IGCSE COMPUTER SCIENCE
NOTES
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,Contents INDEX Page number
-C1. Data Representation 3
-C2. Communication and Internet Technologies 8
-C3. Hardware and Software 12
-C4. Security 20
-C5. Ethics 21
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, Computer Science
Data Representation
Binary to Denary Conversion:
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,Denary to Binary Conversion:
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,Denary to Hex Conversion:
Hex to Denary Conversion:
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,Analogue: This is the smooth stream of data that our senses process.
Digital: Data that is represented in 1 and 0 that a computer can process.
Measuring memory size: Computer storage is usually measured in bytes.
Binary: Is used in robotics, digital instruments and counting systems.
Register: A small piece of memory, built in the CPU, where values and instructions are
temporarily held. Although they are small in capacity, registers have an extremely fast
read and write rate.
Types of registers:
Processor registers: Program counter, the accumulator and the memory address
register. They are used when small amounts of data need to be accessed quickly. E.g.
Calculations.
Hardware registers: Are specific to different types of hardware that are used to
convey a signal. E.g. Robot Arm.
Hex: Computers don’t process hex, they convert it into binary to process it.
Programmers work with hex as it is easier for humans to read and write. Programs
written in hex are easier to debug. Hex is used for a notation of colour in HTML, to
show error messages in windows, in assembly language, machine code and Mac
addresses.
Hex colour notation: They are normally six digits. The system uses 2 hex digits for each
colour.
RGB Colour Model: Is similar to hex colour code. Has values between 0 and 255 for
each shade of green, red and blue.
Images: They are broken down into tiny coloured squares called pixels, each pixel
represents one colour.
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,Image resolution: Is a measure of how much detail there is in the image. It’s defined as
the ability to distinguish between 2 points. In a high-resolution image, it will be easier
to distinguish between 2 dots close together, while in a low resolution it may become
blurry.
Metadata: There is other data that is stored with the image that describes the file. This
type of data is called metadata as it describes the image data. Metadata is what tells
the computer how many pixels wide and how many pixels high an image should be,
the colour depth and the number of colours the image needs.
Sound: Sound waves are analogue; they are converted to digital as binary. Sound is
recorded at set time intervals; this process is called sampling. If set time intervals are
closed together, sound track will be higher in quality. Computer samples the height of
the wave and the number of times per second. Sample sizes are measured in Hertz, Hz.
Vector graphics: Are created in graphic packages and consist of shapes called objects.
You can edit each object separately sand change the shape, colour, size and position.
The file size is very small and they are scalable. When you resize them, they don’t lose
quality.
Bitmap graphics: Is composed of pixels which are of many different colours. You can
edit each individual pixel. The file size is large. When you resize them it loses quality.
Error detection and Correction:
Errors can occur when transmitting and storing data. This is because the channels that
are used to transmit data can be subjected to disturbance. These errors can lead to
inaccuracies in the data. That why we use the following error detection and correction
methods:
Parity Check: Uses a parity bit to make sure that the data has been sent accurately.
Data is sent in bytes, 8 bits. In parity check, the first 7 bits are the data and the 8 th bit is
the parity check. A parity check can use odd or even parity, and depending which
parity is being used, we put a 1 or a 0.
E.g. 0010101, we add this code of 7 bits together. Which is 3. Now we have to add a 1
at the end if it is even parity, or a 0 if it is odd parity.
Disadvantages: If 2 bits transpose and change place, the computer may not detect the
error. If a byte is affected by disturbance and the bits change state but they still have
the same odd or even parity, the computer won’t detect the error.
Checksum: The number of bits being transmitted is counted up and the numeric count
is transmitted with the data. The receiver will do the count again, if it match’s then the
receiver knows the transmission is correct. If the counts don’t match, the receiver will
request to send the data again.
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,Check digits: A check digit is a method used on identification numbers such as ISBNs,
and is used to detect human error when entering the numbers. A calculation is made
using the digits in the identification number and a check digit which is added to the
end as a result. The computer will do again the same calculation and compare the
result, if they match, the data has been sent correctly.
Automatic Repeat Request: The sending computer transmits a block of data. The
sending computer waits a period of time to see if the receiving computer
acknowledges receipt of the data. After a set period of time passes, it makes the data
to be automatically resent. This will continue until the receiving computer
acknowledges the data has been received.
JPEG: Joint Photographic Experts Group, is a compressed bitmap image file format
which is used for photographs. Uses lossy compression.
MP3: Moving Pictures Experts Group is the standard for distributing digital music files
and it uses lossy compression.
MP4: Is a file format created by the Moving Picture Experts Group as a multimedia
container format designed to store audio-visual data.
MIDI: Musical Instrument Digital Interface, is a standard adopted by the electronic
music industry for controlling devices.
Lossless compression: Allows the original file to be recreated exactly from the
compressed file. It works by searching for patterns in the file, that’s why text and
software files compress well.
Lossy compression: It compresses files that contains a lot of unique information, such
as bitmap graphics, sound or video. Lossy compression removes data that is not
needed, either because a drop in quality is acceptable or the difference can’t be
detected.
Communication and Internet Technologies
Data Transmission:
The transfer of data occurs wirelessly or over cable. The data is transmitted as a
stream of bits. The rate at which the transfer of data occurs is called the bit rate. This
is the number of bits that can be transmitted over a period of time. The bit rate is
measured in in bits per second (bps) or megabits per second (Mbps). The more bits or
megabits per second data transfer connection is capable of, the quicker the data is
transmitted.
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, Types of Data Transmission:
Serial data transmission: Uses a single wire to transfer the data. A single wire is cheap
to build and can transmit data over long distances. The bits are transmitted
sequentially, which makes it slower. There is a set time interval before sending each
bit. It is a safer transmission as it is easier to collate the bits together. The time
intervals depend on the speed of transmitting and receiving devices.
Parallel data transmission:
It uses several wires to transfer the data simultaneously, that why it is more expensive
and is used over short distances. The transfer of data is quicker, but the transmission is
less safe as bit are sent simultaneously errors can occur collating them together.
Parallel transmission is simple to implement, but as it has a lot of wires, interferences
can occur. That’s why it is limited to short distances of 5m. Parallel transmission was
used for data transferred between computers and printers. Parallel transmission has
now been replaced by USB. However, parallel transmission is still used for integrated
circuits(IC), microprocessors, the internal components of a computer…
In order for data to be sent by serial transmission it needs to be converted. Data is
converted from parallel to serial at the sender’s side, to be transmitted over a single
wire. It is then converted back from serial to parallel to be processed by the receiver.
The direction in which data is transmitted varies.
Simplex transmission: Data is sent in one direction.
E.g. Television forecast.
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