GCSE Edexcel Computer Science Exam Questions
With Complete Solutions Graded A++
Explain how binary is used to represent data (numbers, text, sound, graphics)
and program instructions
binary is used to represent all elements in a computer
It is stored as a series of 0s and 1s
These relate to the position of transistors (on or off)
All analogue data stored is converted into digital (binary)
Explain the difference between unsigned and signed integers
An unsigned integer can only represent positive values
A signed integer can represent positive and negative values
Both unsigned and signed integers are as accurate
Both unsigned and signed integers can have overflow errors
Unsigned integers store more positive values (unsigned is 0 to 255, signed is -127 to
127)
Both unsigned and signed integers use a parity bit
Parity bits are used to check the binary value transmitted is correct
Explain how sign and magnitude is used to represent positive and negative
values 0
Explain how sign and magnitude is used to represent positive and negative values
Explain how 2's complement is used to represent negative values
To convert a binary value into two's complement
...flip the value of each bit (0 becomes 1, 1 becomes 0)
Add 1 to the flipped value
Explain how to convert from binary to denary (decimal)
Write out the headings for the binary value, starting with 1 on the right-hand side
...usually finishing at 128
...add up the headings with 1 values beneath them
Explain how to convert from denary (decimal) to binary
Write out the headings from 1 to 128, starting on the right-hand side
...assume the denary value you are converting is an amount of money
...You want to purchase the most expensive headings you can
...remember to calculate the remainder (subtraction)
...if you can purchase a heading it gets a 1 below it
...if you cannot purchase a heading it get a 0 below it
...when you finish you should have no money left
Explain how to add two binary values together
Four rules that need to be followed to add binary values
...0 + 0 = 0
...1 + 0 = 1
...1 + 1 = 10 (0, carry 1)
...1 + 1 + 1 = 11 (1, carry 1)
Explain the concept of overflow
,Overflow error occurs when the result cannot fit into the size of the location assigned to
hold it
The computer will ignore the overflow digit
If an overflow error is ignored this compounds errors in subsequent calculations, which
can snowball
Explain what affect left and right shifts have on binary values
Logical shift left multiplies the number by 2 for each shift
Logical shift right divides the number by 2 for each shift
Explain how to convert between hexadecimal and binary values
Each 4 bits of binary represents 1 bit of hexadecimal
If the converted binary value is between 0 and 9, it stays the same (0100 = 4 = 4)
If the converted binary value is between 10 and 15, it changes to a letter (1010 = 10 =
A, 1111 = 15 = F)
Explain why hexadecimal notation is used
Hexadecimal is used as a short-hand for binary
Hexadecimal is used so humans find it easier to read
Hexadecimal is usually used by computers to display error code numbers
Hexadecimal is used to represent numerical values in assembly languages and 24bit
colour codes
Hexadecimal is only used by humans as computers only ever use binary
Explain how computers encode characters using ASCII
Each character has a unique binary code
This unique code is sent by the keyboard to the computer
The ASCII code for A is 65 (in binary 0100 0001)
Codes for other binary numbers can be found from A (B is 66, C is 67, D is 68, etc)
ASCII codes for letters are always represented as 8 bit binary values (e.g. 0100 0001)
Explain the difference between ASCII and UNICODE
ASCII can represent 127 different characters
...the 8th bit it used as a checksum
UNICODE uses 16 bit instead of 8 bit
...this means it can represent 65,536 different characters
...this can include elements from other languages, symbols, and maths characters
Explain what pixels are (in relation to bitmap images)
Pixels are the smallest elements of a bitmap image and the smallest parts that can be
displayed on a screen
Explain what resolution is (in relation to bitmap images)
The resolution of an image is based on the number of elements used to represent the
full image
...the higher the number of elements for a given size, the better the quality of the image
Resolution is the number of pixels per inch when the image is displayed (on a monitor,
or on paper)
Explain what colour depth is (in relation to bitmap images)
Colour depth indicates the number of bits used to represent the colour of a picture
element
...the higher the number of bits used, the greater the range of colours
Explain what RBG colour is and how it works
, All colours are combinations of RGB (red/green/blue)
An equal number of bits is usually given to each colour
Explain how bitmap image file sizes are calculated
