AQA AS Computing - Unit 2

Unit 2 AS Computing - AQA By Naqash

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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 function 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.

De Morgan's Laws: \overline{A \cdot B} = \overline {A} + \overline {B} ( and \overline{A + B} = \overline {A} \cdot \overline {B}  (

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Logic Gate Functions

OR Function: The output is true if either or both inputs are true.

AND Function: The output is true if 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: A computer system can be considered as consisting of three boxes: The Processor (CPU), Storage (Main memory/Memory) and I/O devices. These three boxes are interconnected by wiring known the a bus.

Hardware: Electronic/Electrical circuits that a computer is assembled from. The platform on which the software executes. Simply known as 'The physical components that make up a computing system'.

Main Memory: Memory that is directly addressable by the processor. It is made up of RAM chips but can include ROM chips.

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.

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Computer Architecture

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; It's contents may be altered but writing is about 100 times slower than reading.

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

System Bus/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.

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Computer Architecture

Address bus: Unidirectional, used to address Memory and I/O locations.

Control bus: Bidirectional, used to transport control signals between components of Three-Box Model.

I/O (Input/Output): Allows the CPU to communicate with all the Peripherals.

Peripheral: A computer device that is not part of the CPU. It can be external (e.g. mouse, keyboard, printer, monitor, memory stick or scanner) or internal (e.g. CD-ROM Drive).

I/O Device: A hardware unit that sends/receives/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.

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Computer Architecture

Secondary Storage: Permanent storage memory, It is not connected to the processor directly.

Main Memory Address: A unique numeric code corresponding to a location in Memory.

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

Stored program concept: A program must be resident in Main Memory to 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 fast Memory location in 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.

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Computer Arcitecture

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.

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Basic Machine Code Operations

Machine Code Instruction: A Binary Code that a machine can understand and execute.

Compile High-Level Language: A program translated into Machine Code before it is executed on a digital computer.

Op-Code: The part of a Machine Code instruction that denotes the basic machine operation, e.g. ADD.

Operand: The part of a Machine Code instruction that presents a single item of Binary data or the address of a single item of Binary data.

Instruction Set: The set of Bit patterns or Binary codes for the machine that a Processor has been designed to perform.

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Fetch-Execute Cycle

1) The address of the next instruction to be executed, held in the Program Counter, is copied to the Memory Address Register along the Address Bus.

2) The instruction held at that address is copied to the Memory Buffer Register.

3) Simultaneously, the contents of the Program Counter are incremented by 1 to get ready for the next instruction.

4) The contents of the Memory Buffer Register are copied to the Current Instruction Register along the Data Bus. This frees up the Memory Buffer Register for the execute phase.

5) The instruction held in the Current Instruction Register is decoded then executed.


PC [PC] + 1; MBR [Memory] addressed


[CIR] Decoded & Executed

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Classification Of Software

Software: Consists of sequences of instructions called programs that be understood and executed by Hardware.

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

General-Purpose Software: Software that can be used to perform many different tasks.

Special-Purpose Software: Software designed to support one specific task.

Bespoke Software: Written for a customer's specific need.

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 program (Assembly Code) into Machine Code.

Compiler: Translates High-Level Language code into object code.

Interpreter: Analyses and executes a High-Level Language program a line at a time.

Operating System: Software that runs on the Hardware. It acts as an interface between the user and the Hardware and provides the user with a virtual machine.

Library Programs: A program library is a collection of compiled routines that the other programs can use.

Utility Programs: Utilities are programs that perform a very specific task related to working with computers. For example you may wish to compress a file or format a disk.

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Classification Of Software

First Generation Language: Machine Code is the only program code a computer can execute directly. It is extremely difficult for humans to write programs in Machine Code as it consists entirely of 0s and 1s.

Second Generation Language: Assembly language was invented to make programming slightly easier for humans. It uses the same structure and instruction set as the machine language it is based on. It use mnemonics instead of numbers so has to be translated into Machine code using an Assembler.

Third Generation Languages: 3GL were developed to make programming even easier for humans. They are heavily human oriented. One instruction in a 3GL is many instructions in Machine Code. 

Fourth Generation Language: Also sometimes known as Declarative Languages define what is to computed instead of how to compute it. In a Declarative Language you write or declare the facts and rules that are processed by a standard algorithm to produce the desired results.

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Assembler: Translates Assembly Language program (Assembly Code) into Machine Code.

An assembler creates another file with the output Machine Code so you have the original file with the Source Code and the new file with the Machine Code. One to one relationship

An assembler is a relatively simple piece of software.


The output Machine Code file is not portable.

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Compiler: Translates High-Level Language code into object code.

Takes the whole code and translates it to Machine Code at once then produces an output executable file. One to many relationship.

A Compiler is a relatively complex piece of software.


The code doesn't need to be translated again so execution is relatively fast also no other software is needed to run the program and the source code is protected.


Bad for testing and debugging small changes as you have to wait for the whole code to be translated again also it is not portable.

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Interpreter: Analyses and executes a High-Level Language program a line at a time.

The translated code is not saved so the program has to be translated again every time the program is executed, this makes the execution of the interpreted program relatively slow. One to many relationship.


Good for testing and debugging small changes to code also allows you to translate certain bits of code instead of the whole thing.


If someone else wants to run the program they must use the same interpretor also this means that you have to distribute your source code so users can modify it.

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Structure Of The Internet

Internet: A public network of computer network and computers using unique IP address and TCP/IP.

