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Computer Science GCSE

Terms in this set (96)

Binary Systems
The binary system is based on the number 2
Made up of 1s and 0s
Use of binary numbers in computer systems
Units for Computer memories
A binary digit - BIT,
8 bits = 1 byte
Byte is used to measure memory size
Hexadecimal
Closely related to the binary system
Hexadecimal is a base 16 system
Numbers 0 to 9 and letters A to Fare used to represent each hexadecimal digit
Uses of hexadecimal
Defining colours in Hypertext Markup Language
(HTML)
Media Access Control (MAC) addresses
Assembly languages and machine code Debugging

Memory Dumps
Hexadecimal is used when developing new software or when trying to trace errors
Memory dump is when the memory contents are output to a printer, monitor.

Assembly code and machine code (low level languages)
Computer memory is machine code/ assembly code
Using hexadecimal makes it easier, faster, less error prone to write code compared to binary.
Using machine code (binary) takes a long time to key in values and prone to errors
Parity Checking
Parity checking is used to check weather data has been changed or corrupted following transmission from one device to another
A byte of data is allocated a parity bit
Systems that use even parity have an even number of 1-bits
Systems that use odd parity have an even number of 1-bits
Automatic Repeat Request (ARQ)
ARQ is another method to check if data has been transmitted correctly
It uses and acknowledgement (message sent by the receiver indicating that data has been received correctly)
Checksum
Checksum is another way to see if data has been transmitted correctly
Data is sent in blocks and an additional value sent at the end of the block
Echo check
When data is sent to another device, this data is sent back again to the sender
The sender compares the two sets to check if any errors occurred
Not reliable
Error-Checking
ARQ, Checksum, Echo check, Parity Checking
Musical Instrument Digital Format (MIDI)
Storage of music files

Communications protocol that allows electronic
musical instruments to interact with each other

Stored as a series of demands but no actual music notes

Uses 8-bit serial transmission (asynchronous)

Each MIDI command has a sequence of bytes:
o First byte is the status byte - informs the MIDI device what function to preform
o Encoded in the status byte is the MIDI channel (operates on 16 different channels)

Examples of MIDI commands:
o Note on/off: indicates that a key has been pressed
o Key pressure: indicates how hard it has been pressed (loudness of music)

Needs a lot of memory storage
MP3
Uses technology known as Audio Compression to convert music and other sounds into an MP3 file format

This compression reduces the normal file size by 90%
o Done using file compression algorithms which use Perceptual Music Shaping
o Removes sounds that human ear cannot hear properly
o Certain sounds are removed without affecting the quality too much

CD files are converted using File Compression Software

Use lossy format as the original file is lost following the
compression algorithm
MP4
This format allows the storage of multimedia files rather than just sound

Music, videos, photos and animations can be stored

Videos, could be streamed without losing any real discernible quality
Joint Photographic Experts Group (JPEG)
JPEG is a file formats used to reduce photographic file sizes
Reducing picture resolution is changing the number of pixels per centimetre
When photographic file undergoes compression, file size is reduced
JPEG will reduce the raw bitmap image by a factor between 5 and 15
Lossless File Compression
− The file can be restored to the original state
− Lossy would remove data
− Value is recorded in an index
Lossy File Compression
- Lossy reduces the file size more (than lossless)
- The redundant data can be removed from the files
- Files can be sent quicker
Transmission of Data
Asynchronous data transmission
Synchronous data transmission
Asynchronous data transmission
Data being transmitted in an agreed bit pattern:
o Data bits are grouped together & sent with control bits
o This means the receiver of the data knows when the data starts and ends, prevents data getting mixed up
Synchronous data transmission
A continuous stream of data (not in discrete groups like asynchronous):
o Ensures that the sender and receiver are synchronised with each other
o Faster method
Serial data transmission
When data is sent one bit at a time over a single wire:
o Works well over long distances
o Data transmitted at a slower rate (USB)
Parallel data transmission
When data several bits (1 byte) are sent down several wires at the same time
o Works well over short distance
o Faster method (internal components use parallel for high speed)
Simplex data transmission
One direction only (e.g. computer to printer)
Half-duplex data transmission
Both directions but not at the same time (e.g. phone conversation where only one person speaks)
Full-duplex data transmission
Both directions simultaneously (e.g. broadband connection on phone line)
Universal Serial Bus (USB)
USB is an asynchronous serial data transmission method
USB consists of:
o Four-wire shielded cable
o Two wires used for power and earth o Two wires used in data transmission

Advantages
Automatically detected
Only fit one way, prevents incorrect connections
Different transmission rates

