OCR: GCSE Computing A451 Flashcards
Computer System
A combination of hardware and software components that allow input, processing
and output of data.
Hardware
The physical components that make up a computer system.
Software
The programs that run on a computer system.
Input Device
Hardware that accepts data into the computer. They take real world analogue data and convert it into a digital form that can be stored on a computer. For example: keyboard, mouse, microphone, webcam, scanner, sensors.
Output Device
Hardware that presents the results of processing to the user or actuators that perform a task automatically. They use digital data from a computer and produce it in a form that is understandable or usable. For example: monitors, printers,
speakers, projectors, actuators (motors).
Storage Device
Hardware that is used to store files long term and is non-volatile, such as hard
disks, memory sticks, magnetic tapes and CDs.
System Reliability
How far you can depend on the computer system being available when you need
it. Usually measured in terms of availability.
System Availability
The proportion of time that a system is operational, usually expressed as a percentage over a period of time. E.g. 95% measured over one year.
Mean Time Before Failure (MTBF)
Mean Time Between Failure: a measure of availability often quoted by hardware
manufacturers. For example 2.56 years between failures means that, on average, the hardware can be expected to last 2.56 years before it goes wrong.
System Redundancy
Spare hardware components are built into a system so that, in the event of a component failing, the system can swap over to the spare one.
System Disaster Recovery
Where a company has plans to replace a system quickly if there is a catastrophe (fire, flood, bomb etc). Designed to minimise the time the system is down.
CPU
The central processing unit that contains the processor, main memory and cache.
(sometimes people say CPU when they mean processor so look for the context).
Main Menory /RAM
Also known as Immediate Access Store and Primary Memory The memory in the CPU that is used to temporarily store programs while they are running and the data used by these programs. The processor fetches instructions from main memory. Memory is made up of many addressable locations.
Processor
The component in the computer that fetches, decodes and executes instructions.
Cache
High speed memory in the CPU that is used to store a copy of frequently used
instructions and data. Faster access speed than main memory. Used to improve
CPU performance.
Clock Speed
Measured in hertz or cycles per second, the clock speed represents how many
instructions per second the processor can execute. The higher the clock speed the
faster the CPU can operate.
System Busses
The circuits/internal wiring that connect the processor and main memory.
Fetch-Execute Cycle
The process by which a program is run: instructions are stored in main memory,
fetched by the processor one at a time, decoded and executed.
Dual-Core/Quad Core
A CPU that contains multiple processor components (cores) that can operate independently to process more than one task at a time.
RAM
Random Access Memory: a type of memory that is read-write and volatile.
Used for Main Memory.
ROM
Read Only Memory: memory that is hard-coded at the time of manufacture. Stores the startup program, called the bootstrap loader.
Bootstrap Loader
The first program that is loaded into main memory from ROM when a computer is
switched on. This will load the operating system from secondary storage.
Volatile
Describes memory that loses its contents when the power is turned off, e.g. main memory
Non-Volatile
Describes memory that does not lose its contents when the power is turned off, e.g. hard disk.
Secondary Memory
Long term, non-volatile storage media such as hard disks, memory sticks, magnetic tapes, DVDs and CDs.
Virtual Memory
Part of the hard disk that is configured to behave as an extension to main memory.
Magnetic Media
Secondary storage such as hard disks, tape and floppy disks.
Optical Media
Secondary storage that is read using lasers such as CDs and DVDs.
Solid State / Flash Memory
Secondary storage that has no moving parts. Used in memory sticks, cameras and phones.
Pen Drive
Another term for a USB memory stick
Boolean Expression
An expression that is either true or false, e.g. X=10
Truth Table
A table that shows all the possible combinations of inputs and their logical
output value.
Logical Operators
AND, OR, NOT
Logic Diagram
A diagram of a circuit showing logic gates with inputs and the output these generate.
Software
Programs that run on the computer.
System Software
Programs that are used to run the computer, including the operating system, utilities, library routines and programming language translators.
