In Augment’s notes, a marked term shows its definition when you hover or tap it, and each page ends with the terms it introduces.
212 terms from Computer fundamentals, each defined in a sentence or two, with the part of the syllabus where you first meet it. 44 of them are HL only, and marked.
The rule that a term containing a variable is redundant when it is combined with that variable on its own: and .
The security function that decides which users and programs may use which resources, and in what way (read, write or execute), according to rules set by the owner or administrator.
The operating system function that records how much of each resource (CPU time, memory, storage, network data) each user and process uses, for monitoring, quotas and billing.
A register that holds the result of the most recent ALU operation and supplies one of the inputs to the next.
An output device that converts a control signal into physical action.
The bus that carries memory addresses from the CPU (from the MAR) to main memory, in one direction only.
A technique that prevents starvation by gradually raising the priority of a process the longer it waits in the ready queue.
Compiling a program fully into native machine code for a target platform before it is run, so that no translation is needed while it executes.
A formula that gives the maximum speed-up of a task when part of it is run in parallel on several processors, showing that the part that must run sequentially limits the overall speed-up.
A signal that varies continuously, so it can take any value within a range at any moment in time.
A circuit that measures an analogue signal at regular intervals and outputs each measurement as a binary number.
A logic gate whose output is 1 only when all of its inputs are 1.
A documented set of functions, rules and data formats that one piece of software offers so that other software can use its services without knowing how they are implemented.
Software that performs a specific task for the user, rather than managing the computer itself.
The part of the CPU that performs arithmetic operations, such as addition and subtraction, and logical operations, such as AND, OR, NOT and comparisons.
The American Standard Code for Information Interchange, a 7-bit character set of 128 codes covering the English alphabet, digits, punctuation and control characters.
The rule that the grouping of three or more operands joined by the same operator does not change the result: and .
The process of checking the identity of a user, device or program before it is given access.
A control algorithm in which the actuator is switched fully on or fully off depending on whether the measured value is below or above the set point, often with hysteresis (separate on and off thresholds) to stop rapid switching.
The base-2 number system, which uses only the digits 0 and 1 and column values that are powers of 2 (1, 2, 4, 8, 16, …).
A unit prefix based on powers of 1024, as in kibibyte (KiB, 1024 bytes), mebibyte (MiB, bytes) and gibibyte (GiB, bytes).
A binary digit, the smallest unit of data, which has one of two values, 0 or 1.
The number of bits used to store each sample of a sound, which sets how many different amplitude levels ( for bits) can be recorded.
An image stored as a grid of pixels, with a binary value recording the colour of each pixel.
The algebra of the two values true (1) and false (0), with operations such as AND, OR and NOT and laws for rewriting and simplifying expressions.
An expression built from Boolean variables, the constants 0 and 1, and Boolean operators, whose value is either true (1) or false (0).
An operation that combines or changes Boolean values: AND, OR, NOT, NAND, NOR, XOR or XNOR.
The firmware program, stored in ROM, that runs when a computer is switched on; it checks the hardware, then loads the operating system from storage into RAM and starts it.
A set of parallel wires that carries signals between components of a computer, one bit per wire at a time.
A group of 8 bits, the standard unit for measuring storage and for addressing memory.
A compact, platform-independent set of instructions produced by compiling source code, designed to be executed by a virtual machine rather than directly by a CPU.
The part of a virtual machine that executes bytecode one instruction at a time by fetching each instruction, decoding it and running the matching routine, without converting it into native machine code.
A small amount of very fast memory, inside or next to the CPU core, that holds copies of the instructions and data from RAM that are most likely to be needed next.
A memory access in which the requested instruction or data is found in the cache, so it is supplied quickly without reading RAM.
A memory access in which the requested instruction or data is not in the cache, so it must be fetched from a slower level (a lower cache level or RAM) and is usually copied into the cache for next time.
The component of a computer that fetches, decodes and executes the instructions of a program, processing data and coordinating the other hardware.
The number that a character set assigns to one character, stored in binary to represent that character.
An agreed list of characters in which each character is given a unique number, so that text can be stored and exchanged as binary.
A circuit that produces a steady stream of electrical pulses to synchronise the operations of the CPU; its rate, measured in hertz, is the number of pulses (cycles) per second.
A control system that measures its output with sensors and feeds the measurement back to the controller, which compares it with the desired value and adjusts the output to reduce the difference.
The delivery of computing resources, such as servers, storage, databases, software and processing power, over the internet on demand, with the provider owning and maintaining the hardware and the customer paying according to use.
