IGCSE COMPUTER SCIENCE
1. Data Representation
1.1 Binary systems
● Binary data
○ Computer
■ Electronic machine to process data
■ Data → Process → Information
○ Processor
■ Stores data using electrical switches (on/off)
■ Stores & processes data
○ Binary
■ Base 2 (on/off)
■ All data must be turned into binary for computer to process
■ Used for registers where a certain bit controls a specific thing
■ Disadvantages
● Hard to read/understand
● Easy for errors to occur
● Hard to spot/fix errors
● Writing them takes up a lot of space
● Bits & bytes
○ Bits
■ Binary digIT (1/0)
■ Signal on/off switch in the computer’s electronic memory
○ Byte
■ String of 8 bits
■ Computer organises data into a byte
○ Memory
■ Area of computer that stores data as on/off signals
■ RAM (random access memory)
■ Not enough space in RAM = data stored outside RAM (slower to access)
■ Processors contain registers
● Binary & denary
○ Denary
■ Base 10
○ Binary to denary
■ Eg. 0101011100 to denary (348)
○
512
256
128
64
32
16
8
4
2
1
0
1
0
1
0
1
1
1
0
0
■ Max: 16 bits
■ 4+8+16+64+256 =348
Denary to binary
■ Start with largest value you can subtract from the number
■ Continue to subtract until you have 0
■ Put 1 in the matching columns; 0 in all other columns
■ Eg. 40 to binary (00101000)
● 40-32=8, 8-8=0
128
64
32
16
8
4
2
1
0
0
1
0
1
0
0
0
1.2 Hexadecimal
● Digits
○ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F (Base 16)
○ Usage
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■ Easy conversion (hex to binary vice versa)
■ Easy to read
■ Takes up less space
Hexadecimal & Denary
○ Hex to Denary
■ Place into column
■ Multiply accordingly
■ Add them up
○ Denary to Hex
■ Divide denary number by 16
■ Result goes into the
16s column
■ Remainder goes into
the units
■ Larger numbers (> 225)
● Find largest column that is smaller than X
● Divide X by the common value
● Divide the remainder by the next smallest value
● Continue until you reach the units column
Hexadecimal & Binary
○ Convert: match with the chart
Benefits
○ Can represent 16 bit words as 4 hex digits
○ Easy to convert digit back to binary
Usage
○ Defining colors in HTML
■ 24 bit colour
● Eg. #FF0000 (red), #00FF00 (green), #0000FF (blue)
○ Machine code & assembly language
■ Machine code is turned into hex
● Used to represent binary data & addresses
● Easier, faster, less errors
● Eg. 5F 3A 09 F1
■ Error messages displayed in hex code
● Eg. error #C04 door open
○ Media Access Code (MAC) addresses
■ Identifies a device on the internet
● Refers to network interface card (NIC): part of the device
● Unique address
● Static address (doesn’t change)
● Set by manufacturer
■ 48 bits shown as 6 groups of hex digits (eg. NN:NN:NN:DD:DD:DD)
● 1st half = identity number of manufacturer of the device
● 2nd half = device’s serial number
○ Debugging
■ Error message & debugging software are in hex
○ Main memory & registers (in the RAM)
■ Makes data/addresses easier to read
1.3 Data Storage
● Digital data
○ Digital: Something that has a precise value which can be represented as a number
○ Number values stored in binary/denary/hex
○ Text
■ Stored using basic character code
● American Standard Code for Information Interchange (ASCII)
○ Number code representing all standard keyboard characters
○ File types: .txt (storage for ASCII), .doc (text document format), .exe (instructions to be
executed)
■ Meaning of numbers are different in different formats
● Numbers can be stored in number value/text characters
■ Computer must detect file format
● Digital graphics
○ Images
■ Made out of pixels
● Pixel = 1 spot of light
■ Bitmap graphics (.bmp)
● File stores position & colour of every pixel
○ Each pixel has its own number code (lots of data)
● Full dot to dot detail colours of real life can be stored
● Good way to store real-life images
■ Vector graphic (.svg)
● Made of shapes
● Computer stores math formulas on how to draw it (no pixelation)
● Less information
● For cartoons, diagrams, graphs etc.
● Not for storing pictures
■ Pixelation: individual pixels can be seen
■ Colour depth: No. of colour used in an image
● Digital sound & video
○ High quality sound + video = high bit rate
○ Sample rate * No. of channels * bit depth = No. of bytes
○ File types
File format
MIDI (Musical
instrument digital
interface)
Main uses
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JPEG
(Joint photographic
experts group)
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Advantages
Storage of music files
Uses 8-bit serial
transmission
Stores analogue data
○ Pitch, notation,
velocity, volume
etc.
