as electrical charge in semiconductor chips with no moving parts, making it durable, compact, fast and low in power use, but dearer per gigabyte than magnetic."},{"id":"fc-1.1.3-007","topicId":"1.1.3","front":"Magnetic Storage","back":"Stores data as magnetised areas of a disk or tape, opposite polarities representing 1 and 0. It gives high capacity at a low cost per gigabyte but has moving parts, so it is slower, less durable and uses more power than solid state."},{"id":"fc-1.1.3-008","topicId":"1.1.3","front":"Optical Storage","back":"Data is read from and written to a reflective disc using a laser, and is stored as pits and lands on the surface. Examples are CD and Blu-ray. Discs are cheap to produce and portable, but hold much less than magnetic or solid state media."},{"id":"fc-1.1.3-009","topicId":"1.1.3","front":"Cloud Storage","back":"Data is held on remote servers run by a provider and accessed over the internet. It can be reached from any location and expanded when needed, but depends on an internet connection and on the provider's servers and security."},{"id":"fc-1.1.3-010","topicId":"1.1.3","front":"Volatile memory","back":"Memory that loses its contents when the power is switched off. RAM is of this type, so programs and data in use must be saved to secondary storage if they are to be kept."},{"id":"fc-1.1.3-011","topicId":"1.1.3","front":"Non-volatile memory","back":"Memory or storage that keeps its contents when the power is switched off. ROM and secondary storage (magnetic, optical and solid state) are of this type, so they hold start-up instructions and saved files."},{"id":"fc-1.1.3-012","topicId":"1.1.3","front":"Virtual storage","back":"Remote storage, for example on a network server or in the cloud, that is mounted so it appears and behaves like a local drive. The data is still held on physical media elsewhere and can be accessed from other machines."},{"id":"fc-1.1.3-013","topicId":"1.1.3","front":"Choosing a storage device","back":"A decision made by weighing capacity, read and write speed, cost per gigabyte, durability (moving parts or not), physical size and power consumption against the needs of the scenario, with each reason tied to the context."},{"id":"fc-1.1.3-014","topicId":"1.1.3","front":"Hard disk drive (HDD)","back":"A device with spinning magnetic platters and moving read/write heads. It offers large capacity at a low cost per gigabyte, so suits bulk storage and backup, but is slower than solid state and can be damaged if dropped."},{"id":"fc-1.1.3-015","topicId":"1.1.3","front":"Solid state drive (SSD)","back":"A drive made of flash memory chips that replaces a hard disk as a computer's main secondary storage. No moving parts gives fast access, quick booting and durability, but it costs more per gigabyte and has a limited number of write cycles."},{"id":"fc-1.1.3-016","topicId":"1.1.3","front":"Sensors as input devices","back":"Devices that measure a physical quantity such as temperature, light, movement or heart rate and pass the reading to the computer as data, without anyone entering it. Typical in embedded and control systems, such as a fitness tracker."},{"id":"fc-1.1.3-017","topicId":"1.1.3","front":"Actuators as output devices","back":"Devices that convert an electrical signal from the computer into physical movement or action, such as a motor turning or a valve opening. A control system uses them to respond to the readings from its sensors."},{"id":"fc-1.1.3-018","topicId":"1.1.3","front":"Barcode and RFID readers","back":"Input hardware that identifies items automatically. One type reads a printed pattern of lines using reflected light and needs line of sight; the other reads data from a tag using radio waves, without contact or line of sight."},{"id":"fc-1.1.3-019","topicId":"1.1.3","front":"Storage in embedded systems","back":"The firmware or operating system is held in ROM because it rarely needs to change, cannot be accidentally overwritten and is available as soon as the device is switched on. Any data is saved to flash, which is small, durable and low-power."},{"id":"fc-1.2.1-001","topicId":"1.2.1","front":"Operating System","back":"System software that manages the computer's hardware and provides a platform on which other software runs. It manages memory, processor time, peripherals, files and security, and provides a user interface."},{"id":"fc-1.2.1-002","topicId":"1.2.1","front":"Basic Input Output System (BIOS)","back":"Start-up instructions held in ROM that run when the computer is switched on. They load the stored hardware settings, check that hardware is present and working (the POST) and then find and load the operating system into memory."},{"id":"fc-1.2.1-003","topicId":"1.2.1","front":"Device Drivers","back":"Software that lets the operating system communicate with a hardware device. It translates the operating system's instructions into a form the specific device understands, so hardware from different manufacturers can be used."},{"id":"fc-1.2.1-004","topicId":"1.2.1","front":"Memory Management","back":"The operating system function that allocates RAM to programs and frees it when no longer needed, so memory is used efficiently and several programs can run at once. It also prevents a program accessing memory allocated to another."},{"id":"fc-1.2.1-005","topicId":"1.2.1","front":"Paging","back":"Memory is divided into blocks of equal, fixed size. The division is physical, taking no account of how a program is structured, and the blocks holding one program need not be next to each other in memory."},{"id":"fc-1.2.1-006","topicId":"1.2.1","front":"Segmentation","back":"Memory is divided into blocks of variable size. The division is logical, each block matching a part of the program such as a subroutine or data structure, so block sizes depend on what they contain."},{"id":"fc-1.2.1-007","topicId":"1.2.1","front":"Virtual Memory","back":"An area of secondary storage that the operating system uses as an extension of RAM when RAM is nearly full. Pages not currently needed are moved out to it to free space and moved back when required; excessive swapping causes disk thrashing."},{"id":"fc-1.2.1-008","topicId":"1.2.1","front":"Virtual Machines","back":"Software that takes on the function of a physical computer. It needs a real machine to run on but behaves like separate hardware, so it can run one operating system inside another or execute intermediate code."},{"id":"fc-1.2.1-009","topicId":"1.2.1","front":"Scheduling","back":"The operating system function that decides which of the waiting processes is given processor time next and for how long it runs, with the aim of using the processor efficiently and keeping the system responsive."},{"id":"fc-1.2.1-010","topicId":"1.2.1","front":"First Come First Served (FCFS)","back":"Processes are run in the order in which they arrive, like a queue. Every process has equal priority and runs until it is complete, so a long process delays all those waiting behind it."},{"id":"fc-1.2.1-011","topicId":"1.2.1","front":"Round-Robin","back":"Each process in turn is given the same fixed amount of processor time. A process that has not finished when its time slice ends is suspended and sent to the back of the queue, and the next process starts."},{"id":"fc-1.2.1-012","topicId":"1.2.1","front":"Shortest Job First (SJF)","back":"The waiting process that needs the least total processor time is chosen to run next. Once started, a process runs until it is fully complete and is never suspended for another."},{"id":"fc-1.2.1-013","topicId":"1.2.1","front":"Shortest Remaining Time (SRT)","back":"The process with the least time left until completion is chosen to run next. The running process is suspended if a new process arrives that needs less time than it still has left."},{"id":"fc-1.2.1-014","topicId":"1.2.1","front":"Multi-level Feedback Queues","back":"Processes are placed in several queues, each with a different priority. A process can be moved to a higher or lower queue as its behaviour changes, for example according to how much processor time it uses."},{"id":"fc-1.2.1-015","topicId":"1.2.1","front":"Multi-tasking Operating Systems","back":"Allows more than one program to run at the same time on one computer, by switching processor time between them so quickly that they appear to run simultaneously, for example editing a document while checking email."},{"id":"fc-1.2.1-016","topicId":"1.2.1","front":"Multi-User Operating Systems","back":"Allows several users to use the same computer at the same time, typically from separate terminals, sharing its processor time and resources between them. This is more than one machine simply holding several user accounts."},{"id":"fc-1.2.1-017","topicId":"1.2.1","front":"Distributed Operating Systems","back":"Allows several networked computers to work together on a single task, sharing the processing between them while appearing to the user as one system."},{"id":"fc-1.2.1-018","topicId":"1.2.1","front":"Real-Time Operating Systems","back":"Processes inputs and responds within a guaranteed, predictable time. It is used in safety-critical systems, such as aircraft control or a self-driving car, where a late response could be dangerous."},{"id":"fc-1.2.1-019","topicId":"1.2.1","front":"Embedded Operating Systems","back":"System software built into a device and specific to its hardware and purpose, so it has a limited, dedicated function. It is held in ROM and so cannot be changed, for example in a washing machine."},{"id":"fc-1.2.1-020","topicId":"1.2.1","front":"Interrupts","back":"Signals sent to the processor by hardware or software to show that something needs attention. They are checked for at the end of each Fetch-Decode-Execute cycle; one of higher priority than the current task causes that task to be suspended."},{"id":"fc-1.2.1-021","topicId":"1.2.1","front":"Interrupt Service Routines (ISR)","back":"The code run to deal with a request for the processor's attention. Register contents are first pushed onto a stack and the program counter is set to this code's address; when it finishes, the values are restored and the previous task resumes."},{"id":"fc-1.2.1-022","topicId":"1.2.1","front":"Intermediate Code","back":"Code produced by a compiler that lies between high-level source code and machine code. It is not specific to any processor, so the same code can be run on any computer that has a virtual machine to translate and execute it."},{"id":"fc-1.2.2-001","topicId":"1.2.2","front":"Applications","back":"Software that lets the user carry out a task or solve a problem of their own, rather than maintaining the computer itself, for example a word processor, spreadsheet, database or web browser."},{"id":"fc-1.2.2-002","topicId":"1.2.2","front":"Utilities","back":"Small pieces of system software, each with one purpose, used to maintain, manage or monitor the computer system, for example disk defragmentation, compression, backup or anti-virus software."},{"id":"fc-1.2.2-003","topicId":"1.2.2","front":"Translator","back":"Any program that converts source code, written in a high-level or assembly language, into machine code (object code) that the processor can execute."},{"id":"fc-1.2.2-004","topicId":"1.2.2","front":"Compilers","back":"Translate all of a high-level program into machine code at once, before it is run, producing an executable file that can be run repeatedly without the source code or further translation. All errors are reported together at the end."},{"id":"fc-1.2.2-005","topicId":"1.2.2","front":"Interpreters","back":"Translate and execute high-level source code one line at a time while the program is running, stopping at the first error. No executable file is produced, so the source code must be translated again every time it is run."},{"id":"fc-1.2.2-006","topicId":"1.2.2","front":"Assembler","back":"Translates a program written in low-level mnemonics into machine code. The conversion is one-to-one: each mnemonic instruction becomes exactly one machine code instruction for that particular processor."