A processor that supplements the CPU by carrying out one specific type of task, so that the CPU is free for other work.
The four conditions that must all hold at once for deadlock to occur: mutual exclusion, hold and wait, no pre-emption and circular wait.
The number of bits used to store the colour of each pixel; a colour depth of bits allows different colours.
A user interface in which the user types text commands and the system replies with text output.
The rule that the order of the operands of AND or OR does not change the result: and .
A translator that converts the whole of a program's source code into machine code (or another lower-level form) in one process before the program is executed, reporting the errors it finds in the whole program.
The rule that ANDing a variable with its inverse gives 0, and ORing a variable with its inverse gives 1: and .
Encoding data using fewer bits than the original representation, to reduce the storage space it needs or the time it takes to transmit.
The size of the original data divided by the size of the compressed data, usually written as a ratio such as 10 : 1.
The act of saving the state of the process currently using the CPU and loading the saved state of another process, so that the CPU can be passed between processes without either losing its place.
The set of rules, implemented in software, that a controller uses to calculate its outputs from the sensor inputs and the set point.
The bus that carries control and timing signals between the CPU and other components, such as memory read, memory write, clock pulses and interrupt requests.
A system of hardware and software that manages the behaviour of a device or process by processing inputs and producing outputs that act on it, often using feedback to keep a measured quantity close to a target value.
The part of the CPU that decodes each instruction and sends control signals that coordinate the other components, directing the flow of data between the registers, the ALU, memory and input/output devices.
The component of a control system that receives data from sensors, compares it with the set point using a control algorithm, and sends output signals to the actuators.
The build-up of short processes waiting behind one long process under first-come, first-served scheduling, which raises the average waiting time and leaves devices idle while the long process holds the CPU.
An independent processing unit within a processor, with its own control unit, ALU and registers, able to fetch, decode and execute its own stream of instructions.
Describes software that runs on several operating systems or types of hardware from the same source code or the same distributed file.
The bus that carries data and instructions between the CPU (through the MDR) and main memory, in both directions.
The two rules and , which state that the inverse of an AND is the OR of the inverses, and the inverse of an OR is the AND of the inverses.
A situation in which two or more processes are each waiting for a resource held by another process in the same group, so that none of them can ever continue.
A unit prefix based on powers of 1000, as in kilobyte (kB, 1000 bytes), megabyte (MB, bytes), gigabyte (GB, bytes) and terabyte (TB, bytes).
The base-10 number system, which uses the ten digits 0 to 9 and column values that are powers of 10.
A program that lets the operating system communicate with one particular type of hardware device by translating general requests into the commands that the device understands.
A signal that can take only a limited set of discrete values, such as the binary values 0 and 1, at discrete moments in time.
A circuit that turns a sequence of binary numbers into a continuously varying analogue signal.
A named container in a file system that holds files and other directories, so that files can be organised in a hierarchy.
A mathematical transform that expresses a block of values, such as 8 × 8 pixels, as a sum of cosine waves of different frequencies, giving one coefficient for each frequency.
The rule that an operator applied to a bracket can be applied to each term inside it: and .
The rule that inverting a variable twice gives the original variable: .
Flash storage packaged with its controller in a single chip that is soldered permanently onto a device's circuit board.
A computer system with a dedicated function that is built into a larger mechanical or electronic device, usually with limited memory, processing power and user interface.
The difference between the set point and the measured value of the controlled quantity, which the controller uses to decide its next output.
A file of machine code produced by translation that the operating system can load and run directly on one platform, without the source code or the translator.
A design feature that makes a system move to, or stay in, a safe state when a component fails or power is lost.
Information about the output of a system, measured by sensors and returned to the controller as an input so that the output can be corrected.
The repeated sequence of steps by which the CPU fetches an instruction from main memory, decodes it in the control unit, and executes it, before moving on to the next instruction.
The part of the operating system that organises data on secondary storage into named files and directories, records where each file's data is stored, and controls how files are created, read, changed and deleted.
Software stored in non-volatile memory on a hardware device that controls the device at a low level.
A non-pre-emptive scheduling algorithm that runs processes in the order in which they arrive in the ready queue, each until its CPU burst is complete.
Non-volatile memory with no moving parts that stores each bit as electric charge trapped in a memory cell, and that can be electrically erased and rewritten in blocks.
The number of frames (still images) shown per second of video, measured in frames per second (fps).
A horizontal timeline divided into labelled blocks that shows which task runs during each interval of time; in CPU scheduling, each block is a period in which one process uses the CPU.