Disadvantages
Maximum cable length is about 5 metres
Transmission rate is less than 500 mb/sec
Hacking
The act of gaining illegal access to a computer system

Effect:
o Leads to identity theft, gaining personal information
o Data can be deleted, changed or corrupted

To remove risk:
o Firewalls
o Strong passwords/ user IDs
o Use of anti-hacking software

Difference between hacking and cracking
o Hacking breaks into computer system to steal data
o Cracking is where someone edits a program code, malicious
Viruses
Program that can replicate itself with the intention of deleting or corrupting files, cause computer malfunction

Effect:
o Can cause computer to crash
o Can delete or corrupt files/data

To remove risk:
o Install anti-virus software
o Don't use software from unknown sources
o Be careful when opening emails from unknown
Wardriving
The act of locating and using wireless internet connections illegally

Effects:
o Possible to steal internet time
o Possible to hack into wireless network and steal user's password

To remove risk:
o Use complex passwords
o Firewalls
Spyware
Software that gathers info by monitoring key presses on the user's keyboard and info is sent back

Effects:
o Access to all data entered
o Software is able to install other spyware, read cookie data

To remove risk:
o Use of anti-spyware software
o Use a mouse to select characters from passwords rather than typing them
Internet Service Protocol
These are companies that provide user with access to the internet
Internet Protocol (IP) Address
Each device on the internet is given a unique address known as the IP address

32-bit number written in the form: 109.108.158.1

IP address gives the location of a device on the internet whereas the MAC address identifies the device connected to the internet

IP address changes, MAC address remains unchanged
HTML (HyperText Mark-up Language)
Used when writing and developing pages

Mark-up language is used in the processing

Html use <tags> to bracket piece of codes

Different intensity of colours is determined by its
hexadecimal value
Media Access Control (MAC)
MAC address refers to a number which uniquely identifies a device on the internet

Refers to the network interface card (NIC) which is part of the device

Usually made up of 48 bits shown as six groups of hexadecimal digits

NN:NN:NN:DD:DD:DD
o (NN:NN:NN) first half is the identity number of the manufacturer of the device
o (DD:DD:DD) second half is the serial number of the device

Types of MAC Address:
o Universally Administrated MAC Address (UAA)
o Locally Administrated MAC Address (LAA)
o UAA is the most common type set by the
manufacturer

Reasons to change MAC address using LAA
o To ensure they follow the correct formula
o To bypass MAC address filter on a router or a firewall
o To get past certain types of network restrictions
Web addresses
Each character on the keyboard has its own ASCII code
Can be represented using hexadecimal or decimal values
Hexadecimal addresses are used in the address of files or webpages as a security figure
Takes longer to type in URL but advantage is that you are unlikely to fall into the trap of copying a fake website
Cookies
A packet of information sent by a web server to a web browser
Generated each time the user visits the website
Every time a user visits a website, cookies will have collected some key information about the user
They are able to carry out user tracking and maintain user preferences
Cookies are pieces of data
Information gathered by cookies doesn't contain
personal information
Computer Architecture
Address bus - unidirectional
Data bus - bi-directional
Control bus - unidirectional and bi-directional
Processor
The processor contains the Arithmetic and Logic Unit (ALU)
Control Unit
The control unit controls the operation of the memory, processor and input/output devices
The Fetch-Execute Cycle
- Program counter (PC) holds address / location of the instruction
- The address held in PC is sent to MAR
- Address is sent using address bus
- PC is incremented
- The instruction is sent from address in memory to MDR
- Instruction is transferred using the data bus
- Instruction sent to CIR
Two-dimensional Scanners
Used to input hard-copy documents

The image is converted into an electronic form which can be stored in the computer
o Document is placed on a glass panel
o A bright light illuminates the document
o A scan head moves across the document until the whole page is scanned. And image of the document is produced and sent to a lens using a series of mirrors
o The lens focuses the document image
o The focused image now falls onto a charge couple device (CCD) which consists of a numbers of integrated circuits
o Software produces a digital image from the electronic form

If the original document was a photo/image, then the scanned image forms an image file such as JPEG
Three-dimensional Scanners
3D scanners can scan solid objects and produce a three- dimensional image
Scanners take images at several points, x, y and z (lasers, magnetic, white light)
The scanned images can be used in Computer Aided Design (CAD) or to a 3D printer to produce a working model
Application of 2D Scanners at an Airport
Make use of (OCR) to produce digital images which represent the passport pages
Text can be stored in ASCII format
The 2D photograph in the passport is also scanned and stored as jpeg image
The passenger's face is also photographed using a digital camera and compared using face recognition software
Key parts of the face are compared (distance between eyes, width of nose)
Barcode readers/scanners
− Shines light laser at barcode
− Light is called an illuminator
− Light is reflected back
− Sensors detect the light