},{"id":"fc-1.2.2-007","topicId":"1.2.2","front":"Compilation","back":"The process of translating all of a program's source code into object code before it is run. It takes place in stages: lexical analysis, syntax analysis, code generation and optimisation."},{"id":"fc-1.2.2-008","topicId":"1.2.2","front":"Lexical Analysis","back":"The first stage of compilation. Comments and whitespace are removed, keywords and symbols are replaced with tokens, and identifiers such as variable names are entered into a symbol table."},{"id":"fc-1.2.2-009","topicId":"1.2.2","front":"Syntax Analysis","back":"The second stage of compilation. The tokens are checked against the grammar rules of the language, an error is reported wherever a rule is broken, and an abstract syntax tree is built from them."},{"id":"fc-1.2.2-010","topicId":"1.2.2","front":"Code Generation","back":"The third stage of compilation. The abstract syntax tree is converted into object code, the low-level machine code instructions that the processor can execute."},{"id":"fc-1.2.2-011","topicId":"1.2.2","front":"Optimisation","back":"The stage of compilation that examines the generated code and makes it more efficient without changing what it does, for example by removing unnecessary instructions, so the program runs faster and uses less memory."},{"id":"fc-1.2.2-012","topicId":"1.2.2","front":"Libraries","back":"Collections of pre-compiled, tested routines already written by other programmers that a program can import and call, saving development time because common code need not be rewritten or retested."},{"id":"fc-1.2.2-013","topicId":"1.2.2","front":"Linkers","back":"Combine a program's compiled object code with the library code it uses to form a single executable file, or else add the addresses that let the executable call external library files when it runs."},{"id":"fc-1.2.2-014","topicId":"1.2.2","front":"Loaders","back":"Part of the operating system that copies an executable file from secondary storage into main memory ready to be run, together with any library files that the program requires."},{"id":"fc-1.2.2-015","topicId":"1.2.2","front":"Open Source","back":"Software distributed with its source code freely available, so users have the right to inspect, modify and recompile it to suit their own needs. It is usually free of charge, and anyone can find and fix its bugs."},{"id":"fc-1.2.2-016","topicId":"1.2.2","front":"Closed Source","back":"Software distributed only as executable code, with the source code kept private. Users cannot inspect or modify it or fix bugs themselves, and usually have to pay for a licence and keep to its conditions."},{"id":"fc-1.2.3-001","topicId":"1.2.3","front":"Agile Methodologies","back":"A group of software development approaches that work in short, repeated iterations. Each iteration delivers working software for the user to review, so the project can respond quickly to changing requirements."},{"id":"fc-1.2.3-002","topicId":"1.2.3","front":"Extreme Programming","back":"An agile software development methodology whose emphasis is on the quality of the code. It is iterative, with the program reviewed regularly with the user, so development can respond to changing requirements."},{"id":"fc-1.2.3-003","topicId":"1.2.3","front":"Rapid Application Development","back":"A methodology in which a prototype is built quickly and evaluated by the user, and the feedback is used to produce an improved version. This cycle is repeated until the prototype becomes the final product."},{"id":"fc-1.2.3-004","topicId":"1.2.3","front":"Spiral Model","back":"A risk-driven, iterative methodology. Each cycle passes through four stages: determining objectives, identifying and managing risks, developing and testing, and planning the next cycle, with a fuller prototype produced each time."},{"id":"fc-1.2.3-005","topicId":"1.2.3","front":"Waterfall Lifecycle","back":"A methodology made up of a fixed sequence of stages, such as analysis, design, implementation and testing. Each stage must be completed before the next begins, so the requirements have to be right at the start."},{"id":"fc-1.2.3-006","topicId":"1.2.3","front":"Waterfall: Merits and Drawbacks","back":"Merits: clearly defined sequential stages and responsibilities make it easy to manage, and requirements have to be fully understood at the start. Drawbacks: later changes are hard to make, and the user sees no working software until near the end."},{"id":"fc-1.2.3-007","topicId":"1.2.3","front":"Agile: Merits and Drawbacks","back":"Merits: responds well to changing requirements, and working software is delivered and reviewed by the user frequently. Drawbacks: less documentation is produced, and the final cost and completion date are hard to predict."},{"id":"fc-1.2.3-008","topicId":"1.2.3","front":"Extreme Programming Practices","back":"Programmers work in pairs, write tests before the code they check, release small working versions frequently, constantly refactor the code and share ownership of it, with a customer representative on hand to give feedback."},{"id":"fc-1.2.3-009","topicId":"1.2.3","front":"RAD: Merits and Drawbacks","back":"Merits: a working prototype appears quickly, and user feedback on each version means the final product closely matches what is wanted. Drawbacks: needs users to be available throughout, and suits large or safety-critical systems poorly."},{"id":"fc-1.2.3-010","topicId":"1.2.3","front":"Spiral Model: Merits and Drawbacks","back":"Merits: risks are identified and managed in every cycle, so a project can be changed or dropped early if the risk is too great; suits large, complex projects. Drawbacks: risk analysis needs specialist skill and makes it costly."},{"id":"fc-1.2.3-011","topicId":"1.2.3","front":"Choosing a Methodology","back":"Waterfall suits projects whose requirements are clear and fixed at the start; agile methods, extreme programming and RAD suit requirements that are likely to change; the spiral model suits large projects where risk is high."},{"id":"fc-1.2.3-012","topicId":"1.2.3","front":"Testing Across the Lifecycle","back":"Waterfall tests in a single stage after coding; agile methods test every iteration. Black box testing checks outputs for given inputs without seeing the code, white box testing checks each path through it, and alpha (in-house) precedes beta (real users)."},{"id":"fc-1.2.3-013","topicId":"1.2.3","front":"Writing and Following Algorithms","back":"An algorithm is a finite sequence of unambiguous steps that solves a problem, written in pseudocode or as a flowchart. It is followed by dry-running it with a trace table, recording each variable's value after every step."},{"id":"fc-1.2.4-001","topicId":"1.2.4","front":"Programming Paradigms","back":"Different styles of, or approaches to, writing a program, such as procedural and object-oriented. Each suits particular kinds of problem, so a programmer chooses the one that best fits the task."},{"id":"fc-1.2.4-002","topicId":"1.2.4","front":"Procedural Languages","back":"High-level languages in which a program is a sequence of instructions carried out in the order written. Code is built from sequence, selection and iteration and divided into subroutines that pass data to each other as parameters."},{"id":"fc-1.2.4-003","topicId":"1.2.4","front":"Object-Oriented Languages","back":"Languages in which a program is built from objects created from classes. Each object combines its own data (attributes) with the methods that act on that data, and classes can inherit from one another to reuse code."},{"id":"fc-1.2.4-004","topicId":"1.2.4","front":"Classes","back":"A template from which objects are created. It defines the attributes (data) and the methods (actions) that every object made from it will have."},{"id":"fc-1.2.4-005","topicId":"1.2.4","front":"Objects","back":"An instance of a class, created from it when the program runs. It has the attributes and methods that the class defines, but holds its own values for those attributes."},{"id":"fc-1.2.4-006","topicId":"1.2.4","front":"Methods","back":"A subroutine (procedure or function) defined inside a class that sets out an action an object of that class can perform, such as returning or changing one of the values the object holds."},{"id":"fc-1.2.4-007","topicId":"1.2.4","front":"Attributes","back":"A variable defined inside a class that holds one item of data about an object. Every object of the class stores its own value for it, for example the name held by each customer object."},{"id":"fc-1.2.4-008","topicId":"1.2.4","front":"Encapsulation","back":"Attributes are made private so that they can be read or changed only through the public methods of the class, such as get and set methods. These can validate changes, so data cannot be altered accidentally or in unintended ways."},{"id":"fc-1.2.4-009","topicId":"1.2.4","front":"Inheritance","back":"A child class (subclass) takes on the attributes and methods of its parent class (superclass). It can also add attributes and methods of its own, and override those it has been given."},{"id":"fc-1.2.4-010","topicId":"1.2.4","front":"Polymorphism","back":"Methods and attributes can take different forms in different classes. A subclass can override a method of its superclass, so the same method call behaves differently depending on the class of the object it is used on."},{"id":"fc-1.2.4-011","topicId":"1.2.4","front":"Assembly Language","back":"A low-level language in which mnemonics represent machine code instructions. It is specific to one type of processor, each instruction translates into one machine code instruction, and it gives direct control of hardware and memory."},{"id":"fc-1.2.4-012","topicId":"1.2.4","front":"Direct Addressing","back":"The operand is the memory address of the location that holds the data to be used. An operand of 27 means that the value stored at address 27 is fetched and used."},{"id":"fc-1.2.4-013","topicId":"1.2.4","front":"Immediate Addressing","back":"The operand is the actual value to be used, not an address, so no memory access is needed to fetch the data. An operand of 27 means the number 27 itself."},{"id":"fc-1.2.4-014","topicId":"1.2.4","front":"Indexed Addressing","back":"The operand is added to the contents of the index register to give the address of the data. Changing the value in the register steps through consecutive memory locations, such as the elements of an array."},{"id":"fc-1.2.4-015","topicId":"1.2.4","front":"Indirect Addressing","back":"The operand is the address of a memory location that itself holds the address of the data to be used. An operand of 27 means that the value at address 27 is used as the address from which the data is fetched."},{"id":"fc-1.3.1-001","topicId":"1.3.1","front":"Lossless Compression","back":"Reduces file size without permanently removing any data, by rewriting the data in a more efficient format. The original file can be fully reconstructed, so no quality is lost."},{"id":"fc-1.3.1-002","topicId":"1.3.1","front":"Lossy Compression","back":"Reduces file size by permanently removing some of the data, usually the detail least likely to be noticed. The original file cannot be recreated and quality is reduced, but a much smaller file is produced."},{"id":"fc-1.3.1-003","topicId":"1.3.1","front":"Dictionary Coding","back":"Lossless method in which frequently occurring groups of characters are replaced by shorter tokens. A table stored with the file records which token stands for which group, and is used to restore the original when decompressing."},{"id":"fc-1.3.1-004","topicId":"1.3.1","front":"Run-Length Encoding","back":"Lossless method that relies on consecutive items of data being the same. Each set of repeated consecutive values is stored once, together with the number of times it occurs."},{"id":"fc-1.3.1-005","topicId":"1.3.1","front":"Symmetric Encryption","back":"The same key is used both to encrypt and to decrypt the data. Sender and receiver must both hold that key, so it has to be exchanged securely; best suited to cases where one party does both, such as encrypting a backup."