A user interface in which the user interacts with the computer through visual elements such as windows, icons, menus and a pointer, or by touch.
A processor with a very large number of simple cores designed to carry out the same operation on many data items in parallel, originally for rendering graphics and now also for other highly parallel computations.
An ordering of binary values in which each value differs from the next in exactly one bit.
A logic circuit that adds two single bits, producing a sum bit (A XOR B) and a carry bit (A AND B).
A secondary storage device that records data magnetically on rapidly spinning disks (platters), using read/write heads on a moving arm.
The base-16 number system, which uses sixteen digits, 0 to 9 and A to F (A = 10, B = 11, C = 12, D = 13, E = 14, F = 15), and column values that are powers of 16.
Memory designed to transfer very large amounts of data per second to a GPU or other processor, using a very wide connection and stacked memory chips placed beside the processor.
A programming language whose statements are written in terms of the problem being solved rather than the instructions of a particular processor, so that one statement can stand for many machine instructions.
A part of a program, such as a method or a loop, that runs so often that it accounts for a large share of the program's execution time.
Software that creates and runs virtual machines, sharing the physical computer's CPU, memory, storage and network between them and keeping them isolated from each other.
The rule that ANDing or ORing a variable with itself gives the same variable: and .
The rule that ANDing a variable with 1, or ORing it with 0, leaves it unchanged: and .
The number of pixels in an image, usually given as width × height.
A cloud service model in which the provider supplies virtualised computing resources (virtual machines, storage and networking), and the customer installs and manages the operating system, runtime, applications and data.
A register that holds the instruction currently being decoded and executed.
A translator that analyses and executes a program one statement (or instruction) at a time while it runs, without producing a separate machine-code file.
A signal from a device or program to the CPU indicating that an event needs immediate attention, causing the CPU to suspend its current task and run code that handles the event.
A short routine, usually part of the operating system or a device driver, that the CPU runs in response to a particular interrupt in order to deal with the event that caused it.
A program that loads Java bytecode from .class files, checks it and executes it on a particular computer, by interpreting it and by compiling frequently used parts into native machine code.
Compilation that takes place while a program is running, in which a virtual machine translates frequently executed bytecode into native machine code for the current processor and then runs that machine code instead of interpreting the bytecode.
A grid form of a truth table, with rows and columns labelled in Gray-code order so that adjacent cells differ in one input, used to simplify Boolean expressions by grouping adjacent 1s.
The central part of an operating system that stays in memory and has complete control of the hardware, managing the CPU, memory, devices and the other parts of the system.
The delay between an event or request and the system's response to it.
The tendency of programs to access the same memory locations repeatedly within a short time (temporal locality) and to access locations close to ones recently used (spatial locality).
A diagram of a logic circuit that uses standard gate symbols connected by lines (wires) to show how the inputs are processed to produce the outputs.
An electronic circuit that takes one or more binary inputs and produces a single binary output determined by a Boolean operation.
Compression in which no information is lost, so decompressing the data restores an exact copy of the original.
Compression that permanently removes information that people are unlikely to notice, so the decompressed data is only an approximation of the original.
A programming language that corresponds closely to the instruction set of one processor family, so that each statement usually stands for a single machine instruction.
Instructions written as binary patterns that a particular type of CPU can decode and execute directly, without any further translation.
A register that holds the address of the memory location that is about to be read from or written to.
A small, removable flash storage card designed for portable, battery-powered devices.
A register that holds the data or instruction that has just been read from memory, or that is about to be written to memory.
The arrangement of a computer's memory and storage in levels ordered by speed, size and cost per byte, from small, fast, expensive registers at the top to large, slow, cheap secondary storage at the bottom.
The operating system function that allocates memory to processes when they need it, frees it when they finish and protects each process's memory from access by others.
Data that describes other data, recording the properties needed to interpret, organise or find it.
A small computer on a single integrated circuit, containing a processor, memory and input/output ports, designed to control one device or task.
The leftmost bit of a binary number, which has the largest place value; the rightmost bit, with place value 1, is the least significant bit (LSB).
A single chip containing two or more cores that can execute separate instruction streams at the same time.
A scheduling method that divides ready processes into several separate queues by type of process, schedules each queue with its own algorithm, and shares the CPU between the queues, usually by fixed priority.
The ability of an operating system to run several processes over the same period by rapidly switching the CPU between them, or by running them on different cores, so that all appear to run at the same time.
A logic gate whose output is 0 only when all of its inputs are 1, equivalent to an AND gate followed by a NOT gate.
