# AQA COMP2

Key terms for all chapters of COMP2 syllabus

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• Created by: Choy
• Created on: 27-05-11 13:38

## LOGIC GATES + BOOLEAN ALGEBRA

Boolean variable: may have two discrete possible values, e.g. True or false

Truth table: a table that shows the result of applying the logical functions to all possible combinations of inputs.

Boolean equation: an equation that expresses a Boolean output Q in terms of Boolean inputs, X, Y, Z, etc. To which one or more Boolean functions, such as OR, AND and NOT, are applied.

Logic gate: an electronic circuit that performs a Boolean function.

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## LOGIC GATES + BOOLEAN ALGEBRA

OR function: the output is true is either or both inputs are true.

AND function: the output is true is all inputs are true.

NOT function: the output is the inverse of the input.

Exclusive OR function (XOR): the output is true if either input is true but not if both inputs are true.

NAND function: the output is true if any input is false.

NOR function: the output is true only when all inputs are false.

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## COMPUTER ARCHITECTURE

Three-box model: models computer system as three boxes; the processor, main memory and I/O, interconnected by a bus.

Hardware: Electronic/electrical circuits that a computer assembled from. The platform on which the software executes.

Main memory: memory that is directly addressable by the processor.

Memory location: a separately addressable area of main memory.

RAM: random access memory; volatile main memory in which the locations can be accessed directly in any order with the same access time for all writing and reading operations.

ROM: read-only memory; non-volatile main memory that cannot be written to once it is set up.

EEPROM: electrically erasable programmable read-only memory; its contents may be altered but writing is about 100 times slower than reading.

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## COMPUTER ARCHITECTURE

Computer bus: a set of parallel wires connecting independent components of a computer system.

System bus or external bus:the main highway connecting the processor, main memory and I/O controllers; it is made up of a data bus, an address bus and a control bus.

Data bus: bidirectional, used to transport data between components of the three-box model.

Control bus: bidirectional, used to transport control signals between components of three-box model.

I/O: allows the CPU to communicate with peripherals.

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## COMPUTER ARCHITECTURE

Peripheral: a computer device that is not part of the CPU. It can be external (e.g. Mouse, keyboard, printer, memory stick) or internal, such as a CD-ROM drive.

I/O device: a hardware unit that sends or receives data or stores data by communicating with the processor and main memory through an I/O controller.

I/O controller: an electronic circuit that connects to a system bus and an I/O device; it provides the correct voltages and currents for the system bus and the I/O device.

Secondary storage: permanent storage memory not directly connected to the processor; also called backing store.

Main memory address: a unique numeric code corresponding to a location in memory.

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## COMPUTER ARCHITECTURE

Stored program concept: a program must be resident in main memory o be executed; it is processed by fetching machine code instructions from main memory and executing them, one at a time, in the processor.

Register:a very fast memory location inside the processor or I/O controller.

General-purpose register: a register not assigned a specific role by the processor designer. Programmers may use general-purpose registers.

Dedicated register: a register assigned a specific role by the processor designer. Programmers may use some but not all dedicated registers.

Clock speed: of a processor, the frequency in megahertz or gigahertz at which the processor can carry out instructions.

Word length: the number of digits in a binary word.

Bus width: the number of signal wires or lines allocated to the bus.

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## BASIC MACHINE CODE OPERATIONS +THE FETCH-EXECUTE C

Machine code instruction: a binary code that a machine can understand and execute.

Compiled high-level language program: a program translatedinto machine code before it is executed on a digital computer.

Op-code: the part of a machine code instructions that denotes the basic machine operation, e.g. ADD.

Operand: the part of a machine code instruction that represents a single item of binary data or the address of a single item of binary data.

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## BASIC MACHINE CODE OPERATIONS +THE FETCH-EXECUTE C

Instruction set: the set of bit patterns or binary codes for the machine operations that a processor has been designed to perform.

Fetch-execute cycle: instructions are fetched one at a time from main memory, decoded and executed.

MAR ← [PC]

PC ← [PC] + 1; MBR ← [Memory] addressed

CIR ← [MBR]

[CIR] decoded and executed

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## HARDWARE DEVICES

Track: one of the concentric rings on the platter of a hard disk.

Sector: a subdivision of a track.

Disk block: the smallest unit of transfer between a computer and a disk; a disk sector.

USB: Universal Serial Bus; allows peripherals to be connected using a standard interface socket.

Access time: the time from the start of one storage access to the time when the next access can be started.

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## CLASSIFICATION OF SOFTWARE

Software: consists of sequences of instructions called programs that can be understood and executed by hardware.

Application software: allows users to perform non-computer tasks such as writing a letter or processing orders.

Bespoke software: written for a customer’s specific need.

General- purpose software: software that can be used for many different tasks.

Special-purpose software: software designed to support one specific task.

System software: performs machine-orientated tasks needed to operate the hardware. E.g.operating systems, library programs, utility programs, and language translators.

