Computer science AQA

Probably the only subject i like atm ^^

Abstraction

process of removing unnecessary information from a set of instructions so that it is simpler, more relevant and can speed up the program

Bus

Set of wires connecting components within the CPU

Advantages of networks

• Sharing resources can save money

• data is easy to back up

• Easy collaborative working

Advantages of bus topology

* cheap and easy to install

Advantages of star topology

• reliable, if one device fails the whole network isn’t affected

• High performing as no data collisions occur

• More secure- not all devices receive data sent (unlike in bus topology)

Application software

Computer software that allows the user to carry out a specific task, eg browsers/ games

Arithmetic logic unit

In charge of all maths and logic problems within the processor

ASCII

7 bit character set

Binary search

Takes the middle of an ordered list and removes the lower/ higher half depending on wether the input is lower or higher than the middle value, continues this cycle until only the searched for item is found

Breakpoints

Can be added to code by a programmer, which will pause the programs execution so the programmer can observe results.

Bus topology

• Devices connected via one cable

• As more devices are added, performance decreases

• When data is sent, all devices receive it (only the intended recipient processes it)

• If main cable falls, the whole network will crash

Cache

Memory that is very fast and is easily accessed by the CPU, it allows requests to be served quicker since its quick to read/write

Computer system

A combination of hardware and software

Computer virus

• self replicating

• Damage or delete files on a computer

• Can be spread computer to computer

Control unit

Directs and coordinates most of the operations inside the cpu

CPU

Processes and uses data to perform tasks

Register

Memory locator inside CPU, store information that ALU needs to carry out current instruction

Decomposition

Breaking down a problem into smaller, sub problems, each with an identifiable task

Definite iteration

Repeating a block of code for a finite number of times

Disadvantages of networks

• expensive / may need to hire a network manager

• Hackers / viruses can infiltrate the network more easily

Embedded system

A device developed for a single purpose (which has a system embedded into it in order to fulfil that purpose) , eg washing machine, microwave

Fetch decode execute cycle

• fetch- instructions are fetched from memory, it is them carried to the control unit

• Decode - control unit decodes instructions, if arithmetic or logic is needed, instructions sent to ALU

• The instructions are then executed (may require additional data held in memory)

Field

A single data item within a table

Foreign key

A field in one table, that is a primary key in another table

Primary key

A unique identifier that will have a different value for each record in a table

Function

A name of a section of code that performs tasks and returns a value

General purpose system

A computer system that can change its purpose by adding more software

Global variables

Variables declared in the main body of the program and can be accessed anywhere in the code

Graphics card

Used to process the graphics portion of the processing load

Hard drive

Stores majority of data and houses the hard disk

Hardware

Physical elements of a computer system

Lossless compression

A form of compression where none of the original data is lost, for example Huffman coding

Lossy compression

A compression technique that works by discarding some of the data to reduce the amount stored. (Can result in the deterioration of quality)

Huffman coding

Reduced the number of bits by encoding more frequent characters with shorter bit representations and less frequent ones with longer bit representations

Indefinite iteration

Repeating a block of code until conditions are met, eg while loop

Linear search

Works by starting at the beginning of a list and moving through values until it either finds what it is searching for, or reaches the end of the list

Local variables

A variable declared within a function, and therefore can only be accessed within that function

Logic error

An error in the logic of the code that causes it t run incorrectly, but not to crash. (So the program works, but not as the programmer intended it to)

Magnetic media

• use read write heads

• Contain electromagnets on the surface

• Split into sectors of magnetised regions

• Require movements (therefore not as robust as SSDs

• Have a spiral track

Optical media

• disks eg cds and dvds

• Light is shined onto them

• Have a spiral track and rotate

• Consists of a series of pits and lands each of which reflect differently when the light is shone onto it, and can either represent a 1 or 0

Parameters

A variable that can be passed through a subroutine

Phishing

Usually an email that falsely claims to be from a legitimate enterprise, in order to trick the user into sending private information

Blagging

The fabrication of an elaborate story in order to trick a user into sending money/ giving away personal information

Network Protocol

Rules for communication between two or more computers

Records

Complete sets of information that consist of fields.

Run length encoding

Works by reducing the physical size of a repeating string of characters by representing the number of times the character repeats, followed by the character. Eg AAHHH would be 2A3H

Sample resolution

The number of bits used to represent each sample

Secondary storage

Non volatile memory that is used to store data permanently, not directly accessible by the CPU

Solid state drive

A form of flash memory that is retained even when there is no power- more robust than magnetic and optical due to no moving parts.

Spyware

A computer program which is installed without permission, sometimes through a virus, it collects data and sends it back to another source

SQL

Structured query language, a program language that is designed to manage data within a database.

Syntax error

An error that occurs when the rules of the programming language is broken, results in the program crashing.

Trojan

A piece of software that is ‘disguised’ as another application (eg utility/ game) and performs malicious activities like releasing viruses or installing spyware whilst the user is unaware.