},{"id":"fc-1.3.1-006","topicId":"1.3.1","front":"Asymmetric Encryption","back":"Uses two different but related keys, one to encrypt and one to decrypt. Data encrypted with the recipient's public key can only be decrypted with their private key, so no secret key needs to be exchanged."},{"id":"fc-1.3.1-007","topicId":"1.3.1","front":"Hashing","back":"Applying a one-way algorithm to a value to produce a fixed-size result that cannot be reversed to recover the original. Used to store passwords securely and to calculate the address of a record for direct access."},{"id":"fc-1.3.1-008","topicId":"1.3.1","front":"Compression","back":"The process of making a file smaller so that it takes up less storage space, and so that it can be sent more quickly while using less bandwidth."},{"id":"fc-1.3.1-009","topicId":"1.3.1","front":"When to use lossless compression","back":"When the original must be restored exactly because any loss would be noticeable or make the data unusable, as with text, program code or data files, where losing even a few characters makes them unreadable."},{"id":"fc-1.3.1-010","topicId":"1.3.1","front":"When to use lossy compression","back":"When a small file matters more than exact reproduction and some loss of quality is acceptable, as with photographs on a web page or streamed video and sound, where it cuts bandwidth use, download time and buffering."},{"id":"fc-1.3.1-011","topicId":"1.3.1","front":"Run-length encoding example","back":"AAAABBBCC is stored as 4A3B2C: each group of identical consecutive characters becomes a count followed by the character. Works well on simple images with blocks of one colour, but poorly on ordinary text, which has few such repeats."},{"id":"fc-1.3.1-012","topicId":"1.3.1","front":"Dictionary coding example","back":"In 'the cat and the dog and the bird', 'the' could be replaced by 1 and 'and' by 2, giving '1 cat 2 1 dog 2 1 bird' plus a table mapping 1 and 2 back to the words. Suits text, where the same words recur often."},{"id":"fc-1.3.1-013","topicId":"1.3.1","front":"Encryption","back":"Scrambling data with an algorithm and a key into ciphertext that cannot be understood if it is intercepted or stolen. It is two-way: the original can be restored, but only by someone with the correct key."},{"id":"fc-1.3.1-014","topicId":"1.3.1","front":"Public key","back":"In asymmetric encryption, the key of the pair that can be shared openly with anyone. Others use it to encrypt data for its owner, which can then be decrypted only with the owner's matching private key."},{"id":"fc-1.3.1-015","topicId":"1.3.1","front":"Private key","back":"In asymmetric encryption, the key of the pair that its owner keeps secret and never shares. It decrypts data encrypted with the owner's public key, and can be used to sign data so the sender's identity can be verified."},{"id":"fc-1.3.1-016","topicId":"1.3.1","front":"Key distribution problem","back":"Symmetric encryption needs sender and receiver to hold the same secret key, so it must be passed between them without being intercepted. Asymmetric encryption avoids this, because only the public key is ever shared."},{"id":"fc-1.3.1-017","topicId":"1.3.1","front":"Hashing for passwords","back":"Only the hash of each password is stored, never the password itself. At login the entered password is hashed and compared with the stored hash. As the function is one-way, anyone who gains access to the database cannot recover the passwords."},{"id":"fc-1.3.1-018","topicId":"1.3.1","front":"Properties of a hash function","back":"The output has a fixed length whatever the size of the input, the same input always gives the same output, it cannot be reversed to find the input, it is quick to calculate, and different inputs rarely give the same output."},{"id":"fc-1.3.1-019","topicId":"1.3.1","front":"Hash collision","back":"Occurs when two different inputs produce the same hash value. A good function makes this rare; in a hash table it means two items map to the same position, so one must be stored elsewhere, for example in the next free space."},{"id":"fc-1.3.1-020","topicId":"1.3.1","front":"Digital signature","back":"A hash of a message encrypted with the sender's private key and sent with the message. The receiver decrypts it with the sender's public key and compares it with their own hash of the message, confirming the sender and that nothing was altered."},{"id":"fc-1.3.2-001","topicId":"1.3.2","front":"Atomicity","back":"A transaction is either completed in full or not carried out at all; it can never be left partially complete. If any part of it fails, the whole transaction is cancelled and the database is left unchanged."},{"id":"fc-1.3.2-002","topicId":"1.3.2","front":"Consistency","back":"A transaction can only take the database from one valid state to another: every change must obey the rules defined for the database, such as validation rules and referential integrity, or the transaction is rejected."},{"id":"fc-1.3.2-003","topicId":"1.3.2","front":"Isolation","back":"Concurrent transactions cannot interfere with each other: the outcome is the same as if they had been carried out one after another. Achieved by locking a record so that only one transaction can use it at a time."},{"id":"fc-1.3.2-004","topicId":"1.3.2","front":"Durability","back":"Once a transaction has been committed its changes are not lost, even if there is a power cut or system failure, because they are written to secondary storage rather than being held only in RAM."},{"id":"fc-1.3.2-005","topicId":"1.3.2","front":"Relational Databases","back":"Store data in several tables, one for each entity, linked by relationships made with primary and foreign keys. Each item of data is stored only once, giving less redundancy, better consistency and more complex queries than a single table allows."},{"id":"fc-1.3.2-006","topicId":"1.3.2","front":"Entity-Relationship Modelling","back":"Drawing a diagram of a database design in which each entity (table), named in the singular, is a box and the lines joining the boxes show whether they are related one-to-one, one-to-many or many-to-many."},{"id":"fc-1.3.2-007","topicId":"1.3.2","front":"One-to-One Relationship","back":"Each record in one table is linked to no more than one record in the other table, and the same is true in reverse, for example one employee and one staff parking space."},{"id":"fc-1.3.2-008","topicId":"1.3.2","front":"One-to-Many Relationship","back":"A single record in one table can be linked to several records in a second table, but each of those links back to only one record in the first, for example one customer with many bookings. The first table's primary key is a foreign key in the second."},{"id":"fc-1.3.2-009","topicId":"1.3.2","front":"Many-to-Many Relationship","back":"A record in either table can be linked to several records in the other, for example students and courses. It cannot be implemented directly, so a link table is placed between the two, holding the primary key of each as a foreign key."},{"id":"fc-1.3.2-010","topicId":"1.3.2","front":"Primary Key","back":"A field whose value is unique for every record in a table and so identifies each record; the same value can never appear twice, for example a customer ID."},{"id":"fc-1.3.2-011","topicId":"1.3.2","front":"Foreign Key","back":"A field in one table that links to the primary key of a second table, creating the relationship between the two. Its value does not have to be unique and may appear many times in its own table."},{"id":"fc-1.3.2-012","topicId":"1.3.2","front":"Secondary Key","back":"A field, which need not be unique, that records are likely to be searched for or sorted by, such as a surname. It is indexed so that searches on it are faster."},{"id":"fc-1.3.2-013","topicId":"1.3.2","front":"Flat File Database","back":"Holds all the data in a single table, often a plain text file with one record per line. Easy to set up, but data is repeated, which wastes storage, slows searches and allows copies of the same data to become inconsistent."},{"id":"fc-1.3.2-014","topicId":"1.3.2","front":"Indexing","back":"Keeping a separate, ordered list of the values in a chosen field, each with a pointer to its record, so that records can be found without searching the whole table. Searches are faster, but the list takes space and must be updated with the data."},{"id":"fc-1.3.2-015","topicId":"1.3.2","front":"Record Locking","back":"While one user or process is accessing or changing a record, no other can modify it until it is released. This stops simultaneous updates overwriting each other, but users may be delayed and deadlock can occur."},{"id":"fc-1.3.2-016","topicId":"1.3.2","front":"Redundancy","back":"The same data being stored more than once in a database when it does not need to be. It wastes storage space and can lead to inconsistencies if one copy is changed and the others are not."},{"id":"fc-1.3.2-017","topicId":"1.3.2","front":"Referential Integrity","back":"Keeping the links between tables valid: every foreign key value must match an existing primary key value in the related table. If a record is deleted or changed, all references to it must be deleted or changed too."},{"id":"fc-1.3.2-018","topicId":"1.3.2","front":"Database Normalisation","back":"The process of arranging data into linked tables according to a set of rules so that repeated data and unwanted dependencies are removed. The data is stored once, stays consistent and is easier to maintain."},{"id":"fc-1.3.2-019","topicId":"1.3.2","front":"First Normal Form (1NF)","back":"A table that has a primary key, a unique name for every field, no repeating fields or groups of fields, and atomic data in every field, meaning each value cannot be split any further."},{"id":"fc-1.3.2-020","topicId":"1.3.2","front":"Second Normal Form (2NF)","back":"The table is already in first normal form and every non-key field depends on the whole of the primary key, so there are no partial dependencies on only part of a composite key."},{"id":"fc-1.3.2-021","topicId":"1.3.2","front":"Third Normal Form (3NF)","back":"The table is already in second normal form and no non-key field depends on another non-key field, so there are no transitive dependencies: every non-key field depends only on the primary key."},{"id":"fc-1.3.3-001","topicId":"1.3.3","front":"Local Area Networks (LANs)","back":"A collection of connected computers and devices covering a small geographical area, such as one building or site, normally using cabling and hardware owned and maintained by the organisation itself."},{"id":"fc-1.3.3-002","topicId":"1.3.3","front":"Wide Area Networks (WANs)","back":"A collection of connected computers and devices spread over a large geographical area, often using third-party communication channels such as leased telephone lines, undersea cables or satellite links."},{"id":"fc-1.3.3-003","topicId":"1.3.3","front":"Client-Server Network","back":"Client computers connect to a central server, which provides access to resources and services such as shared files, logins and backups. Users, security and backups are managed centrally, but it is expensive to set up and needs specialist staff."},{"id":"fc-1.3.3-004","topicId":"1.3.3","front":"Peer-to-Peer Network","back":"All computers have equal status, with no central server: each shares its own files and hardware directly with the others and is responsible for its own security, logins and backups. Simple and cheap to set up, with little administration."},{"id":"fc-1.3.3-005","topicId":"1.3.3","front":"Circuit Switching","back":"A dedicated path is set up between two devices and kept for the whole communication, with all the data following that one route in order. The hardware along it is tied up and cannot be used by other connections until it ends."},{"id":"fc-1.3.3-006","topicId":"1.3.3","front":"Packet Switching","back":"Data is split into packets that are sent independently, possibly by different routes, and reassembled at the destination. The hardware is shared by many connections, failed links can be routed around and only faulty packets are resent."