Application Software
Programs that perform a task for a user, such as word processing, timetabling, accounts or payroll.
Operating Software
System software that is necessary for the computer to function.
Utility Software
System software that provides other useful functions for operating the computer or performing computer-related tasks such as anti-virus
programs.
Library Software
System software modules that perform frequently required tasks. They can be built into or called from other programs.
Translators
System software that translates high-level programming languages into
machine code. Includes compilers and interpreters. (An assembler translates low-level assembly code into machine code.)
Memory Management
One of the main functions of the operating system – allocating space in main memory to all currently running programs and their associated data, and recycling the space when they have finished.
Peripheral
A device (hardware) that is connected to the CPU to provide input, output or storage
Peripheral Management
One of the main functions of the operating system – managing the input to and the output from the CPU.
Multi-tasking
One of the main functions of the operating system – managing how several tasks or programs, which are all running at the same time, share the processor.
Security
One of the main functions of the operating system – protecting the computer system from various hazards such as unauthorised users, viruses, hackers and accidental damage.
User Interface
The method of communication between the computer and its user. Sometimes called HCI (Human-Computer Interface) or MMI (Man-Machine Interface).
HCI
Human-Computer Interface: another term for user interface.
MMI
Man-Machine Interface: another term for user interface.
GUI
Graphical User Interface. A style of user interface, which is based on icons rather than text.
WIMP Interface
Stands for: Windows, Icons, Menus and Pointers. It describes a type of user interface where the user selects icons and menu items with a pointer of some kind (with a mouse, stylus or finger).
Command-Line Interface
A style of user interface that is only text-based. Commands are typed in at a text prompt.
Address
A numerical reference to a location in memory.
Process
A program that is running in main memory.
Virus
A program that is installed on a computer without your knowledge or permission with the purpose of doing harm. It includes instructions to replicate automatically on a computer and between computers.
Antivirus Software
A utility program that prevents harmful programs being installed and important files being changed. The antivirus software scans all the files
on the computer periodically and if a virus does install itself, it detects and removes it.
Firewall
A utility program that prevents unauthorised access to computers or a LAN from the Internet and controls what sites computers on the LAN can access.
Spyware
A program that secretly records the user’s actions on the computer including passwords and personal details they type when accessing a secure site.
Disk Defragmenter
A utility program that optimises the use of the hard disk space by collecting together the separate segments of each file into contiguous blocks (i.e. blocks next to each other) on the disk as well as grouping together the blocks of free space so newly saved files do not have to be fragmented (split up).
System Cleanup
A utility program that deletes unnecessary files and settings to optimise the computer’s performance.
Automatic Update
A utility program that, for any software already installed on the computer, will regularly check on the Internet for newer versions and updates, then download and install them.
System Information and Diagnosis
A utility program that presents information about the computer hardware and usage as well as information to help diagnose problems.
Formatting
A utility program that formats secondary storage devices such as hard disks, preparing them for use.
File Transfer and File Management
A utility program that allows the user to create a logical view of how their files are organised using folders. Allows the user to move files
and folders, copy, paste, name and delete files and folders.
Open Source
Software that is supplied with its source code. The source code can be modified and the software redistributed (as long as this too is open source).
Licence
An agreement that defines the conditions for using the software.
Freeware
Software that is provided free of charge under licence but without the source code. It is copyrighted and cannot be modified.
Proprietary Software
Software that is copyrighted and the licence sold under a patented or trademarked name.
Off-the-Shelf Software
Software that can be purchased from a High Street or online store. Not custom-written.
Custom-Written Software
Software that is written for one customer’s specific requirements.
What is a bit?
A single binary digit: 1 or 0
How many bits in a byte?
8 bits
How many bits in a nibble?
4 bits
How many bytes in a kilobyte?
1024 bytes
How many kilobytes in a megabyte?
1024 kilobytes
How many megabytes in a gigabyte?