A storage device, usually containing several drives, that connects to a local network so that many users and devices can store and share files on it.
A group of 4 bits, half a byte, which can represent 16 values and is written as one hexadecimal digit.
Memory that keeps its contents when the power supply is removed.
A logic gate whose output is 1 only when all of its inputs are 0, equivalent to an OR gate followed by a NOT gate.
A logic gate with one input whose output is the opposite of the input, 1 for an input of 0 and 0 for an input of 1.
The rule that ANDing a variable with 0 always gives 0, and ORing it with 1 always gives 1: and .
The part of a machine-code instruction that specifies the operation to perform, such as load, add, store or jump.
A control system in which the output is decided by the input and the control algorithm alone, with no measurement of the result fed back to the controller.
The part of a machine-code instruction that gives the data to be used, or the memory address or register where that data is found.
System software that manages a computer's hardware and resources and provides common services to the programs that run on it.
A device that reads (and sometimes writes) data on optical discs such as CDs, DVDs and Blu-ray discs, using a laser to detect the pattern of tiny pits and flat areas (lands) along a spiral track.
A logic gate whose output is 1 when at least one of its inputs is 1.
The error that occurs when the result of a calculation is too large to be represented in the fixed number of bits available.
A memory management technique that divides a process's memory into fixed-size pages and physical memory into frames of the same size, so that any page can be placed in any free frame and pages can be moved between RAM and secondary storage.
Carrying out several computations at the same time, using more than one processing unit, so that a task finishes sooner or more tasks finish in the same time.
A situation in which the next instruction in a pipeline cannot proceed in the following clock cycle, either because it needs a result that an earlier instruction has not yet written back (a data hazard) or because a branch means the wrong instructions have been fetched (a control hazard).
A technique in which the stages of instruction processing are carried out by separate parts of the processor, so that several instructions are processed at once, each at a different stage.
The smallest single-coloured element of a bitmap image or display; its name is short for "picture element".
The value of a digit's position in a number, equal to the base raised to the power of the position, counting from 0 at the right.
A cloud service model in which the provider supplies and manages a complete platform (servers, operating system, runtime and development tools), and the customer only develops, deploys and manages their own applications and data.
A method of handling input and output in which the CPU repeatedly checks the status of a device, at regular intervals or in a continuous loop, to find out whether it needs attention.
The ease with which software can be moved to a different hardware platform or operating system and still run correctly, with little or no change.
Scheduling in which the operating system can interrupt a running process and move it back to the ready queue, so that another process can use the CPU; in non-pre-emptive scheduling a process keeps the CPU until it finishes or waits.
Memory that the CPU can access directly, holding the instructions and data in current use; it includes registers, cache, RAM and ROM.
A situation in which a high-priority process is forced to wait for lower-priority processes, because a low-priority process holds a resource it needs and is itself kept from running by a medium-priority process.
A scheduling algorithm that always gives the CPU to the ready process with the highest priority; it can be non-pre-emptive, or pre-emptive, so that a newly arrived process with a higher priority takes the CPU from the running process.
A program that is being executed, together with its current state: its code, data, memory space, register values and the resources it is using.
The current condition of a process as recorded by the operating system, such as new, ready (waiting for the CPU), running (using the CPU), blocked (waiting for an event or resource) or terminated.
Collecting measurements about a program while it runs, such as how often each method is called, how many times each loop repeats and how long each part takes.
A register that holds the memory address of the next instruction to be fetched.
A rugged industrial computer that monitors inputs from sensors and controls machinery and processes by running a program in a fast, repeating cycle.
Reducing the precision of values by rounding them to a smaller set of levels, so that many small values become zero.
Volatile primary memory that can be read from and written to, holding the operating system, running programs and the data they are using.
Non-volatile primary memory whose contents are written during manufacture or by a special update process and are not changed during normal use; it holds firmware such as the bootstrap program.
An interactive programming environment that reads one statement or expression typed by the user, evaluates it, prints the result and then waits for the next input.
A computer system that must respond to inputs within a guaranteed time limit, because a late response counts as a failure.
A very small, very fast storage location inside the CPU that holds a single value, such as an instruction, an address or a piece of data, while it is being used.
A diagram that shows processes and resources as nodes, with assignment edges from each resource to the process holding it and request edges from each process to the resource it is waiting for.
A conflict that arises when several processes or threads need the same limited resource at the same time.
The time from when a process arrives until it first starts running on the CPU.