},{"id":"fc-1.3.3-007","topicId":"1.3.3","front":"Domain Name System (DNS)","back":"Translates the domain name in a URL into the matching IP address. The request goes to a resolver, which checks its cache and otherwise passes the query on to name servers until the authoritative one returns the address to the client."},{"id":"fc-1.3.3-008","topicId":"1.3.3","front":"Protocol Layering","back":"Dividing network communication into separate layers, each with one well-defined job and exchanging data only with the layers next to it. A layer can be changed without affecting the others, and faults are easier to trace to one layer."},{"id":"fc-1.3.3-009","topicId":"1.3.3","front":"TCP/IP Stack","back":"A set of networking protocols for communicating across a network or the internet, arranged in four layers: application, transport, internet and link. Data passes down through the layers when sent and back up them when received."},{"id":"fc-1.3.3-010","topicId":"1.3.3","front":"Encryption","back":"Scrambling data with a key before it is sent across a network, so that anyone who intercepts it cannot understand it. Only a recipient with the correct key can turn it back into the original message."},{"id":"fc-1.3.3-011","topicId":"1.3.3","front":"Firewalls","back":"Hardware or software that monitors the traffic going into and out of a network and filters or blocks packets according to a set of rules, preventing unauthorised access and restricting the sites and applications that can be used."},{"id":"fc-1.3.3-012","topicId":"1.3.3","front":"Proxies","back":"Servers that sit between a user's device and the internet. Requests are sent to them and forwarded on, so the destination never sees the user's own IP address, keeping the user anonymous; they can also filter requests and cache pages."},{"id":"fc-1.3.3-013","topicId":"1.3.3","front":"IP Address","back":"A logical numeric address given to each device on a network, for example by a router, and used to route packets between networks at the internet layer. It can change when the device joins a different network."},{"id":"fc-1.3.3-014","topicId":"1.3.3","front":"MAC Address","back":"A unique identifier given to a device's network interface card when it is manufactured. It is added to packets at the link layer, and a switch uses it to forward data to the correct device on a local network."},{"id":"fc-1.3.4-001","topicId":"1.3.4","front":"HyperText Markup Language (HTML)","back":"Defines the structure and content of a web page using tags written in angle brackets, which mark up elements such as headings, paragraphs, links, images and forms for the browser to display."},{"id":"fc-1.3.4-002","topicId":"1.3.4","front":"Cascading Style Sheets (CSS)","back":"Defines the style and appearance of a web page, such as fonts, colours and borders, using selectors such as tag names, classes and IDs. It can be written inside the page or in an external file shared by many pages, keeping a site consistent."},{"id":"fc-1.3.4-003","topicId":"1.3.4","front":"JavaScript","back":"A scripting language interpreted by the browser to run client-side code that makes a web page interactive, for example validating a form before it is submitted. It will not run if the user has disabled it."},{"id":"fc-1.3.4-004","topicId":"1.3.4","front":"Client-Side Processing","back":"Code is run by the browser on the user's own computer, for example a script validating a form. No request has to be sent to the server, so responses are quicker and server load is reduced, but the code can be seen, altered or disabled."},{"id":"fc-1.3.4-005","topicId":"1.3.4","front":"Server-Side Processing","back":"Code is run on the web server and only the result is sent to the browser, for example querying a database. It is away from the user's control, so cannot be altered or disabled, and is used to repeat validation and for secure tasks."},{"id":"fc-1.3.4-006","topicId":"1.3.4","front":"Search Engine Indexing","back":"Building a database of the words found on web pages, each stored with links to the pages that contain it and its position on those pages. A search is answered from this database instead of by scanning the live web."},{"id":"fc-1.3.4-007","topicId":"1.3.4","front":"PageRank Algorithm","back":"Gives each web page a score used to order search results. The score depends on the number of links to the page from other pages and on the scores of those linking pages; a damping factor allows for users not following links for ever."},{"id":"fc-1.3.4-008","topicId":"1.3.4","front":"Common HTML Tags","back":"Page structure uses , , and <body>. Content uses <h1> to <h3> for headings, <p> for a paragraph, <a href> for a link, <img src> for an image, <ol> or <ul> holding <li> items, <div>, and <form> holding <input> boxes."},{"id":"fc-1.3.4-009","topicId":"1.3.4","front":"CSS Selectors and Rules","back":"A rule is a selector then declarations in curly brackets, each written property: value; for example h1 { color: blue; } (the property is spelt color). A tag name selects every such element, a full stop prefix a class (usable on many elements) and a hash (#) prefix an id (unique to one)."},{"id":"fc-1.3.4-010","topicId":"1.3.4","front":"JavaScript in the Browser","back":"Runs client-side in response to events such as a button click. document.getElementById finds an element by its id so that its value or innerHTML can be read or changed, letting input be validated and the page updated without reloading."},{"id":"fc-1.3.4-011","topicId":"1.3.4","front":"Search Engine Indexing","back":"Programs called crawlers or spiders visit web pages, moving from page to page by following hyperlinks. For each page they record information such as its text, meta tags and the position of each word, and store it in the search engine's index."},{"id":"fc-1.3.4-012","topicId":"1.3.4","front":"PageRank Formula","back":"PR(A) = (1 - d) + d x (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn)), where T1 to Tn are the pages that link to A, C(T) is the number of outbound links on page T and d is the damping factor, a value between 0 and 1 (about 0.85)."},{"id":"fc-1.4.1-001","topicId":"1.4.1","front":"Binary","back":"A base-2 number system that uses only the digits 0 and 1, with place values that are powers of 2. Computers store all data in this form because their circuits are switches (transistors) with two states, on and off."},{"id":"fc-1.4.1-002","topicId":"1.4.1","front":"Denary","back":"The base-10 number system used in everyday counting. It has ten digits, 0 to 9, and place values that are powers of 10: units, tens, hundreds and so on."},{"id":"fc-1.4.1-003","topicId":"1.4.1","front":"Hexadecimal","back":"A base-16 number system using the digits 0 to 9 then the letters A to F for the values 10 to 15. Each digit represents four bits (a nibble), so it is a shorter way to write binary that people can read and copy with fewer mistakes."},{"id":"fc-1.4.1-004","topicId":"1.4.1","front":"Boolean","back":"A data type that can hold only one of two values, TRUE or FALSE (1 or 0). It is used for flags and for the result of a comparison or condition, such as 10 > 5."},{"id":"fc-1.4.1-005","topicId":"1.4.1","front":"Character","back":"A data type that holds a single letter, digit, punctuation mark or other symbol, such as 'j'. It is stored as that symbol's binary code from a set such as ASCII or Unicode."},{"id":"fc-1.4.1-006","topicId":"1.4.1","front":"String","back":"A data type that holds a sequence of characters treated as text, such as 'Hello World'. Digits stored this way must be converted (cast) to a numeric type before they can be used in calculations."},{"id":"fc-1.4.1-007","topicId":"1.4.1","front":"Integer","back":"A data type that holds a whole number, which can be positive, negative or zero, with no fractional part, such as -42 or 23."},{"id":"fc-1.4.1-008","topicId":"1.4.1","front":"Real/Floating Point","back":"A data type that holds a number with a fractional part, such as 3.14 or -0.5. It is stored in binary as a mantissa and an exponent."},{"id":"fc-1.4.1-009","topicId":"1.4.1","front":"ASCII","back":"A character set that maps each character to a unique binary code, using 7 bits (128 characters) or 8 bits in its extended form (256). It covers English letters, digits and common symbols but not most other alphabets."},{"id":"fc-1.4.1-010","topicId":"1.4.1","front":"Unicode","back":"A character set that can use 16 or 32 bits per character instead of 7 or 8, so it can represent far more characters, including the alphabets of other languages and emojis. The same text therefore takes more storage. Its first 128 codes match ASCII."},{"id":"fc-1.4.1-011","topicId":"1.4.1","front":"AND (∧)","back":"A logical operation whose output is 1 (TRUE) only when both of its inputs are 1; otherwise the output is 0. Used with a mask, it keeps the bits where the mask has a 1 and sets all the other bits to 0."},{"id":"fc-1.4.1-012","topicId":"1.4.1","front":"OR (∨)","back":"A logical operation whose output is 1 (TRUE) when at least one of its inputs is 1, and 0 only when every input is 0. Used with a mask, it sets to 1 the bits where the mask has a 1 and leaves the other bits unchanged."},{"id":"fc-1.4.1-013","topicId":"1.4.1","front":"XOR (⊻)","back":"A logical operation whose output is 1 (TRUE) when exactly one of its two inputs is 1, and 0 when both inputs are the same. Used with a mask, it flips the bits where the mask has a 1."},{"id":"fc-1.4.1-014","topicId":"1.4.1","front":"NOT (¬)","back":"A logical operation with a single input whose output is the opposite of that input: 1 becomes 0 and 0 becomes 1. Applied to a binary number, it inverts every bit."},{"id":"fc-1.4.1-015","topicId":"1.4.1","front":"Bitwise Manipulation","back":"Working directly on the individual bits of a binary value, by shifting them left or right or by combining the value with a mask using AND, OR or XOR, in order to extract, set, clear or flip chosen bits."},{"id":"fc-1.4.1-016","topicId":"1.4.1","front":"Shifts","back":"Moving every bit of a binary number left or right by a given number of places. Each place moved left multiplies the value by 2 and each place moved right divides it by 2, so three places right divides by 8."},{"id":"fc-1.4.1-017","topicId":"1.4.1","front":"Two's Complement","back":"A way of storing signed binary integers in which the most significant bit has a negative place value (-128 in 8 bits). It has only one representation of zero, and subtraction is done by adding a negative number, so no extra hardware is needed."},{"id":"fc-1.4.1-018","topicId":"1.4.1","front":"Sign and Magnitude","back":"A way of storing signed binary numbers in which the most significant bit shows the sign (0 for positive, 1 for negative) and the remaining bits hold the size of the number. It has two representations of zero and makes arithmetic harder."},{"id":"fc-1.4.1-019","topicId":"1.4.1","front":"Exponent","back":"The part of a floating point number that gives how many places the binary point in the mantissa must be moved: to the right if it is positive, to the left if it is negative. Giving it more bits increases the range of values that can be stored."},{"id":"fc-1.4.1-020","topicId":"1.4.1","front":"Mantissa","back":"The part of a floating point number that holds its significant digits, stored as a two's complement binary fraction with the binary point after the first bit. Giving it more bits increases the precision of the number."},{"id":"fc-1.4.1-021","topicId":"1.4.1","front":"Normalisation in Binary","back":"Adjusting a floating point number so that the first two bits of the mantissa differ, 01 for a positive number or 10 for a negative one, with the exponent changed to match. This gives the most precision for the bits available and a unique form for each value."},{"id":"fc-1.4.1-022","topicId":"1.4.1","front":"Floating Point Arithmetic","back":"Adding or subtracting numbers stored as a mantissa and an exponent: the binary point of each mantissa is moved by its exponent so that the two numbers line up, they are added or subtracted, and the result is normalised to give the new mantissa and exponent."