Advantages
− barcode can identify a product
− barcode can be used to look up product
− data about stock levels can be stored on a system
Quick Response (QR) Codes
Made up of a matrix of filled in dark squares on a light background
Can hold more storage (7000 digits)
Read/scanned using app
The app/device processes the image
Each small square/pixel is converted to a binary value
Advantages of QR codes:
o No need for the user to write down website address
o QR codes can store website addresses
Digital Cameras
Controlled by microprocessor which automatically adjusts the shutter speed, focus the image, etc.
Photo is captured when light passes through the lens onto a light sensitive cell
Cell is made up of pixels
Number of pixels determines size of the file
Keyboards
Connected to computer with a USB connection or by wireless connection
Each character has an ASCII value and is converted into a digital signal
Slow method
Prone to errors
Pointing devices
Mouse/trackball
o Traditional; mechanical ball, connected by USB port
Modern type; red LEDs to detect movement
Microphones
Used to input sound to a computer
When a microphone picks up sound, a diaphragm
vibrates producing an electric signal
The signal goes to a sound card and is converted into
digital values and stored in computer
Voice recognition, voice is detected and converted into
digital
Touchscreens
Capacitive (medium cost tech)
- An electrostatic/electric field is created
- Sensor(s) (around the screen) monitor the electrostatic field
- When touched (electrostatic) charge is transferred to finger
- Location of touch is calculated // Co-ordinates used to calculate touch

Disadvantages
Gloves are an insulator
The charge can be blocked from finger

- use capacitive gloves: this will allow the charge to be charged/disturbed


Infra-red heat (expensive)
o Use glass as the screen material
o Needs warm object to carry an input operation

Infra-red optical (expensive)
− Rays are sent across screen
− Has sensors around edge
− Ray is broken by a finger
− Calculation is made to locate the 'touch'

Resistive (inexpensive)
Uses multiple layers
When top layer is touched, two layers make contact
Circuit is completed when the layers touch
Point of contact is calculated

Resistant to weather
Less chance of input error
Does not require a mouse or keyboard
Sensors
Devices which read or measure physical properties
Data needs to be converted to digital
Analogue to Digital Converter (ADC) converts physical values into digital
Control of Street Lighting
Light sensor sends data to the ADC
Digitises data and sent to the microprocessor
Microprocessor samples data every minute
If data from sensor < value stored in memory:
o Signal sent from microprocessor to street lamp
o Lamp switched on
Inkjet Printers
Used to print one-off pictures and documents
1. Data from document sent to printer driver
2. Printer driver ensures data is in correct format
3. Check made by printer driver that chosen printer is available
4. Data is sent to printer, stored in a temporary memory (printer buffer)
5. Sheet of paper is fed; sensor detects if paper is available in paper tray
6. Print head moves across paper printing text/image, four ink colours sprayed in exact amount
7. Paper is advanced so next line is printed
8. Repeated until buffer is empty
9. Once it is done, printer send an interrupt to the
processor (request for more data to be sent)
Laser Printers
Used to print flyers, high quality
Use dry powder ink (toner) and static electricity to produce text and images
Prints the whole page in one go 1. (steps 1-4 same as inkjet)
2. Printing drum is given a positive charge; as the drum rotates, a laser beam is scanned across it removing the positive charge leaves negatively charged areas which match the text/image
3. Drum is then coated with positively charged toner, it only sticks to negatively charged parts of the drum
4. A negatively charged sheet is rolled over the drum
5. Toner on the drum now sticks to the paper to produce copy of page
6. Paper finally goes through a fuser (set of heated
rollers); heat melts the ink so it is permanent
7. Discharge lamp removes all electric charge from the drum, ready to print next page
3D Printers
Used for models of cars
Produce solid objects that work
Built up layer by layer, using powdered resin, ceramic powder
A design is made using Computer-aided Design (CAD)
2D and 3D Cutters
3D cutters can recognise objects in x, y, z direction
3D laser cutters can cut; glass, crystal, metal, wood
Actuators
Used in many control applications involving sensors and devices (ADC and DAC)
Loudspeakers/Headphones
Sound is produced by passing the digital data through a DAC then through amplifier and then emerges from loudspeaker

Produced by voltage differences vibrating a cone in the speaker at different frequencies
LCD and LED Monitors
Front layer of monitor is made up of Liquid Crystal Display (LCD), these tiny diodes are grouped together in threes as pixels (LCD doesn't emit any light)