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Used to reduce
photographic file sizes
Lossy compression for
images
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Smaller file size
Easily modified
No background noise
Can create music
Disadvantages
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Reduces photographic file
sizes
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Dependent on
quality of sound
card for overall
sound
Can’t store vocals
Effects are limited
Reduction in
quality
Only supports
8-bit images
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MP3
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MP4
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Lossy compression for
audio
Audio format for websites
Removes sounds human
ear can’t hear
If 2 sounds are played
simultaneously, the softer
sound is removed
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Small file size
Widely accepted format
Good for online audio
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Standard video format
Allows the storage of
multimedia files rather than
just sound
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Video streaming
Store other data types
(video, audio, text,
images)
Separate hint track
Can be played on
anything
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Changing the
number of pixels
per cm
●
Some softwares
don’t accept
No quality
Hard to edit
Can't be streamed
live
Compression
○ Stores the same data but using fewer bytes
○ Lossy compression
■ Eliminates unnecessary bits of data
■ Can’t return to its original state when uncompressed
■ Data quality is reduced
■ Detail in file is lost = blurry image
○ Lossless compression
■ All data bits from the original file are reconstructed when the file is
uncompressed
● No data is removed (original data can be restored)
● Patterns in the data are identified & are indexed & put into a table
■ Reduces size of data file
○ Compressed image files
■ Reducing image quality (lossy compression methods)
● Using fewer larger pixels to make the image (decrease resolution)
● Using fewer bytes to store colour code (decrease colour depth)
■ Removing repetition
● Same colour stored twice gets removed
■ File format
● JPEG (.jpg/.jpeg), TIFF, GIF, PNG
○ Compressed sound = reduction of sound quality
■ Fewer channels
■ Lower sample rate
■ Reduces bit depth
○ Compressed video
■ Reduces number of audio channels, sample rate & image quality
○ Compressed text files
■ Lossless compression using a compression algorithm
■ Repeated words/word sections can be indexed/replaced by numeric value
■ Compressed file is a zip file
■ Can be saved/converted into pdf
2. Communications & the internet
2.1 Data transmission
● Physical transfer of data over a communications channel
● Signals are sent from one place to another
○ Transmission content
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Electrical pulses that travel down metal cables
Pulses of light that travel down a fibre optic cable
Wireless signals
● Radio signals, microwave, infrared waves
● Bits are sent as electromagnetic waves; move through space
Asynchronous data transmission: data being transmitted in an agreed bit pattern
○ Data bits are grouped together & sent with control bits
○ Receiver of the data knows when the data starts & ends
■ Prevents data from getting mixed up
Synchronous data transmission: a continuous stream of data
○ Ensures that the sender & receiver are synchronised with each other
○ Faster method
Serial & parallel transmission
○ Serial: when data is sent one bit at a time over a single wire
■ Eg. connecting computer to a modem
Advantages
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Disadvantages
Works well over long distances
Data will not be out of
synchronisation (reliable)
Cheaper (fewer wires)
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Slower
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Universal serial bus (USB)
● Peripheral that uses asynchronous serial data transmission
● Bytes of data are broken up & sent along the bus one bit at a time
● Advantages
○ Devices are automatically detected & configured when initially
attached
○ Impossible to connect device incorrectly (connector only fits
one way)
○ Supports multiple data transmission speeds
○ Supported by many OS
Parallel: when data is sent several bits (1 byte) at a time over several wires (normally 8)
at the same time (eg. used by internal components)
■ Eg. Sending data from a computer to a printer
Advantages
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Works well over short
distances
Faster rate of transmission
Disadvantages
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Higher chance of error
Bits can become ‘skewed’ & out of
synchronisation
More expensive (more wires)
Integrated circuit (IC)
● Collection of microscopic electronic circuits sealed into a single
plastic/ceramic package
● Uses parallel transmission
○ Simplex & Duplex
■ Simplex data transmission: one direction only
● Eg. computer to printer
■ Half-duplex data transmission: both directions but not at the same time
● Eg. walkie-talkie system
■ Full-duplex data transmission: both directions simultaneously
● Eg. Phone call
Transmission errors
○ Data that is corrupted during transmission
○ Errors caused by flaws of the transmission medium
■ Imperfections of the wires