},{"id":"fc-1.4.2-001","topicId":"1.4.2","front":"Arrays","back":"A static data structure of fixed size holding items of a single data type under one identifier. Each item is accessed directly by its index, the contents can be changed, and it can have more than one dimension, such as rows and columns."},{"id":"fc-1.4.2-002","topicId":"1.4.2","front":"Lists","back":"A data structure holding an ordered sequence of items whose size can change while the program runs (dynamic). Its items can be of different data types and can be changed, added or removed."},{"id":"fc-1.4.2-003","topicId":"1.4.2","front":"Linked Lists","back":"A dynamic data structure made of nodes, each holding a data item and a pointer to the next node. A start pointer gives the first node, the last node holds a null pointer, and items can only be reached by following the pointers in order."},{"id":"fc-1.4.2-004","topicId":"1.4.2","front":"Tuples","back":"An ordered collection of items, which can be of different data types, that is immutable: once it has been created its contents cannot be changed while the program runs."},{"id":"fc-1.4.2-005","topicId":"1.4.2","front":"Records","back":"A data structure that groups related items of data, called fields, under one identifier. The fields can be of different data types, and each one is accessed by its field name."},{"id":"fc-1.4.2-006","topicId":"1.4.2","front":"Stacks","back":"A last in, first out (LIFO) data structure. Items are pushed onto and popped from the same end, the top, which is tracked by a single pointer, so the most recently added item is the first to be removed."},{"id":"fc-1.4.2-007","topicId":"1.4.2","front":"Queues","back":"A first in, first out (FIFO) data structure. Items are added (enqueued) at the rear and removed (dequeued) from the front, so they leave in the order they arrived. A head pointer and a tail pointer mark its two ends."},{"id":"fc-1.4.2-008","topicId":"1.4.2","front":"Circular Queues","back":"A first in, first out structure held in a fixed-size array in which the head and tail pointers wrap round to index 0 after the last index (using MOD). Space freed at the front is reused, so items never have to be moved along."},{"id":"fc-1.4.2-009","topicId":"1.4.2","front":"Trees","back":"A hierarchical data structure of nodes joined by branches (edges), starting from a single root node. Each node can have child nodes, a node with no children is a leaf, and there are no cycles."},{"id":"fc-1.4.2-010","topicId":"1.4.2","front":"Binary Search Tree","back":"A tree in which each node has at most two children and the nodes are kept in order: values less than a node go to its left and greater values to its right. A search can therefore ignore one subtree at every step."},{"id":"fc-1.4.2-011","topicId":"1.4.2","front":"Graphs","back":"A data structure made of nodes (vertices) connected by edges (arcs), used to model how things are linked. Unlike a tree it has no root node and can contain cycles, and its edges can be directed or undirected and can carry weights."},{"id":"fc-1.4.2-012","topicId":"1.4.2","front":"Directed Graphs","back":"A graph in which every edge (arc) has a direction, so it can only be followed one way: an edge from A to B does not allow movement from B to A."},{"id":"fc-1.4.2-013","topicId":"1.4.2","front":"Undirected Graphs","back":"A graph in which the edges (arcs) have no direction, so every edge can be followed both ways: an edge between A and B links A to B and also B to A."},{"id":"fc-1.4.2-014","topicId":"1.4.2","front":"Breadth First Traversal","back":"Visits the start node, then all the nodes directly connected to it, then the nodes connected to those, working outwards one level at a time. It uses a queue to hold the nodes waiting to be visited."},{"id":"fc-1.4.2-015","topicId":"1.4.2","front":"Depth First Traversal","back":"Follows one branch as far as it can go, then backtracks to the most recent node with unvisited children and follows the next branch. It uses a stack to remember where to return to, and on a tree outputs the nodes in post-order, so the root is last."},{"id":"fc-1.4.2-016","topicId":"1.4.2","front":"Hash Table","back":"A data structure in which a hashing function is applied to an item's key to calculate the position where it is stored. This gives direct access without searching, so lookup time stays constant, O(1), unless two keys give the same position (a collision)."},{"id":"fc-1.4.2-017","topicId":"1.4.2","front":"Indexing","back":"Keeping an ordered list of the values in a chosen field, each with a pointer to its record, so that a database can find records by that field without checking every record. It makes searching faster but uses extra storage."},{"id":"fc-1.4.2-018","topicId":"1.4.2","front":"Dynamic Data Structures","back":"Data structures whose size is not fixed in advance: they can grow and shrink while the program runs, taking and releasing memory as needed. A linked list is one example; an array is static because its size is set when it is declared."},{"id":"fc-1.4.3-001","topicId":"1.4.3","front":"Boolean Logic","back":"A system of logic in which every value is either TRUE or FALSE (1 or 0), and values are combined using operators such as AND, OR, NOT and XOR. It is the basis of logic gates and so of all digital circuits."},{"id":"fc-1.4.3-002","topicId":"1.4.3","front":"Boolean Expressions","back":"A statement made of variables joined by logical operators such as ¬, ∧ and ∨ that evaluates to TRUE or FALSE. It can describe the output of a logic circuit, for example Q = (A ∧ B) ∨ (C ∧ D)."},{"id":"fc-1.4.3-003","topicId":"1.4.3","front":"Association Laws","back":"When the operators are all the same, moving the brackets does not change the result: (A ∧ B) ∧ C ≡ A ∧ (B ∧ C) and (A ∨ B) ∨ C ≡ A ∨ (B ∨ C)."},{"id":"fc-1.4.3-004","topicId":"1.4.3","front":"Commutation Laws","back":"The order of the operands either side of an operator does not change the result: A ∧ B ≡ B ∧ A and A ∨ B ≡ B ∨ A."},{"id":"fc-1.4.3-005","topicId":"1.4.3","front":"Distribution Laws","back":"A term outside a bracket can be combined with each term inside it, or a common term taken out as a factor: A ∧ (B ∨ C) ≡ (A ∧ B) ∨ (A ∧ C) and A ∨ (B ∧ C) ≡ (A ∨ B) ∧ (A ∨ C)."},{"id":"fc-1.4.3-006","topicId":"1.4.3","front":"De Morgan's First Law","back":"Negating an AND of two terms is the same as an OR of the two terms each negated: ¬(A ∧ B) ≡ ¬A ∨ ¬B. So the expression ¬A ∨ ¬B simplifies to ¬(A ∧ B), which is a NAND."},{"id":"fc-1.4.3-007","topicId":"1.4.3","front":"De Morgan's Second Law","back":"Negating an OR of two terms is the same as an AND of the two terms each negated: ¬(A ∨ B) ≡ ¬A ∧ ¬B. So the expression ¬A ∧ ¬B simplifies to ¬(A ∨ B), which is a NOR."},{"id":"fc-1.4.3-008","topicId":"1.4.3","front":"Double Negation Law","back":"Negating a value twice gives back the original value, so two NOTs in a row cancel each other out: ¬(¬A) ≡ A."},{"id":"fc-1.4.3-009","topicId":"1.4.3","front":"D-Type Flip Flops","back":"A logic circuit that stores one bit. It has a data input (D) and a clock input: when the clock signal goes high, the output Q changes to the value of D and holds it until the next clock pulse. A second output, NOT Q, is always the inverse of Q."},{"id":"fc-1.4.3-010","topicId":"1.4.3","front":"Half Adders","back":"A logic circuit that adds two single bits, A and B, and gives two outputs: the sum, S = A XOR B, and the carry, C = A AND B. It has no carry input, so it cannot add in a carry from a previous column."},{"id":"fc-1.4.3-011","topicId":"1.4.3","front":"Full Adders","back":"A logic circuit that adds three bits: A, B and a carry in from the previous column, giving a sum and a carry out. It is made from two half adders joined by an OR gate, and several can be chained, carry out to carry in, to add multi-bit numbers."},{"id":"fc-1.4.3-012","topicId":"1.4.3","front":"Karnaugh Maps","back":"A grid of the outputs of a logic expression for every combination of its inputs. The 1s are put into the largest possible rectangular groups of 1, 2, 4, 8 or 16, wrapping round the edges if needed, and each group gives one term of a simplified expression."},{"id":"fc-1.4.3-013","topicId":"1.4.3","front":"Logic Gate Diagrams","back":"A drawing of a circuit that uses the standard symbols for gates such as AND, OR, NOT and XOR, with lines carrying each labelled input through the gates to the output. It shows how a logical expression is built from gates."},{"id":"fc-1.5.1-001","topicId":"1.5.1","front":"Computer Misuse Act (1990)","back":"UK law that criminalises hacking. Its three offences are unauthorised access to a computer system, unauthorised access with intent to commit further offences, and unauthorised modification of computer material."},{"id":"fc-1.5.1-002","topicId":"1.5.1","front":"Unauthorised Access","back":"The basic offence under the Computer Misuse Act: knowingly gaining entry to a computer system or its data without permission, for example by using someone else's password, even if nothing is changed or stolen."},{"id":"fc-1.5.1-003","topicId":"1.5.1","front":"Cybercrimes","back":"Illegal acts carried out using computers or networks, such as hacking, spreading malware, denial-of-service attacks and online fraud. The Computer Misuse Act is the main UK law aimed at them."},{"id":"fc-1.5.1-004","topicId":"1.5.1","front":"Electronic Vandalism","back":"The Computer Misuse Act offence of unauthorised modification of computer material: deliberately changing, deleting or corrupting programs or data without permission, for example by releasing a virus or altering files."},{"id":"fc-1.5.1-005","topicId":"1.5.1","front":"Information Theft","back":"Entering a system without permission to obtain confidential data such as login or bank details. When the data is wanted for a further crime such as fraud, this is the Computer Misuse Act offence of access with intent to commit further offences."},{"id":"fc-1.5.1-006","topicId":"1.5.1","front":"Copyright, Designs and Patents Act (1988)","back":"UK law that gives the creator of an original work, such as software, music, video or images, automatic ownership of it. Others may not copy, use or distribute the work without permission, which can be granted, bought or licensed."},{"id":"fc-1.5.1-007","topicId":"1.5.1","front":"Intellectual Property Protection","back":"Original creative work, such as program code, music, images and writing, is treated in law as belonging to its creator. Under the Copyright, Designs and Patents Act the creator decides who may copy, adapt or sell it."},{"id":"fc-1.5.1-008","topicId":"1.5.1","front":"Copyright Duration","back":"Under the Copyright, Designs and Patents Act, protection is not permanent: for most works, including software, it lasts for the creator's lifetime plus 70 years, after which the work may be used freely."},{"id":"fc-1.5.1-009","topicId":"1.5.1","front":"Illegal Distribution","back":"Copying, sharing or selling a protected work without the owner's permission, such as pirating software or uploading films for others to download. Under the Copyright, Designs and Patents Act the offender can be fined or prosecuted."},{"id":"fc-1.5.1-010","topicId":"1.5.1","front":"Digital Copyright","back":"Under the Copyright, Designs and Patents Act, works held or sent electronically, such as software, streamed video and online images, are protected like any other. A subscription or licence only gives permission to view or use them."},{"id":"fc-1.5.1-011","topicId":"1.5.1","front":"Data Protection Act (2018 - UK GDPR)","back":"UK law governing how organisations collect, store and use personal data. Data must be processed fairly and lawfully, used only for the purpose it was collected for, kept accurate and secure, and not kept longer than necessary."},{"id":"fc-1.5.1-012","topicId":"1.5.1","front":"Regulation","back":"The Data Protection Act controls the processing of personal data: an organisation needs a lawful reason to collect it, may use it only for that stated purpose, and may hold only data that is adequate, relevant and not excessive."