1024 megabytes
How many gigabytes in a Terabyte?
1024 gigabytes
Binary
Base 2 number system, used by computers, uses the digits 0 and 1 only.
Denary/Decimal
Base 10 number system, how we normally count, uses digits 0 to 9.
Hexadecimal
Base 16 number system used by humans to represent groups of four bits at a time. Uses digits 0 to F.
Overflow
When the result of a numeric calculation is too large to be stored in the space reserved for that type of data.
Character set
The set of symbols that can be represented by a computer. The symbols are called characters and can be letters, digits, space, punctuation marks and some control characters such as “Escape”. Each character is represented by a numerical code that is stored as a binary integer.
ASCII
American Standard Code for Information Interchange: a 7-bit character set used by PCs. (There is also an extended ASCII character set that uses 8 bits.)
EBCDIC
Extended Binary Coded Decimal Interchange Code: an 8-bit character set used by older mainframes.
Unicode
A 16-or 32-bit character set that allows many more characters to be coded.
Bitmap Image
An image that has been stored as a series of values per pixel. The colour of each individual pixel is stored in a file.
Vector Graphic
An image file that is made up of lines and shapes that have certain properties,for example, a line may have the following properties: start-point, end-point, line colour, line thickness, line style. The properties of each shape are stored in a file to make up the image.
Pixel
Short for picture element. It is the smallest component of a bitmapped image.
Colour Depth
The number of bits used to represent the colour of a single pixel in a bitmapped
image. Higher colour depth gives a broader range of distinct colours. For example,
an image stored as a .gif file uses 8 bits per pixel so the image could use 256
different colours.
Resolution
The number of pixels in an image expressed as:
the-number-of-pixels-across x the-number-of-pixels-down e.g. 400 x 600. Effectively this describes the pixel density.
Metadata
Data about data. In the case of image files metadata is the data the computer needs to interpret the image data in the file, for example: resolution, colour depth and image dimensions.
Analogue
A continuously changing wave such as natural sound.
Digital
Data that is made up of separate values. How data is stored on a computer.
Sample Rate
The number of times per second that the sound wave is measured. The higher the rate the more accurately the sound wave is represented.
Sample Interval
The time gap between measurements of the sound wave being taken. Another
way of expressing the sampling rate.
Sample Resolution
The number of bits used to store the value of each sample. The greater the number of bits the more accurately the value is stored.
ADC
Analogue to Digital converter: takes real-world analogue data and converts it to a binary representation that can be stored on a computer.
Instruction Set
The group of instructions available for a specific processor to use. The number of instructions available will depend on the number of bits used. For example, with 4
bits there could potentially be 16 different instructions.
Opcode
The group of bits in a machine code (binary) instruction that represents the operation (instruction) such as ADD, STORE or BRANCH.
Operand
A data value or an address that is part of a machine code instruction.
Compiler
Systems software that converts a program written in a high level programming
language into machine code (binary).
Machine Code
Program instructions that have been converted into a form that the computer can execute. A machine code instruction typically has an opcode and an operand in binary.
High Level Programming Language
A programming language written in constructs using language we can understand.
Languages include Delphi, Visual Basic, Java and C++.
Database
A persistent organised store of data.
Persistent Storage
Non-volatile storage on a secondary storage medium such as a hard disk.
Data Duplication/
Data Redundancy
Where the same data is stored more than once, unnecessarily.
Data Inconsistency
Where different versions of the same data have different values because duplicate
versions have been stored and updated differently.
Program-Data Independence
Where the applications that use a shared database are separated from the actual data by a database management system. Changes can be made to one application without it affecting another.
DBMS
Stands for Database Management System, the system that separates the
applications from the data and provides features that allow database systems to be
created, interrogated and maintained.
Views
A feature of a DBMS that provides each application or user with specific access
rights and views of the database.
Flat File Database
A persistent organised store of data where data is stored in a single file organised into fields and records.