Intranet: A private computer network used to share information or operations or transfer data within an organisation.

Gateway: Connects networks that use different Link Layer Protocols.

Router: Connects networks that use the same Link Layer Protocol.

IP Address: A unique address that identifies a computer of another electronic device on a network.

World Wide Web (WWW): A system of interlinked hypertext documents accessed via the internet, also called the Web.

Domain Name: A name that identifies one or more IP address, e.g. '' represents several IP addresses.

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Structure Of The Internet

Fully Qualified Domain Name (FQDN): It consists of a host ID and domain name.

DNS Server: A server that translates FQDNs into IP addresses.

Uniform Resource Locator (URL): A URI that identifies a resource by its network location.

Uniform Resource Identifier (URI): 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 part of the process governed by the stack.

TCP/IP Protocol Stack: A Protocol Stack for TCP/IP; it has an application later, a transport layer; a network layer 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.

Client Port Numbers: Temporary Port numbers used by client processes; they lie in the range 1024-4095:

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Structure Of The Internet

Well-Known Port Numbers: Universally known Port numbers used by servers; they lie in the range 0-1023.

Well Known Ports & Their Servers:

20 - FTP (File Transfer Protocol) server, File transfer data

21 - FTP (File Transfer Protocol)  server, File transfer control

23 - Telnet server

25 - SMTP (Simple Mail Transfer Protocol) server

110 - POP3 (Post Office Protocol 3) server

80 & 8080 - Web server

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TCP/IP Stack

Application Layer: Handles the details of a particular networking application, e.g a Web Browser using HTTP.

Transport Layer: Adds source port, destination port and packet sequence number to packets on their way from the Transport Layer to the Link Layer. and removes the source port, destination port and packet sequence number from packets on their way from the Link Layer to the Transport Layer.

Network Layer: Adds source and destination IP address to packets on their way from the Transport Layer to the Link Layer. and removes the source and destination IP address from packets on their way from the Link Layer to the Transport Layer.

Link Layer: Handles all the physical details of interfacing with the cable, including the network interface card and a device driver.

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Web Design

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

Hypertext Markup Language (HTML): 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.

Hyperlink: A link from one page to the other; a hypertext link.

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 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.

Structure: Headings, paragraphs, line breaks, hypertext links, lists and embedded media.

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.

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Web Design

Class Selector: Used to select an HTML element that has a class attribute.

The class selector is used to specify a style for a group of elements. 

This allows you to set a particular style for many HTML elements with the same class.

The class selector uses the HTML class attribute, and is defined with a "."

ID Selector: Identifies a specific HTML element and can only be used once per page.

The id selector is used to specify a style for a single, unique element.

The id selector uses the id attribute of the HTML element, and is defined with a "#".

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Consequences Of Computer Use

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.

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Consequences Of Computer Use

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

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

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.

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Consequences Of Computer Use

Copyright, Design & Patents Act 1988: Protects the intellectual property of an individual.

  • Copyright (Any Medium): Means you can only copy or use copyrighted work with the owners permission unless it is for educational purposes.
  • Design Rights: Gives you protection from copying of your original design, You can register your design to protect it for up to 25 years.
  • Patent: If you invent something that can be made or used, you may wish to patent it. This will protect your idea for up to 20 years.

Computer Misuse Act 1990: Punishable by a fine or prison sentence. The act distinguishes between three offences in order of severity.

  • Unauthorised access to computer material, i.e. data or programs
  • Unauthorised access with intent to commit or facilitate commission of further offences
  • Unauthorised modification of computer material
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Consequences Of Computer Use

The Data Protection Act 1984 & 1998: Protects the identity of individuals. There are 8 principles that govern personal data and data processors.

  • Data Gathering: Data shall be processed fairly and lawfully.
  • Data Purpose: Data to be processed for specific purpose.
  • Data Quantity: Only data relevant to purpose may be collected.
  • Data Quality: Data shall be accurate and kept up to date.
  • Data Lifetime: Data shall not be kept longer than necessary.
  • Data Subject's Right: Data subject should be told what's held.
  • Internal Data Security: Data should be protected.
  • External Data Security: Data to kept within the EU.

Under this act, every data controller must register with the information commissioner. There are exemptions such as security.

Digital Right Management: DRM is controversial. Copyright owners of digital media try limit aces to media. e.g. When/How often you may watch a film.

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Consequences Of Computer Use

Health and Safety (Display Screen Equipment) Regulations 1992: If display screen equipment use is a significant part of their work activity, employees must have training and information on how to use the equipment safely. They should have suitable work stations with adjustable chair, a footrest if required and a detachable keyboard if a keyboard is required. Students are not classed as employees.

Regulations Of Investigatory Powers Act 2000: It is an offence to intercept a message sent via a public or private telecommunication device (telephone calls, emails, etc.). There are important exemptions to the act such as security and intelligence services who have the right to intercept messages. ISP may be required to make this possible and RIPA may request an encryption key to an encrypted message. RIPA no longer applies once messages are in a mailbox, and the DPA requires ISP and network administrators to grant privacy to users.

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Bill Scott

Very good, definitions. Maybe more in depth power points to follow? 

SGT Griffy

Very useful thank you!


very useful, thanks

zohaib khan

very good thank you so much desver 5 stars 

James Harrison

so useful thanks you have just saved me from failing the exam a second time :D




Awesome revison cards. I am glad to be in the same 6th form as you. SHOUT OUTS TO NAQASH!

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