},{"id":"fc-1.5.1-013","topicId":"1.5.1","front":"User Rights","back":"Under the Data Protection Act, an individual may see the personal data an organisation holds about them, have it corrected if it is wrong, and in some cases have it erased or have a decision made by computer alone reviewed by a person."},{"id":"fc-1.5.1-014","topicId":"1.5.1","front":"Security Requirements","back":"The Data Protection Act principle that personal data must be kept safe from unauthorised access, loss or damage, for example by encryption and access rights, and must not be sent to countries without equivalent legal protection."},{"id":"fc-1.5.1-015","topicId":"1.5.1","front":"Accountability","back":"The organisation that holds personal data is responsible for obeying the Data Protection Act and must be able to prove that it does, for example by recording what it holds and why. It can be fined or prosecuted for a breach."},{"id":"fc-1.5.1-016","topicId":"1.5.1","front":"Regulation of Investigatory Powers Act (2000)","back":"UK law that regulates how public bodies such as the police and security services may carry out surveillance and monitor or intercept people's electronic communications, in order to detect crime and protect national security."},{"id":"fc-1.5.1-017","topicId":"1.5.1","front":"Surveillance Powers","back":"Under the Regulation of Investigatory Powers Act, authorised public bodies may watch individuals covertly, monitor their internet activity, carry out mass monitoring of communications and make ISPs fit equipment to enable it."},{"id":"fc-1.5.1-018","topicId":"1.5.1","front":"Interception of Communications","back":"Under the Regulation of Investigatory Powers Act, authorised bodies can obtain the content of private emails, text messages and phone calls, and can demand that ISPs and businesses hand over their customers' messages and records."},{"id":"fc-1.5.1-019","topicId":"1.5.1","front":"Data Access","back":"Under the Regulation of Investigatory Powers Act, authorities can demand protected data and force a person or organisation to decrypt it or hand over the encryption keys. Refusing can lead to a prison sentence."},{"id":"fc-1.5.1-020","topicId":"1.5.1","front":"Public Safety & Privacy","back":"The debate over the Regulation of Investigatory Powers Act: monitoring helps to detect and stop serious crime and terrorism, but it intrudes on innocent people's private lives and has been used by many bodies for minor offences."},{"id":"fc-1.5.2-001","topicId":"1.5.2","front":"Artificial Intelligence & Bias","back":"Systems that perform tasks normally needing human intelligence can make unfair decisions, because they inherit prejudice from their programmers or from unrepresentative training data. Thorough testing and scrutiny of data sets reduce this."},{"id":"fc-1.5.2-002","topicId":"1.5.2","front":"Automation & Job Loss","back":"Computers and robots replace people in repetitive, manual or dangerous work because they need no wages or breaks and can run 24/7, causing unemployment. New jobs appear in designing and maintaining the systems, so workers must retrain."},{"id":"fc-1.5.2-003","topicId":"1.5.2","front":"Privacy Issues","back":"Concerns that personal data, such as images, location and browsing history, is collected, stored or shared without a person's knowledge or consent. Individuals are entitled to keep such information private and to know how it is used."},{"id":"fc-1.5.2-004","topicId":"1.5.2","front":"Environmental Impact","back":"Making, powering and disposing of digital devices uses raw materials and energy and leaves toxic e-waste, often in landfill. Against this, computers cut paper use, reduce travel through home working and control heating efficiently."},{"id":"fc-1.5.2-005","topicId":"1.5.2","front":"Digital Divide","back":"The gap between people who have access to modern technology, a fast internet connection and the skills to use them and those who do not, because of cost, location or age. Those without are disadvantaged in education, work and services."},{"id":"fc-1.5.2-006","topicId":"1.5.2","front":"Monitor Behaviour","back":"Using technology to watch and record what people do, such as CCTV, tracking location or logging employees' email and web use. It can deter crime and improve safety, but it intrudes on privacy and may happen without consent."},{"id":"fc-1.5.2-007","topicId":"1.5.2","front":"Analyse Personal Information","back":"Processing large amounts of data about individuals to find patterns and build profiles, for example for targeted advertising or medical research. It can improve services, but people may not have agreed and the results can be misused."},{"id":"fc-1.5.2-008","topicId":"1.5.2","front":"Piracy and Offensive Communications","back":"Illegally copying or sharing copyrighted software, music or film, which costs creators income, together with sending abusive, threatening or hateful messages online. The internet makes both easy to do and hard to trace or police."},{"id":"fc-1.5.2-009","topicId":"1.5.2","front":"Layout, Colour Paradigms and Character Sets","back":"Cultural factors in interface design: some languages are read right to left, so the screen arrangement must change; red and green carry different meanings in different cultures; and Unicode, not ASCII, is needed to display every language."},{"id":"fc-1.5.2-010","topicId":"1.5.2","front":"Computers in the Workforce","back":"Digital technology changes how and where people work: staff can work remotely and are freed from dangerous or menial tasks, and output rises, but skills can be lost, employers can track staff closely and new technical skills are demanded."},{"id":"fc-1.5.2-011","topicId":"1.5.2","front":"Automated Decision Making","back":"A computer reaches an outcome that affects people with no human involved, such as approving a loan or steering a driverless car. It is fast and consistent but may be wrong or unfair, and people have a legal right to a human review."},{"id":"fc-1.5.2-012","topicId":"1.5.2","front":"Censorship and the Internet","back":"Governments, ISPs or other organisations deliberately block or remove online content. This can protect people from illegal or harmful material, but it restricts freedom of speech and access to information and can be abused for political control."},{"id":"fc-2.1.1-001","topicId":"2.1.1","front":"Abstraction","back":"The removal of unnecessary detail from a problem so that only the features essential to solving it remain. This simplifies a complex problem and lets the programmer focus on its core aspects."},{"id":"fc-2.1.1-002","topicId":"2.1.1","front":"Data Abstraction","back":"Hiding how data is actually stored so that it is used only through its logical operations, for example a stack used through push and pop without knowing whether it is built from an array or a linked list."},{"id":"fc-2.1.1-003","topicId":"2.1.1","front":"Graph Theory","back":"Modelling a problem as a set of nodes joined by edges and leaving out all other detail, for example towns as nodes and roads as weighted edges, so that standard algorithms can find solutions such as the shortest path."},{"id":"fc-2.1.1-004","topicId":"2.1.1","front":"Abstraction vs Reality","back":"The real world contains every detail, much of it irrelevant to a program, whereas a computer model deliberately leaves features out or replaces them with symbols. The model is therefore simpler to build but less realistic."},{"id":"fc-2.1.1-005","topicId":"2.1.1","front":"Representational Abstraction","back":"Producing a simplified picture of something real in which irrelevant details are left out and what remains is shown with symbols or shapes, as an Underground map shows stations and connections but not true distances or geography."},{"id":"fc-2.1.1-006","topicId":"2.1.1","front":"Abstraction by Generalisation","back":"Grouping things by their shared characteristics so that one category, class or solution covers them all, as when cars, vans and lorries are all treated as vehicles. Recognising such common patterns reduces the complexity of the code."},{"id":"fc-2.1.1-007","topicId":"2.1.1","front":"Procedural Abstraction","back":"Using a subroutine through its name, parameters and result without needing to know how it works inside, as when calling sort(). The detail of its implementation is hidden from the programmer who calls it."},{"id":"fc-2.1.1-008","topicId":"2.1.1","front":"Need for Abstraction","back":"Reduces the complexity of design and code, so development time is shorter; makes the problem easier to understand and solve; and produces a program that needs less memory and processing power than one modelling every real detail."},{"id":"fc-2.1.1-009","topicId":"2.1.1","front":"Abstract Model","back":"A simplified representation of a real system, held as data and rules that a program can work with, such as a road network stored as junctions and distances. It includes only the features that affect the solution."},{"id":"fc-2.1.1-010","topicId":"2.1.1","front":"Devising an Abstract Model","back":"Deciding what the solution is for, identifying the real-world features and data that affect it, discarding everything else, and choosing how to represent what is kept, for example as symbols, variables or a data structure."},{"id":"fc-2.1.1-011","topicId":"2.1.1","front":"Layers of Abstraction","back":"A complex system is split into levels, each hiding the detail of the level below and offering a simple interface to the one above, as in the TCP/IP stack or a high-level language above assembly and machine code."},{"id":"fc-2.1.1-012","topicId":"2.1.1","front":"Example: Chess Program","back":"The board is modelled as an 8 by 8 two-dimensional array and each piece as an object with a type, colour and position. The material and size of the real set, the players' names and the surroundings are left out as irrelevant."},{"id":"fc-2.1.2-001","topicId":"2.1.2","front":"Caching","back":"Storing data or instructions that have already been used in a location that can be accessed quickly, such as RAM, in case they are needed again, so that later retrieval is faster."},{"id":"fc-2.1.2-002","topicId":"2.1.2","front":"Inputs","back":"Data that a system is given to process, such as a username typed by a user, a key pressed to steer a vehicle or a sensor reading. It is the data itself in the context of the problem, not the device used to enter it."},{"id":"fc-2.1.2-003","topicId":"2.1.2","front":"Outputs","back":"The results a system produces from its processing and passes to the user or another system, such as a message that a login succeeded or an updated score on screen. It is the information itself, not the device that presents it."},{"id":"fc-2.1.2-004","topicId":"2.1.2","front":"Preconditions","back":"Requirements that must be met before an algorithm or subroutine runs for it to work correctly, for example the data must be sorted before a binary search. Stating them tells whoever uses the code what it assumes."},{"id":"fc-2.1.2-005","topicId":"2.1.2","front":"Reusable Program Components","back":"Self-contained pieces of code, such as subroutines, classes and library routines, that are written once and can then be called many times or used in other programs without being rewritten."},{"id":"fc-2.1.2-006","topicId":"2.1.2","front":"Identifying Inputs and Outputs","back":"Working out, before designing a solution, exactly what data it must be given and what results it must produce in the context of the problem. This is where recognising the problem starts, and it fixes what the processing must achieve."},{"id":"fc-2.1.2-007","topicId":"2.1.2","front":"Benefits of Caching","back":"Data needed again is read from fast memory instead of being fetched again from secondary storage or over a network, so access is faster, the program performs better and less bandwidth and processor time is used."},{"id":"fc-2.1.2-008","topicId":"2.1.2","front":"Drawbacks of Caching","back":"Keeping copies of used data only helps if the same data is requested again. The copies take up memory and may become out of date, and the technique is more complicated to design and implement, adding development time."