Relational Database
A persistent organised store of data where data is stored as a collection of related tables to minimise data redundancy.
Entity
A category of, for example, person (e.g. student, customer), object (e.g. classroom, stock item) or event (e.g. holiday booking, TV program) about which data is stored in a database, and which corresponds to a table in the relational database.
Table
A collection of data organised into records and fi elds within a relational database. A table represents a real world entity.
Record
Data stored about one instance of an entity: for example, one particular person or object
Field
One specific data item being stored such as surname or date.
Primary Key
A field in a table that uniquely identifies a record.
Foreign Key
A field in one table that is the primary key in another table and is used to create a relationship between those two tables.
Relationship
The logical connection created between two tables using a primary and foreign
key pair. It allows related data about a record to be accessed from another table. A relationship between two tables can be one-to-one, one-to-many or many-tomany.
Form
Input. An interactive window used for data entry that usually includes validation
routines, and uses controls such as combo boxes and radio buttons. Data input is usually saved to a database.
Validation
A check made by the computer to make sure the data is sensible.
Verification
A check to ensure that data has been input correctly. Sometimes this is done by
prompting the user to read the data they have input and confirm it is correct. Other times the data has to be entered twice and one version is compared against
the other to make sure it is the same.
Query
A feature of a DBMS that allows the database to be interrogated. It selects
records from the database based on specifi ed criteria.
Database Report
Output. A snapshot in time of the data from a database that can be printed. Data is formatted on a page and may be sorted or grouped. It may include totals.
Database Module
Section of code within a DBMS that allows the user interface to be tailored.
LAN
A collection of computers and peripheral devices connected together within a single site.
WAN
A collection of computers and LANs connected together over a geographically remote area, using leased infrastructure.
Topology
A description of how devices are connected together. Does not necessarily represent physical layout.
Bus
A topology where each device is connected to a main cable, referred to as the bus. Any device can transmit at any time but only one transmission can occur on the main bus at any one time.
Ring
A topology where each device is connected to the next in a loop. Uses a tokenpassing
protocol to allow transmission by one device at a time.
Star
A topology where each device has its own cable connecting it to a central device, which can be a switch or a server.
Peer-to-Peer
A method of organising devices in a network where devices are all of equal status rather than having specialised roles. Each computer can access resources on another computer, assuming access rights have been granted by the other computer.
Client-Server
A method of organising devices in a network where some computers have
specialised roles: servers. The servers provide resources and services to the other computers, known as clients. Management of the network and shared resources/
files is centralised at the server.
Hub
A hardware device that provides connectivity to a LAN cable. A multiport box that has a connection to the LAN from one side and several computers on the other. Can be wireless or cabled.
Switch
A hardware device that is similar to a hub but it has built-in intelligence to direct
traffic to the right place. Computers connected to a switch form a star topology
LAN.
Wireless Access Point
The device to which a computer connects wirelessly. Can be a wireless hub or a
wireless switch.
NIC
Network Interface Card: the card that plugs into a computer to provide a connection to a LAN. Can be wireless or cabled. Holds the MAC address.
MAC Address
A unique hardware number allocated to every NIC. It is a 48-bit address, usually
written in hex, e.g. 00-09-7C-F1-F7-85
Message
A communication between devices. Split into packets for sending over a network
and put back together again at the other end.
Packet
A fixed size chunk of a message created to send a message over a network. It
has its own header containing data such as the destination address and packet number (so the message can be put back together in the right order).
Protocol
A set of rules that defi nes how devices communicate. E.g. IP, HTTP, HTTPS
Internet
A public worldwide network where computers and networks in geographically
separate locations are connected together using a variety of communication links.
Devices communicate using Internet Protocol (IP).
Routers
The hardware devices that make up the backbone of the Internet as well as (smaller ones) providing connectivity from a LAN to the Internet. Use Internet Protocol to communicate with each other.
Modem
The hardware device used to convert the digital transmission from a computer
into an analogue signal that can be carried over the analogue telephone network.