Clock

A part of the cpu that synchronises all instructions, each instruction is carries out on an electrical impulse that is controlled by the clock

Types of social engineering

• blagging

• Phishing

• Shouldering

Shouldering

Observing someone enter passwords/ personal information. Eg watching someone enter their bank pin in order to steal money later.

Compilers

• translates whole code into machine code before it is run

• Can store compiled code as an executable file

Interpreted code

• translate code into machine code line by line (so makes errors easier to spot)

• Does NOT save machine code

Advantages of compiled code

• makes it easier to distribute code, since the executable file can be shared rather than the source code (less risk of companies stealing machine code)

• faster since they’re not line by line

• code only has to be translated one instead of each time it is needed to be used

• No installation needed.

Advantages of interpreted code

* convenient when developing a program (debugging is often easier)

TCP/IP stack

• application

• transport

• Internet

• Link

Application layer

• http, https, smtp, IMAP and Ftp operate at this layer

• This is where network applications eg browsers/ email programs operate

Transport layer

• sets up communication between two hosts (computers)

• Agrees settings eg size of packets

• TCP and UDP operate at this layer

Internet Layer

• where addresses are added to packets

• IP operates at this layer

Link layer

• where network hardware is located

• WIFI and Ethernet operate at this layer

HTTPS

allows computers to send/ receive web pages

IP

Adds addresses to packets of data so that they arrive at the correct destination when sent over the internet

SMTP

Protocol to send email

IMAP

Used when receiving emails

FTP

• Protocol used when files are transferred from one computer to another over a network

• Used by web devs when uploading web pages to web server

Ethernet

• Family of related protocols rather than a single protocol

• Used by LANs

Wi-Fi

• similar to Ethernet - family of related protocols

• WLAN (used by comps in a wireless network)

PAN

• personal area network

• Group of computers connected around an individual (usually using Bluetooth)

WAN

• wide area network

• Group of computers connected over a large geographical area/ global scale

• Collective or distributive ownership

LAN

• Local area network

• Connected over one building/ site

• Uses Star/ bus topology

Benefits of standalone computer

• less malware risks/ hacking

• Faster speed (since networks can become slow as usage increases)

• No need to employ network manager (so cheaper)

Benefits of networks

• hardware devices can be charged (eg printers) - so cheaper

• Data/ files can be shared easily, so collaborative working is easier

• Central management can handle backups

• Work can be accessed from any computer on the network

• Computer usage can be monitored by employers

Advantages of wireless networks

• usage anywhere in range

• Not limited by cables

• No cable cost

• No trip hazards

• Very easy to add new computers to network

Diss advantage of wireless networks

• slower network speed than wired

• Not as secure

• Signals can be affected by other electrical devices

How is CPU performance affected

• by clock speed - higher clock speed = more executes per second

• Increase num of cores = more executes at same time

• Increase cache memory - more data/ instructions can be fetched from fast memory

Embedded system

computer system inside another device usually designed for a specific purpose

Converting a sound to digital format

• Analogue sound is converted to an electronic signal

• Strength of signal is measured at regular intervals (samples)

• Each sample is rounded to nearest lvl and represented as a bit pattern

Misconfigured access rights

access rights that have been set up incorrectly, giving the wrong people access to things they shouldn’t have access to

Pharming

A cyber attack where traffic for a website is redirected to a fake website

Removable media

• devices used to store data that can be moved from one machine to another

• Not very safe - could get lost/ carry sensitive data

White box penetration testing

• done by someone who has some access rights/ knowledge of system, but not all

• Used to simulate an internal attack (eg rouge employee)

Black box penetration testing

• done by a person who has no knowledge of system/ no access rights

• Used to cumulate external attack

Biometric measures

• Examples include fingerprint scans, face recognition, retina cans

• Used to biologically identify an individual

Automatic software updates

• improve security - allow programs to be patched

• Does not rely on person remembering to update

Captcha

* Completely automated public Turing test to tell computers and humans apart

Inserting using SQL

INSERT INTO (specifies tables)

VALUES (states new data value)

Deleting data with SQL

DELETE FROM (specify tables)

WHERE (conditions)

Using SQL to edit data

UPDATE (specifies tables)

SET (specify fields)

WHERE (conditions)

Cloud storage

Uses magnetic/ SS in a remote location

Can be accessed from any computer with internet connection

Storing data on optical media

Surface has pits and lands that reflect light differently each represents a 0 or 1. Light sensors detect this and it is converted to binary and processed by cpu

Storing data on a hard disk

Surface of each disk is divided into concentric circles called tracks. Intersection of track and sector is called a block. Block =magnetised region. The magnetism represents a 0 or 1, this is detected by a read write head, converted to binary and processed by cpu.

Comparison of all storage types

• SS is faster than magnetic, is also smaller, lighter and generates less heat, more robust

• Magnetic is cheaper per byte and has large capacity

• Optical is very cheap but low capacity and slow access time