},{"id":"fc-2.1.2-009","topicId":"2.1.2","front":"Web Caching Example","back":"A browser keeps copies of a site's images, style sheets and scripts on the local device, so that on a return visit they load from local storage instead of being downloaded again: the page appears sooner and less bandwidth is used."},{"id":"fc-2.1.2-010","topicId":"2.1.2","front":"Reusable Components: Benefits","back":"The code does not have to be written again, which saves development time; it has already been tested, so less testing is needed and it is more reliable; and it can be kept in a library for use in other programs."},{"id":"fc-2.1.2-011","topicId":"2.1.2","front":"Reusable Components: Drawbacks","back":"Code written for general use takes longer to plan and design, may not exactly fit a new program so needs adapting, and when it comes from a third party it may be less efficient and depends on others to maintain it."},{"id":"fc-2.1.2-012","topicId":"2.1.2","front":"Preconditions in Practice","back":"Requirements on a subroutine's arguments are either documented, so the caller must guarantee them, or tested in code before the main work begins, as when a function that pops a stack first checks that the stack is not empty."},{"id":"fc-2.1.3-001","topicId":"2.1.3","front":"Procedural Abstraction","back":"Hiding the steps of a computation inside a named subroutine, so that the rest of the program needs to know only what it does, what it is passed and what it returns, not how it is carried out."},{"id":"fc-2.1.3-002","topicId":"2.1.3","front":"Components of a Problem","back":"The distinct sub-problems found when a task is analysed, each a single job that must be dealt with before the whole is solved, such as rolling dice, moving a counter and checking for a winner in a board game."},{"id":"fc-2.1.3-003","topicId":"2.1.3","front":"Decomposition","back":"Breaking a problem down into smaller sub-problems that can each be solved independently. It is repeated until every sub-problem is simple enough to solve, and each can then be written as a subroutine or module."},{"id":"fc-2.1.3-004","topicId":"2.1.3","front":"Sub-procedure","back":"A named block of code that carries out one specific task within a larger program, such as checking a password against the stored one. It is called by name when needed and can be written, tested and reused separately."},{"id":"fc-2.1.3-005","topicId":"2.1.3","front":"Components of a Solution","back":"The subroutines, data structures and other building blocks chosen to solve each sub-problem and then combined into the finished program, for example separate routines to load questions, check an answer and update a score."},{"id":"fc-2.1.3-006","topicId":"2.1.3","front":"Order of Steps","back":"Deciding the sequence in which the sub-tasks of a solution must be carried out, since some need the results of others, as a password can be checked only after it is entered. Tasks that do not depend on each other could run concurrently."},{"id":"fc-2.1.3-007","topicId":"2.1.3","front":"Top-down Design","back":"A design approach that begins with the whole system as a single task and works downwards one level at a time, postponing detail until the lowest-level modules are simple enough to code. Also known as stepwise refinement."},{"id":"fc-2.1.3-008","topicId":"2.1.3","front":"Structure (Hierarchy) Chart","back":"A diagram showing a problem broken down level by level: the overall task is in a box at the top, and beneath each box are the smaller sub-tasks it is split into, joined by lines to show which module each belongs to."},{"id":"fc-2.1.3-009","topicId":"2.1.3","front":"Benefits of Thinking Procedurally","back":"Each sub-problem is smaller, so it is easier to solve, test and debug on its own; the work can be divided between programmers and done at the same time; and subroutines can be reused instead of being written again."},{"id":"fc-2.1.3-010","topicId":"2.1.3","front":"Interface of a Sub-procedure","back":"The name of a subroutine together with the parameters it must be passed and the value it returns. Agreeing this in advance tells each programmer the requirements for their part and lets modules be called without seeing their code."},{"id":"fc-2.1.3-011","topicId":"2.1.3","front":"Example: Checkout Sub-procedures","back":"A supermarket till program split into separate routines: one to scan an item, one to apply discounts, one to calculate the total, one to take payment and one to print the receipt, each called in turn by the main program."},{"id":"fc-2.1.3-012","topicId":"2.1.3","front":"Prerequisite Step","back":"A task that must be finished before another can begin because the later one needs its result, as data must be validated before it is saved. Finding these dependencies fixes the sequence in which a solution is carried out."},{"id":"fc-2.1.4-001","topicId":"2.1.4","front":"Flowcharts","back":"Diagrams that show the steps of an algorithm as standard symbols joined by arrows: rectangles for processes, parallelograms for input and output, and diamonds where a condition sends the flow down one of two paths."},{"id":"fc-2.1.4-002","topicId":"2.1.4","front":"Logical Conditions","back":"Tests that evaluate to either true or false, such as whether a stack is full or a timer has reached zero; a program checks them to decide which action to carry out next."},{"id":"fc-2.1.4-003","topicId":"2.1.4","front":"Decision Points in a Solution","back":"The places in an algorithm where a question with a true or false answer has to be asked, such as whether a chosen space is already taken, with one action following if it is true and a different one if it is false."},{"id":"fc-2.1.4-004","topicId":"2.1.4","front":"Selection (Branching)","back":"A programming construct in which the result of a condition decides which block of code is run, so only one of the possible paths is executed, and only once. Written with if, elseif and else, or with switch and case."},{"id":"fc-2.1.4-005","topicId":"2.1.4","front":"Boolean Operators in Conditions","back":"AND, OR and NOT, used to join or reverse the tests inside one condition: AND is true only when both parts are true, OR is true when at least one part is true, and NOT turns true into false and false into true."},{"id":"fc-2.1.4-006","topicId":"2.1.4","front":"Nested Decisions","back":"One if statement placed inside a branch of another, so the inner condition is tested only when the outer condition has already given a particular result."},{"id":"fc-2.1.4-007","topicId":"2.1.4","front":"Decisions and Program Flow","back":"The way the outcome of each decision changes the order in which instructions are executed: one set is followed when the condition is true and another when it is false, so different inputs take different routes through the code."},{"id":"fc-2.1.4-008","topicId":"2.1.4","front":"Loop Conditions","back":"The tests that control repetition. A while loop checks its test at the start (pre-condition), so its body may never run; a do-until loop checks at the end (post-condition), so its body always runs at least once."},{"id":"fc-2.1.4-009","topicId":"2.1.4","front":"Trace Table","back":"A grid with a column for each variable and output, filled in one row at a time as an algorithm is followed by hand, recording every change of value so that its logic can be checked and errors found."},{"id":"fc-2.1.4-010","topicId":"2.1.4","front":"Validation as a Decision","back":"A test that input data is sensible and obeys set rules, such as a range, type, length, presence or format check, made before the data is used; data that fails is rejected and the user is asked to enter it again."},{"id":"fc-2.1.4-011","topicId":"2.1.4","front":"Comparison Operators","back":"Symbols that compare two values and give a result of true or false: == equal to, != not equal to, < less than, <= less than or equal to, > greater than, >= greater than or equal to. Also called relational operators."},{"id":"fc-2.1.4-012","topicId":"2.1.4","front":"Flowchart Decision Symbol","back":"A diamond containing a condition, with one arrow entering and two labelled arrows leaving it, Yes and No or True and False, showing the two paths that the algorithm can take from that point."},{"id":"fc-2.1.5-001","topicId":"2.1.5","front":"Concurrent Processing","back":"Several processes are in progress at the same time or at overlapping times, so one can start before another has finished. Each is given a slice of processor time, or different processes are run by different cores."},{"id":"fc-2.1.5-002","topicId":"2.1.5","front":"Concurrent vs Parallel Processing","back":"In the first, processes overlap in time, which a single core can achieve by giving each a slice of processor time; in the second, instructions are executed at the very same instant, which needs more than one core or processor."},{"id":"fc-2.1.5-003","topicId":"2.1.5","front":"Thread","back":"A single sequence of instructions within a program that runs independently and has its own lifetime. A concurrent program consists of several of these, which may all need access to the same data."},{"id":"fc-2.1.5-004","topicId":"2.1.5","front":"Identifying Parts That Can Run Concurrently","back":"Breaking a problem into components and working out which of them do not rely on each other's results, so they can be tackled at the same time; a part that needs another's output has to wait for it."},{"id":"fc-2.1.5-005","topicId":"2.1.5","front":"Dependency Between Tasks","back":"Where one process cannot start or continue until another has produced its result, so the two must be carried out in order. This limits how much of a problem can be worked on at the same time."},{"id":"fc-2.1.5-006","topicId":"2.1.5","front":"Benefits of Concurrent Processing","back":"More efficient use of the processor, with less idle time and greater throughput; long-running tasks do not delay short ones, and the user can carry on interacting with the computer while other tasks are running."},{"id":"fc-2.1.5-007","topicId":"2.1.5","front":"Trade-offs of Concurrent Processing","back":"Programs are more complex to write and test, because only one process at a time can be allowed to change shared data, which makes the others wait; and the speed gained is less than the number of processors suggests."},{"id":"fc-2.1.5-008","topicId":"2.1.5","front":"Race Condition","back":"A fault in which two processes read and change the same data at overlapping times, so the result depends on which happens to run first and one update can overwrite the other, such as the same seat being sold twice."},{"id":"fc-2.1.5-009","topicId":"2.1.5","front":"Deadlock","back":"A situation in which two or more processes are each waiting for a resource that another of them is holding, so none of them can ever continue. It is a risk whenever processes are given exclusive access to shared records."},{"id":"fc-2.1.5-010","topicId":"2.1.5","front":"Lock (Mutual Exclusion)","back":"Allows only one user or process at a time to access or modify a shared record or resource; all others must wait until it is released. This stops updates overwriting each other but can cause delays."},{"id":"fc-2.1.5-011","topicId":"2.1.5","front":"Concurrency in a Multi-user System","back":"A server may receive a large number of requests at once and must respond to each in reasonable time, so requests are handled as separate overlapping processes, with access to shared data restricted so users cannot overwrite each other's changes."},{"id":"fc-2.1.5-012","topicId":"2.1.5","front":"When Concurrency Is Not Worth It","back":"When the steps of a task rely on each other and cannot be split up, as in a binary search, or when a bottleneck such as access to a single storage device stops extra processors from reducing the time taken."},{"id":"fc-2.2.1-001","topicId":"2.2.1","front":"Branching","back":"A construct in which a condition decides which block of code is run next, so program control passes down one of several possible paths and that code is run once rather than repeatedly. Also called selection."