A pre-emptive scheduling algorithm in which each process in the ready queue receives the CPU for at most one fixed time slice in turn; a process that has not finished at the end of its slice goes to the back of the queue.
A lossless compression method that replaces each run of identical consecutive values with a single pair: the number of repetitions and the value.
Measuring the amplitude of an analogue signal at regular, equally spaced moments and storing each measurement, called a sample, as a binary number.
The number of samples taken per second, measured in hertz (Hz).
The operating system function that determines the order in which ready processes are given the CPU, and for how long, to make the best use of the processor and give each process fair service.
Non-volatile storage that keeps programs and data permanently, whether or not the computer is switched on; the CPU cannot access it directly, so data is first copied into RAM.
An input device that measures a physical quantity and converts it into an electrical signal that a controller can process.
A contract between a service provider and a customer that defines the level of service promised, such as the percentage of time the service will be available, and what compensation is given if it is not met.
The desired value of the quantity that a control system is trying to maintain.
A form of parallel processing in which one instruction is applied to many data items at the same time by many processing units.
A processor with one core, which executes one instruction stream at a time.
A cloud service model in which the provider supplies a complete application over the internet, managing everything from the hardware to the software itself, and the customer simply uses it, usually through a web browser or app, for a subscription.
A secondary storage device that stores data in flash memory chips managed by a controller, with no moving parts.
The instructions of a program as written by a programmer, usually in a high-level language, before any translation.
A situation in which a ready process waits indefinitely for a resource because other processes are always given it first.
A form of Boolean expression made of AND terms (products) joined by OR (a sum).
A request from a running program to the kernel for a service that the program is not allowed to perform itself.
A single integrated circuit that contains most of the components of a computer, typically the CPU cores, GPU, other co-processors, memory controller and input/output controllers.
Software that runs and manages the computer system itself and provides a platform for other software, rather than doing a task for the user.
A single sequence of instructions within a process that can be scheduled to run independently, sharing the process's memory and resources with the other threads of the same process.
The amount of work completed per unit of time.
The fixed maximum length of time for which a process may use the CPU before it is pre-empted under round robin scheduling.
A device that converts one form of energy or signal into another.
A compression method that converts data into a different representation, usually a set of frequency components, so that the components people perceive least can be stored with less precision or discarded.
A tiny electronic switch, controlled by a voltage, that is either on or off; it is the basic building block of processors and memory chips.
A program that converts code written in one programming language into another form, usually machine code or an intermediate code, so that it can be executed.
A table that lists every possible combination of input values of a logic circuit or Boolean expression, with the output for each combination.
The total time from when a process arrives in the ready queue until it completes, including the time spent running and the time spent waiting.
A way of storing signed integers in which the most significant bit has a negative place value, so an -bit number holds values from to .
A universal character set that gives every character of the world's writing systems, together with symbols and emoji, its own number, called a code point.
A design in which the CPU, GPU and other processors on a chip share a single pool of RAM, so that data does not have to be copied between separate memories.
A logic gate from which any other logic gate, and therefore any logic circuit, can be built; NAND and NOR are the two universal gates.
A whole number stored in binary without a sign, so it can only be zero or positive; with bits it can hold values from 0 to .
A small, portable storage device containing flash memory and a controller, which plugs into a USB port.
A variable-length encoding of Unicode that stores each code point in one, two, three or four bytes, using a single byte for the 128 ASCII characters.
An image stored as a list of shapes (lines, curves, polygons and text) described by coordinates and properties such as colour and line width, which the computer draws when the image is displayed.
A situation in which a customer finds it difficult or expensive to move to a different provider, because their systems and data depend on features, formats or services that only the current provider offers.
A software-based computer that runs its own operating system and applications, using hardware resources shared out by a hypervisor on a physical computer.
A technique in which the operating system uses part of secondary storage as if it were extra RAM, so that programs can use more memory than is physically installed.
The use of software to create virtual versions of hardware resources, so that one physical computer can behave as several separate computers, or so that resources appear larger or simpler than they really are.
Memory that loses its contents when the power supply is removed.
The total time a process spends in the ready queue waiting for the CPU, not counting the time it spends running.
The number of bits a CPU processes or moves as one unit, which is usually the size of its registers and the width of its data bus.
The final stage of a pipelined instruction, in which the result produced by the execute stage is written into its destination register (or memory), where later instructions can use it.
A logic gate whose output is 1 when its two inputs are equal (both 0 or both 1), equivalent to an XOR gate followed by a NOT gate.
A logic gate whose output is 1 when exactly one of its two inputs is 1, that is, when the inputs are different.