Bitmap image files are a combination of image size and colour depth
Bitmap image files are calculated by using width x height x colour depth
The better the image quality the larger the file size
Explain how sound (an analogue signal) is represented in binary
The amplitude of the signal is recorded digitally
This is done at regular intervals, which is the sample frequency
The greater the sample frequency the more samples
...the smoother the digital representation of the original audio
...this results in less distortion of the original audio
...this makes the resulting digital representation more accurately match the original
audio
Explain the limitations of binary representation of audio
Sampling is the process of recording analogue signals at regular intervals
Sampling frequency is the average number of samples obtained per second
...measured in Hertz (Hz)
Sampling is done by taking a sample of the amplitude of the sound signal at regular
intervals
The more frequent the samples, the truer the representation is to the original analogue
signal
When sampling sound, the more bits used the better the quality
Explain how to convert between the terms bit (b) and byte (B)
To convert from byte to bit multiply by 8 (1 byte is 8 bits)
To convert from bit to byte divide by 8 (8 bits are 1 byte)
Explain how to convert between the terms byte (B), kilobyte (KB), megabyte (MB),
gigabyte (GB) and terabyte (TB)
Size sequence is bit (b) - byte (B) - kilobyte (KB) - megabyte (MB) - gigabyte (GB) -
terabyte (TB)
To move from TB down to GB (and beyond) multiply by 1024 for each level/unit
To move up from B to KB (and beyond) divide by 1024 for each level/unit
Explain the difference between lossy and lossless compression
Lossy compression data is permanently lost (during the compression process)
Lossless compression not data is lost (during the compression process)
A lossless compressed file can be returned to its original format with no data loss
Explain how lossy compression can be acceptable for audio compression
Lossy compression for audio can be acceptable because most people do not notice
...most people have a limited hearing range
...sounds (frequencies/amplitudes) can be masked by other sounds
...people's brains will 'fill in the blanks'
Explain the advantage of compressing files
Compressed files will reduce the amount of space needed to store the file
Compressed files will mean the file will take less time to download
Explain how run
With Complete Solutions Graded A++
Explain how binary is used to represent data (numbers, text, sound, graphics)
and program instructions
binary is used to represent all elements in a computer
It is stored as a series of 0s and 1s
These relate to the position of transistors (on or off)
All analogue data stored is converted into digital (binary)
Explain the difference between unsigned and signed integers
An unsigned integer can only represent positive values
A signed integer can represent positive and negative values
Both unsigned and signed integers are as accurate
Both unsigned and signed integers can have overflow errors
Unsigned integers store more positive values (unsigned is 0 to 255, signed is -127 to
127)
Both unsigned and signed integers use a parity bit
Parity bits are used to check the binary value transmitted is correct
Explain how sign and magnitude is used to represent positive and negative
values 0
Explain how sign and magnitude is used to represent positive and negative values
Explain how 2's complement is used to represent negative values
To convert a binary value into two's complement
...flip the value of each bit (0 becomes 1, 1 becomes 0)
Add 1 to the flipped value
Explain how to convert from binary to denary (decimal)
Write out the headings for the binary value, starting with 1 on the right-hand side
...usually finishing at 128
...add up the headings with 1 values beneath them
Explain how to convert from denary (decimal) to binary
Write out the headings from 1 to 128, starting on the right-hand side
...assume the denary value you are converting is an amount of money
...You want to purchase the most expensive headings you can
...remember to calculate the remainder (subtraction)
...if you can purchase a heading it gets a 1 below it
...if you cannot purchase a heading it get a 0 below it
...when you finish you should have no money left
Explain how to add two binary values together
Four rules that need to be followed to add binary values
...0 + 0 = 0
...1 + 0 = 1
...1 + 1 = 10 (0, carry 1)
...1 + 1 + 1 = 11 (1, carry 1)
Explain the concept of overflow
,Overflow error occurs when the result cannot fit into the size of the location assigned to
hold it
The computer will ignore the overflow digit
If an overflow error is ignored this compounds errors in subsequent calculations, which
can snowball
Explain what affect left and right shifts have on binary values