A method of accessing the Internet.
Digital
A transmission signal that is made up of separate values (numbers), as opposed to
the continuously changing signal in analogue transmissions.
Analogue
A transmission signal that is continuously changing, as opposed to being made up
of separate values (numbers). Sound in the real world is analogue.
Broadband
A digital method of connecting to the Internet that allows more than one transmission at the same time, e.g: phone and computers. It may use the site’s
normal phone line or a fibre optic cable to carry transmissions.
WWW
World-Wide Web: a collection of pages distributed on servers connected to the
Internet. Uses HTTP to request and send pages to browsers.
HTTP
HyperText Transfer Protocol: the protocol used by a browser to send page requests
to a server and also by the server to send back the required page.
HTTPS
A secure version of HTTP where transmissions are encrypted.
IP Addressing
A method of labeling any device connected to the network with a unique
numerical value. Uses four bytes usually expressed in this notation: 123.123.003.243
Domain Name
The text label for a website in the Internet: www.bbc.co.uk It corresponds to an IP address for that site.
DNS Servers
Domain Name System server: a database of domain names and associated IP addresses stored on servers. There are many DNS servers distributed across the Internet, which communicate with each other.
HTML
HyperText Markup Language: the programming language used to define the layout and content of a webpage. Uses tags in conjunction with a CSS to control how content is displayed.
CSS
Cascading Style Sheet: defines the formatting and layout of the content defi ned by the HTML code. E.g. may be 32pt Arial in Green.
Tags
Labels that go around the content (text, pictures etc) to define the page layout.
Eg: A heading
User Access Levels
A network policy that defines which users can see which folders and files and the type of access they have to them. Eg: Read-Only or Read-Write.
Encryption
Where the data is changed, using a key, before it is transmitted so that it can
only be deciphered by another device with the appropriate key. To anyone
intercepting the message it would be unintelligible.
Acceptable Use Policy
An agreement that computer users will sign/agree to before being allowed access to a computer or the network.
Failover
When a hardware component fails, the computer switches over to a redundant
component without the service to the user being interrupted.
Redundant
Spare, ready to be used if another component fails. Relates to spare hardware
components in fault-tolerant systems that use failover.
Fault-Tolerant
A system that has been designed to cope with hardware failures. Uses
redundant hardware and failover usually.
Backup
A copy of data is taken from a live computer system as a precaution against system failure or corruption/deletion of individual files/folders. To be restored in the event of data loss.
Archiving
Files are removed from the main computer system but kept in long-term storage, just in case they are needed in the future or because the law requires they be kept. Creates space on main system.
Disaster Recovery
A collection of precautions that ensures the computer system can be reestablished very quickly after a catastrophe. Includes backup policy, complete hardware system available offsite at short notice and policies to restore data and applications on the replacement hardware.
Compression
Making files smaller for quicker transmission over a network.
Lossless compression
File is compressed with no loss of essential data.
Lossy compression
Files are compressed by removing some data that is less essential for the purpose. For example, using fewer colours in a picture (reduce colour depth).
Algorithm
A series of steps to solve a problem. Can be expressed in structured English, pseudocode or as a flowchart (also called flow diagram).
Flowchart/Flow Diagram
A diagram using commonly defi ned symbols to express an algorithm.
Structured English
A way of writing an algorithm in natural language using some basic programming
constructs such as IF…THEN…ELSE and loops. More structured than just natural language/prose.
Pseudocode
A way of writing an algorithm that is close to actual programming language, using
coding-style constructs such as IF…THEN…ELSE, loops and array notation as appropriate.
Hungarian Notation
The convention of prefi xing identifiers to indicate what type of object they are.