LCD monitors are back lit using Light Emitting Diode (LED) because:
o LEDs reach their maximum brightness immediately
o LEDs sharpens image (higher resolution), CCFL has yellow tint
o LEDs improve colour image
o Monitors using LED are much thinner than CCFL
o LEDs consume very little power

Before LEDs, LCD monitors were backlit using CCFL

CCFL uses two fluorescent tubes behind the LCD screen which supplies the light source
Light projectors
Two common types of light projectors:
o Digital Light Projector (DLP)
o LCD Projector

Projectors are used to project computer output onto larger screens/interactive whiteboards
Digital Light Projectors (DLP)
Uses millions of micro mirrors
the number of micro mirrors and the way they are arranged on the DLP chip determines the resolution of the image
When the micro mirrors tilt towards the light source they are on
When the micro mirrors tilt away from the light source they are off
This creates a light or dark pixel on the projection screen
A bright white light source passes through a colour filter on its way to the DLP chip
White light splits into primary colours
LCD Projectors
Older technology than DLP
A powerful beam of white light is generated from a bulb
This beam of light is then sent to a group of chromatic-coated mirrors; these reflect the light back at different wavelengths
When the white light hits the mirrors, the reflected light has wavelengths corresponding to red, green and blue
These three different light pass through three LCD screens; these screens show the image to be projected as millions of pixels in grayscale
When the coloured light passes through the LCD screens, a red, green and blue version of the grey image emerges
Finally, the image passes through the projector lens onto the screen
Random Access Memory (RAM)
Features of RAM
o Volatile (contents lost if RAM is turned off)
o Used to store; data, files
o It can be written to or read from and the contents from the memory can be changed

Larger the size of the RAM, faster the computer will operate

RAM never runs out of memory, continues to run slow

As RAM becomes full, the processor has to continually access the hard drive to overwrite old data on RAM with new data
Read Only Memory (ROM)
Features of ROM:
o Non-volatile (contents remain even when ROM is turned off)
o Used to store start up instruction (basic input/output systems)
o Data/contents of a ROM chip can only be read, cannot be changed
Hard Disk Drives (HDD)
Data is stored in a digital format on the magnetic surface of the disks (platter)

Number of read/write heads can access all of the surfaces of the disk

Each platter will have two surfaces which can be used to store the data

Data is stored on the surfaces in sectors and tracks

HDD have very slow data access compared to RAM
Solid-State Storage (SSD)
No moving parts and all data is received at the same time (not like HDD)

Store data by controlling the movement of electrons within NAND chips, as 1s and 0s

Non-volatile rewritable memory

Benefits of using SSD rather than HDD:
o More reliable (no moving parts)
o Considerably lighter (suitable for laptops)
o Lower power consumption
o Data access if faster than HDD
o Very thin
o Run much cooler than HDDs

Drawback - questionable longevity (20GB per day
CD/DVD Disks
Laser (red) light is used to read and write data in the surface of the disk
Use a thin layer of metal alloy to store data Interpreter
Both systems use a single, spiral track which runs from the centre of the disk to the edge
DVD uses Dual-Layering which increases the storage capacity (two individual recoding layers)
Blu-ray Disks
Uses blue laser to carry out read and write operations
Wavelength of laser light is less than CD and DVD (stores up to five times more data than DVD)
Automatically come with secure encryption (prevent piracy and copyright infringement)
Used as back-up systems
USB Flash Memories
Very small, lightweight suitable from transferring files
Small back-up devices for photo, music
Solid state so need to be treated with care
High-Level Languages
Easier to read and understand as the language is closer to human language
Easier to write in shorter time
Easier to debug at the development stage
Easier to maintain once in use
Low-Level Languages
Refer to machine code
Binary instructions that computer understands
Translators
A program must be translated into binary before a computer can use it