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## CLASSIFICATION OF SOFTWARE

Assembler: translates assembly language programs into machine code.

Compiler: translates high-level language code into object code.

1.       Object code can be saved and run whenever required without the need to recompile.

2.       Object code executes faster than interpreted code.

3.       Object code is more secure as it cannot be without a great deal of reverse engineering

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## CLASSIFICATION OF SOFTWARE

Interpreter: analyses and executes a high-level language program a line at a time.

1.       Useful for program development

2.       Easier to partially test and debug programs.

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## STRUCTURE OF THE INTERNET

Internet: a network of computer networks and computers using unique IP addresses and TCP/IP.

Gateway: connects networks that use different layer protocols.

Router: connects networks that use the same link layer protocol.

IP address: a unique address that identifies a computer or other electronic device on a network.

WWW: World Wide Web; a system of interlinked hypertext documents accessed via the Internet; also called the Web.

Intranet: a private computer network used to share information or operations within an organisation.

Domain name: a name that identifies one or more IP addresses, e.g. the domain name Microsoft.com represents several IP addresses.

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## STRUCTURE OF THE INTERNET

FQDN: fully qualified domain name; it consists of a host ID and domain name.

DNS server: a server that translates fully qualified domain names into IP addresses.

URL: uniform resource locator; a URI that identifies a resource by its network location.

URI: uniform resource identifier; specifies how to access a resource on the Internet.

Server: a software process that provides a service requested by a client.

Client: a software process that requests and uses the services provided by a server.

Client-server model: a client software process initiates a request for a service from a server software process, and the server responds to that request.

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## STRUCTURE OF THE INTERNET

Protocol: a set of agreed signals, codes and rules for data exchange between systems.

Protocol stack: a collection of protocol layers; each layer is responsible for a part of the process governed by the stack.

TCP/IP protocol stack: a protocol stack for TCP/IP; it has an application layer, a transport layer, a network layers and a link layer.

Port: an address for a software process.

Socket: a combination of host IP address and a port number.

Bidirectional: data passes both ways at the same time.

Network layer: adds source and destination IP addresses to packets on their way from the transport layer to the link layer, and removes source and destination IP addresses from packets on their way from the link layer to the transport layer.

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## WEB SITE DESIGN

Home page: the starting page for a web site; it often has links to other parts of the site.

HTML: Hypertext Markup Language; the language used to write web pages. It consists of text that defines the content of the page and tags that define how the content should be structured.

Hypertext: a body of text, graphics etc., stored in a machine-readable form and structured so that a reader can cross-refer between related items of information.

Source: the HTML code to create a web page.

Web browser: software that defines a web page by rendering the HTML elements.

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## WEB SITE DESIGN

Web page: a document on the World Wide Web written in HTML and displayed in a web browser.

Web site: a set of linked documents associated with a particular person, organisation or topic that is held on a computer system and can be accessed as part of the world wide web.

Attribute: a named field that appears inside a HTML tag.

Style: a visual effect such as text colour, font size or background colour.

Type selector: a selector that is an HTML tag; it targets every instance of an element.

Class selector: used to select an HTML element that has a class attribute.

ID selector: identifies a specific HTML element and can only be used once per page.

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## CONSEQUENCES OF USES OF COMPUTERS

Copyright: protects material, such as literature, art, music, sound recordings, films and broadcasts.

Design right: protects how something looks.

Patent: protects how an invention works or what it does.

Trademark: protects the name or logo used to identify a business or product.

Cracking: illegally breaking into a computer system.

Hacking: illegally breaking into a computer system; programming in an unstructured way.

Personal data: data that relates to a living individual who can be identified from it.

Data subject: an individual who is the subject of personal data.

Data controller: someone who determines why and how personal data is processed.

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## CONSEQUENCES OF USES OF COMPUTERS

Data processor: someone who processes data on behalf of a data controller.

Digital rights management: application of control technologies to limit the use of digital media.

Proprietary software: software where the owner places restrictions on its use, copying and modification.

Copyright, Design + Patent Act 1988: Protecting intellectual property of an individual

•  copy software
• run pirated software
• transmit software over a telecommunications line, thereby creating a copy
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## CONSEQUENCES OF USES OF COMPUTERS

The Computer Misuse Act 1990: Maximum fine is prison or a fine

•  unauthorised access with intent to commit or facilitate commission of further offences
•   unauthorised modification of computer material

The Data Protection Act 1984 + 1998: Protecting the identity of individuals and how firms can handle/ use individuals’ information. Main principles include stated personal data must be:

•  Fairly and lawfully obtained and processed
•  Processed for specified purposes
• Accurate and kept up to date
• Not kept longer than necessary
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## CONSEQUENCES OF USES OF COMPUTERS

Health and Safety (Display Screen Equipment) Regulations 1992: Employers providing the necessary support to ensure that all employees who use computers are well equipped and comfort

Regulation of Investigatory Powers Act 2000: Illegal to intercept a private message before it arrives in the recipient’s inbox

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