Logical shift left multiplies the number by 2 for each shift
Logical shift right divides the number by 2 for each shift
Explain how to convert between hexadecimal and binary values
Each 4 bits of binary represents 1 bit of hexadecimal
If the converted binary value is between 0 and 9, it stays the same (0100 = 4 = 4)
If the converted binary value is between 10 and 15, it changes to a letter (1010 = 10 =
A, 1111 = 15 = F)
Explain why hexadecimal notation is used
Hexadecimal is used as a short-hand for binary
Hexadecimal is used so humans find it easier to read
Hexadecimal is usually used by computers to display error code numbers
Hexadecimal is used to represent numerical values in assembly languages and 24bit
colour codes
Hexadecimal is only used by humans as computers only ever use binary
Explain how computers encode characters using ASCII
Each character has a unique binary code
This unique code is sent by the keyboard to the computer
The ASCII code for A is 65 (in binary 0100 0001)
Codes for other binary numbers can be found from A (B is 66, C is 67, D is 68, etc)
ASCII codes for letters are always represented as 8 bit binary values (e.g. 0100 0001)
Explain the difference between ASCII and UNICODE
ASCII can represent 127 different characters
...the 8th bit it used as a checksum
UNICODE uses 16 bit instead of 8 bit
...this means it can represent 65,536 different characters
...this can include elements from other languages, symbols, and maths characters
Explain what pixels are (in relation to bitmap images)
Pixels are the smallest elements of a bitmap image and the smallest parts that can be
displayed on a screen
Explain what resolution is (in relation to bitmap images)
The resolution of an image is based on the number of elements used to represent the
full image
...the higher the number of elements for a given size, the better the quality of the image
Resolution is the number of pixels per inch when the image is displayed (on a monitor,
or on paper)
Explain what colour depth is (in relation to bitmap images)
Colour depth indicates the number of bits used to represent the colour of a picture
element
...the higher the number of bits used, the greater the range of colours
Explain what RBG colour is and how it works
, All colours are combinations of RGB (red/green/blue)
An equal number of bits is usually given to each colour
Explain how bitmap image file sizes are calculated
Bitmap image files are a combination of image size and colour depth
Bitmap image files are calculated by using width x height x colour depth
The better the image quality the larger the file size
Explain how sound (an analogue signal) is represented in binary
The amplitude of the signal is recorded digitally
This is done at regular intervals, which is the sample frequency
The greater the sample frequency the more samples
...the smoother the digital representation of the original audio
...this results in less distortion of the original audio
...this makes the resulting digital representation more accurately match the original
audio
Explain the limitations of binary representation of audio
Sampling is the process of recording analogue signals at regular intervals
Sampling frequency is the average number of samples obtained per second
...measured in Hertz (Hz)
Sampling is done by taking a sample of the amplitude of the sound signal at regular
intervals
The more frequent the samples, the truer the representation is to the original analogue
signal
When sampling sound, the more bits used the better the quality
Explain how to convert between the terms bit (b) and byte (B)
To convert from byte to bit multiply by 8 (1 byte is 8 bits)
To convert from bit to byte divide by 8 (8 bits are 1 byte)
Explain how to convert between the terms byte (B), kilobyte (KB), megabyte (MB),
gigabyte (GB) and terabyte (TB)
Size sequence is bit (b) - byte (B) - kilobyte (KB) - megabyte (MB) - gigabyte (GB) -
terabyte (TB)
To move from TB down to GB (and beyond) multiply by 1024 for each level/unit
To move up from B to KB (and beyond) divide by 1024 for each level/unit
Explain the difference between lossy and lossless compression
Lossy compression data is permanently lost (during the compression process)
Lossless compression not data is lost (during the compression process)
A lossless compressed file can be returned to its original format with no data loss
Explain how lossy compression can be acceptable for audio compression
Lossy compression for audio can be acceptable because most people do not notice
...most people have a limited hearing range
...sounds (frequencies/amplitudes) can be masked by other sounds
...people's brains will 'fill in the blanks'
Explain the advantage of compressing files
Compressed files will reduce the amount of space needed to store the file
Compressed files will mean the file will take less time to download
Explain how run