Types of translators; Compiler, Interpreter and Assembler
Compiler
Translates a program written in high-level language into machine code
Used without compiler
Executable file of machine code produced
One high-level language translated into several machine code instructions
Used for general use
Provides an executable file
interpreter
Executes a high-language program a statement at a time
No executable file of machine code produced
One high-level language program statement may require several machine code instructions to be executed
Cannot be used without interpreter
Used when program is being developed
Assembler
Translates a low-level language program into machine code
Executable file of machine code produced
One low-level language translated into one machine code instructions
Can be used without assembler
Used for general use
Syntax Errors
When program is being compiled, if any syntax errors are found no translated program is produced
Instead, a list of all errors in program is produced
Programmer corrects program and recompiles
When a program is being interpreted, the interpreter
preforms the action until syntax error is found
The programmer is the alerted to the place in the program where error was found
The error is corrected and interpretation continues
Security
Need to keep data safe from accidental damage, including corruption and human errors
Need to keep data safe from malicious actions, including unauthorised viewing, deleting, copying and corruption
Firewall
A firewall sits between the user's computer and an external network (internet) and filter information in and out of the computer
Tasks carried out by firewall:
o Examining 'traffic'
o Checking weather incoming or outgoing data meets criteria
o If data fails the criteria, the firewall blocks 'traffic'
o Firewall can keep a list of all undesirable IP addresses o Helping to prevent viruses or hackers entering the user's computer
Proxy server
Proxy servers act as an intermediary between the user and a web server
Functions of proxy servers:
− Prevents direct access to the webserver
− If an attack is launched it hits the proxy server instead
− Used to direct invalid traffic away from the webserver
− Filters traffic
Secure Sockets Layer (SSL)
Type of protocol that allows data to be sent and received securely over the internet
When a user logs onto a website, SSL encrypts the data
https or padlock in the status bar
When user wants to access a secure website:
o User's web browser sends a message so it can connect with required website which is secured by SSL
o Web browser requests that the web server identifies itself
o Web server responds by sending a copy of its SSL certificate
o Web browser checks if certificate is authentic
o Sends signal back to web browser
o Starts to transmit data once connection is established o If not secure, browser will display an open padlock
Transport Layer Security (TLS)
Form of protocol that ensures the security and privacy of data between devices and users when communicating over the internet
Designed to provide encryption, authentication and data integrity in a more effective way than SSL
Possible to extend TLS by adding new authentication methods
Encryption
Used to protect data in case it has been hacked
Doesn't prevent hacking, makes data meaningless
Need a key to decrypt the text
Symmetric Encryption
A secret key which can be a combination of characters
If this key is applied to a message, its contents is
changed
One key is needed to encrypt a message and another key is needed to decrypt a message
Increasing the length of the key increases the strength of the encryption
Plain text
Is the normal representation of data before it goes through an encryption algorithm
Cypher Text
Cypher text is the output from an encryption algorithm
Authentication
Authentication is used to verify that data come from a trusted source
Works with encryption to strengthen internet security
Passwords: usually a user id/name and password are used to log on to systems
Digital Signatures: public key encryption; ensure an electronic document is authentic
Biometrics: relies on the unique characteristics of human beings (fingerprint scans, retina scans)
Phishing
Creator sends out a legitimate-looking email; as soon as recipient clicks on link, user is sent to a fake website

Effect:
o Creator of email can gain personal data; bank account
o Can lead to fraud

To remove risk:
o Many ISPs filter out phishing emails
o User should be cautious
Pharming
Malicious code installed on a user's hard drive or on the web server, code will redirect the user to a fake website

Effect:
o Creator of malicious code can gain personal data; bank account
o Can lead to fraud or identity theft

To remove risk:
o Some anti-spyware can identify and remove pharming code from hard drive
o User should be alert
Denial of Service Attacks (DoS)
An attempt at preventing users from accessing part of a network
Usually temporary but may be damaging Attacker me be able to prevent user from:
o Accessing their emails
o Accessing websites
o Accessing online services
Ways bank protect their costumers
o Banks use 10/12-digit code unique to the costumer

o May be asked to input three random numbers from a four-digit PIN

o Some use a hand-held device into which a customer inserts their card. Device will generate an eight-digit code which the customer types into web page

o Some ask to key in parts of their passwords using
drop-down boxes (using a mouse)
Ethics
A set of principles set out to regulate the use of computers
Plagiarism is when a person takes another person's idea/work and claims it as their own
Free Software
Software source code can be freely accessed and modified as required
Run, copy, change or adapt free software (e.g. Scribus, Abiword)
Possible to distribute modified versions of software to friends and family
Freeware
A software a user can download from the internet free of charge
Once it has been downloaded, there are no fees associated (e.g. Adobe, Skype)
Subject to all copyright laws
User is not allowed to study, modify code
Shareware
All the features of the full version of software are not made available; full version needs to be purchased first
Subject to all copyright laws
Permission needs to be obtained before software is copied and given to friends or family
Optical Character Recognition (OCR)
A software which converts scanned documents into a text file format
Internet Service Provider (ISP)
- Provide access to the internet
- Give users an IP address
- Supports domain names
- Provide security services
- Provide web hosting facilities
- Provide access to Email / Mailbox
- Provides online data storage
Converting into digital photos
- Image is converted from analogue to digital (using ADC)
- Image is turned into pixels
- Each pixel is given a binary value
- Pixels form a grid (to create the image)
- Each pixel has a colour
anti-virus software
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