Commonly used with forms where, for example the prefix txt might indicate a textbox and lst might prefix a list box. The prefix is conventionally in lowercase.
camelCase
The use of capital letters in an identifier to make them more readable. With camelCase the first word is not capitalised:
txtCustomer, lstCounty, frmAddCustomer, btnSubmit
PascalCase
The use of capital letters in an identifier to make it more readable. For example,
variable and procedure identifiers: StudentNumber, ProductID, StartPoint, CalcTotal
(programming) Sequence
Where instructions are executed one after another in series.
(programming) Selection
Where the program will execute certain instructions based on conditions. Selection
statements include: IF…THEN…ELSE and CASE…OF to select which commands to execute.
(programming) Iteration
Where a program will execute a group of instructions zero or more times based on a condition. FOR loops will execute instructions a specific number of times, REPEAT…UNTIL loops one or more times and WHILE…DO loops zero or more times.
(programming) Condition
A boolean expression that controls an iteration or selection statement. For
example, REPEAT…UNTIL X=10, where X=10 is the condition.
(programming) Boolean Expression
An expression that is true or false. For example: continue=”Y”
Expressions can be more complex, containing several parts: ((continue=”Y”) or (continue=”y”)) and (tries
High Level Programming Language
A programming language where programming constructs are written in a way
that is close to natural language instead of in mnemonics or machine code. For example, Delphi, Pascal, Visual Basic, Java, C++.
Imperative Language
Programming language such as Python or Delphi which uses a sequence of statements to determine how to reach a certain goal or solve a problem.
Assembly Language
Second generation programming language where instructions are in the form of
mnemonics.
Mnemonics
Abbreviations representing commands used in assembly language programming.
For example, LDA, STO, ADX.
Machine Code
First generation code; binary instructions where some bits are used to define the
operation (called the “opcode”) and some bits define the data to be used.
Translator
The piece of systems software used to convert different programming languages
into machine code. Three types: assembler, interpreter and compiler.
Interpreter
A translator that converts high level languages into machine code one line at a time, checking syntax, converting to machine code and executing the code.
Compiler
A translator that converts high level languages into machine code. Works through the whole program (source code) checking the syntax, then converting to machine code and creating the executable object code, which can be saved. The object code is executed, not the source code.
Source Code
The original high level program.
Object Code
The executable version of the program after it has been compiled.
Assembler
The translator that converts assembly language programs into machine code.
Identifier
A unique name for something (variable, constant, program, procedure etc.) within
the program.
Constant
A named value within a program that has a specific value. Its value does not
change while the program is running.
Variable
An identifier associated with a particular memory location, used to store data. Its
value may change as the program is run and new values are assigned to it.
Data Type
A formal description of the type of data being stored in a variable. It defines the
amount of memory required and the type of operations that can be performed on
that variable. Typical data types include Integer, real, string, boolean
Integer
Data type for whole numbers. Typically uses 2 bytes.
Real
Data type for fractional numbers. Typically uses 4 bytes.
Char
Data type for a single character. Typically uses 1 byte.
String
Data type for text, zero or more characters. Typically uses 1 byte per character.
(programming) Operations
The actions that can be performed on a variable.
Arithmetic Operations
Add, subtract, multiply, divide, integer division (DIV) and modulus (MOD)
Comparison Operations
= =
Logical Operators
NOT AND OR
Array
A group of data items of the same data type that use a single identifi er. Individual
data items are accessed using a subscript.
Syntax
A set of rules that defi nes how program statements must be written in order for the translator to understand them.
Syntax Errors
An error in the format of the program statements such as missing semicolons or
keywords spelt incorrectly.
Logic Errors
An error in the algorithm that means the outcome is not as expected, even though
the program will run.
Valid Data
Data that should be allowed by the program. For example, if a range of 1 to 10 is
allowed, then valid data will be any number between 1 and 10 inclusive.
Invalid Data
Data that is not valid and should be rejected by the program. For example, if a
range of 1 to 10 is allowed, then invalid data will include abc, -251, 0, and 11
Boundary Data
Data either side of the range extremes. For example in a range of 1 to 10 boundary
data will include 0, 1, 10 and 11