},{"id":"fc-2.2.1-002","topicId":"2.2.1","front":"Functions","back":"Named subroutines that always return a value to the code that called them, so a call can be used in an expression or assigned to a variable. Simply outputting a result is not the same as returning one."},{"id":"fc-2.2.1-003","topicId":"2.2.1","front":"Global Variables","back":"Variables that can be accessed from anywhere in the program, so any subroutine can use or change them without them being passed to it. They stay in memory for the whole run-time, and a change in one place can have unwanted side effects elsewhere."},{"id":"fc-2.2.1-004","topicId":"2.2.1","front":"Integrated Development Environment (IDE)","back":"Software that combines the tools for writing and testing code: an editor with features such as auto-complete and syntax highlighting, a translator and run-time environment, and debugging tools such as breakpoints, stepping and a variable watch window."},{"id":"fc-2.2.1-005","topicId":"2.2.1","front":"Iterations","back":"A construct in which the same block of code is run repeatedly, either a fixed number of times (count-controlled, as in a for loop) or while or until a condition is met (condition-controlled, as in a while or do-until loop)."},{"id":"fc-2.2.1-006","topicId":"2.2.1","front":"Local Variables","back":"Variables that exist only within the subroutine in which they are declared and cannot be accessed from outside it. They are removed from memory when the subroutine ends, so data has to be passed into the subroutine and returned from it."},{"id":"fc-2.2.1-007","topicId":"2.2.1","front":"Modularity","back":"Splitting a program into self-contained modules that each carry out one task, so that each can be written by a different programmer, tested on its own, and reused in this or other programs."},{"id":"fc-2.2.1-008","topicId":"2.2.1","front":"Parameter Passing by Reference","back":"The subroutine is given the memory address of the original variable rather than a copy, so any change made inside the subroutine alters the original and is kept after the subroutine ends; less memory is used, as nothing is copied."},{"id":"fc-2.2.1-009","topicId":"2.2.1","front":"Parameter Passing by Value","back":"The subroutine is given a copy of the variable's contents, so any changes are made to the copy and do not overwrite the original variable; the copy is discarded when the subroutine ends."},{"id":"fc-2.2.1-010","topicId":"2.2.1","front":"Parameters","back":"Items of data passed into a subroutine when it is called, named in the brackets after the subroutine's identifier, which allow values to be sent to it. Inside the subroutine each behaves as a variable that exists only there."},{"id":"fc-2.2.1-011","topicId":"2.2.1","front":"Procedures","back":"Named subroutines that carry out a task when called but do not return a value to the calling code, although they can still produce output, such as printing to the screen."},{"id":"fc-2.2.1-012","topicId":"2.2.1","front":"Recursion","back":"Where a subroutine calls itself, with a base case that stops the calls. Each call puts a new copy of its variables on the stack, so it uses more memory than a loop and can run out of stack space."},{"id":"fc-2.2.1-013","topicId":"2.2.1","front":"Sequences","back":"A construct in which instructions are executed one after another, each exactly once, in the order in which they are written, with nothing skipped or repeated."},{"id":"fc-2.2.1-014","topicId":"2.2.1","front":"Subroutines","back":"Named, self-contained blocks of code that carry out one task and can be called from many places in a program, so the code is written and tested once and then reused; some return a value and some do not."},{"id":"fc-2.2.2-001","topicId":"2.2.2","front":"Backtracking","back":"Following one path towards a solution until it can go no further, then returning to the most recent point that still has an unexplored option and trying that instead, as a depth-first traversal does on reaching a leaf node."},{"id":"fc-2.2.2-002","topicId":"2.2.2","front":"Computable Problems","back":"Problems for which an algorithm exists that produces the correct answer in a finite number of steps. To be worth solving by computer they must also be tractable, finishing in reasonable time and memory as the input grows."},{"id":"fc-2.2.2-003","topicId":"2.2.2","front":"Computational Methods","back":"Techniques for analysing a problem and designing a solution that a computer can carry out, such as problem recognition, decomposition, abstraction, heuristics, data mining and visualisation; programming constructs such as iteration do not count."},{"id":"fc-2.2.2-004","topicId":"2.2.2","front":"Data Mining","back":"Analysing very large quantities of data to turn them into useful information, by finding patterns, trends, anomalies and relationships that are not otherwise obvious and that can be used to make predictions and inform decisions."},{"id":"fc-2.2.2-005","topicId":"2.2.2","front":"Divide and Conquer","back":"Repeatedly breaking a problem down into smaller sub-problems of the same kind, finding a solution to each one separately, then combining those solutions to give the solution to the whole problem, as merge sort and quick sort do."},{"id":"fc-2.2.2-006","topicId":"2.2.2","front":"Heuristics","back":"Rules of thumb or educated guesses, such as an estimate of the distance from a node to the goal in pathfinding, used to reach a good-enough solution more quickly, although not guaranteed to be the best one."},{"id":"fc-2.2.2-007","topicId":"2.2.2","front":"Performance Modelling","back":"Simulating or testing the behaviour of a system before it is used, particularly under heavy load such as very large numbers of users or data items, to predict how well it scales when testing the real system would be too expensive or unsafe."},{"id":"fc-2.2.2-008","topicId":"2.2.2","front":"Problem Decomposition","back":"Splitting a problem into smaller sub-problems, and splitting those again, until each is manageable enough to be solved independently; this reveals the subroutines needed and lets the work be shared between programmers."},{"id":"fc-2.2.2-009","topicId":"2.2.2","front":"Problem Recognition","back":"Identifying that there is a problem to be solved and working out exactly what it is from the scenario, including its inputs and required outputs, and whether it can be solved by a computer at all."},{"id":"fc-2.2.2-010","topicId":"2.2.2","front":"Visualisation","back":"Presenting a problem or data in graphical form, using symbols or objects such as the nodes and edges of a graph, so that it is simplified and easier to understand; it is for the benefit of people rather than the computer."},{"id":"fc-2.2.2-011","topicId":"2.2.2","front":"Pipelining","back":"Arranging a task as a series of stages in which the output of one process is the input of the next, so that different stages can overlap; in a processor, one instruction is fetched while another is decoded and a third is executed."},{"id":"fc-2.2.2-012","topicId":"2.2.2","front":"Abstraction in Computational Methods","back":"Removing unnecessary detail from a problem so that only the features needed to solve it remain, for example leaving the scenery out of a simulation; this simplifies the problem and reduces the memory and processing a solution needs."},{"id":"fc-2.3.1-001","topicId":"2.3.1","front":"A* Algorithm","back":"Finds the shortest path between two nodes by always visiting next the node with the lowest total of distance travelled so far plus a heuristic estimate of the distance remaining to the goal, so fewer nodes need to be explored."},{"id":"fc-2.3.1-002","topicId":"2.3.1","front":"Big O Notation","back":"A way of expressing how the number of steps an algorithm takes, or the memory it uses, grows as the amount of data increases. It shows the order of growth rather than an actual time, and is written in a form such as O(n log n)."},{"id":"fc-2.3.1-003","topicId":"2.3.1","front":"Binary Search","back":"Needs sorted data. Compares the middle item with the target and, if they differ, discards the half that cannot contain it by moving the lower or upper bound, repeating until the target is found or no items remain. Worst-case time O(log n)."},{"id":"fc-2.3.1-004","topicId":"2.3.1","front":"Bubble Sort","back":"Compares each pair of adjacent items in turn and swaps them if they are in the wrong order. At the end of the list it starts another pass, stopping when a flag shows that a pass made no swaps. Worst-case time O(n²); space O(1)."},{"id":"fc-2.3.1-005","topicId":"2.3.1","front":"Dijkstra's Shortest Path Algorithm","back":"Finds the shortest path from a start node to the other nodes of a weighted graph by repeatedly visiting the unvisited node with the smallest distance from the start and updating its neighbours whenever a shorter route is found. Uses no heuristic."},{"id":"fc-2.3.1-006","topicId":"2.3.1","front":"Insertion Sort","back":"Treats the first item as a sorted list, then takes each item in turn from the unsorted part and places it at its correct position in the sorted part. Time is O(n²) at worst and O(n) if already in order; space is O(1)."},{"id":"fc-2.3.1-007","topicId":"2.3.1","front":"Linear Search","back":"Compares the first item with the target and, if they are not equal, moves on to the next item, repeating until the target is found or the end of the list is reached. The data does not need to be sorted. Worst-case time O(n)."},{"id":"fc-2.3.1-008","topicId":"2.3.1","front":"Merge Sort","back":"Repeatedly splits the list in half until each sublist holds one item, then combines pairs of sublists by comparing their first items and writing the smaller to a new list, until one sorted list remains. Time O(n log n); space O(n)."},{"id":"fc-2.3.1-009","topicId":"2.3.1","front":"O(1) (Constant Time/Space)","back":"The time taken, or the memory needed, stays the same however many items of data there are, as when an array element is accessed by its index or a sort works in place and needs no extra storage."},{"id":"fc-2.3.1-010","topicId":"2.3.1","front":"O(2ⁿ) (Exponential Time/Space)","back":"The time or memory multiplies by a fixed factor, for example doubling, with every extra item of data, so it grows at an ever-increasing rate and soon becomes unmanageable; not the same as growth in proportion to n squared."},{"id":"fc-2.3.1-011","topicId":"2.3.1","front":"O(log(n)) (Logarithmic Time/Space)","back":"The time or memory grows at a decreasing rate as the number of items increases, so doubling the amount of data adds only one more step; typical of algorithms that halve the data at each stage."},{"id":"fc-2.3.1-012","topicId":"2.3.1","front":"O(n) (Linear Time/Space)","back":"The time or memory increases in direct proportion to the number of items, so doubling the data doubles the number of steps; typical of a single loop through the data. Proportional to n does not mean equal to n."},{"id":"fc-2.3.1-013","topicId":"2.3.1","front":"O(nᵏ) (Polynomial Time/Space)","back":"The time or memory is proportional to the number of items raised to a fixed power, such as n squared when one loop through the data is nested inside another, as in the worst case of the simpler sorting algorithms."},{"id":"fc-2.3.1-014","topicId":"2.3.1","front":"Quick Sort","back":"Selects a pivot, compares every other item with it and places smaller items to its left and larger items to its right, then repeats the process on each sublist until all items are in place. Average time O(n log n); worst case O(n²)."},{"id":"fc-2.3.1-015","topicId":"2.3.1","front":"Space Complexity","back":"How the amount of memory an algorithm needs, over and above the data itself, grows as the number of items increases. A sort that builds new lists has a higher one than a sort that works in place."},{"id":"fc-2.3.1-016","topicId":"2.3.1","front":"Time Complexity","back":"How the number of steps an algorithm takes grows as the number of items to process increases. 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