GCSE Edexcel Computer Science Exam
Questions And Complete Solutions
1. 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)
2 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
3 Explain how sign and magnitude is used to represent positive and negative
values
The leftmost bit is known as the most significant bit (MSB) The rightmost bit is known as
the least significant bit (LSB) With Sign and Magnitude, the MSB represents the sign
With Sign and Magnitude 0 indicates positive, and 1 indicates negative 0110 1010 = a
positive value (64 + 32 + 8 + 2 = 106) 1110 1010 = a negative value (64 + 32 + 8 + 2 = -
106)
4 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 001011 = 8 + 2 + 1 = 11 Flipped = 110100
Add 1 = 110101 = -11 (in 2's complement)
5 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
6 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
7 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) If you prefer, you could count the 1s in each
column to get the same result 0 ones = 0 1 one = 1 2 ones = 10 (0, carry 1) 3 ones = 11
(0, carry 1)
8 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
9 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 001010 = 8 + 2 = 10 Left shifted = 010100 = 16 + 4 = 20
(value is doubled) Right shifted = 000101 = 4 + 1 = 5 (value is halves)
10 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)
11 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
12 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)
13 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
14 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
15 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)
16 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
17 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
18 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 e.g. if the colour depth is 4 bits, then each pixel will need 4 bits of
memory to store If the image has a width and height of 10x10, then it will have 100
pixels Therefore, the amount of memory needed would be 100 x 4 = 400 bits (divided by
8 = 50 bytes)
19 Explain how sound (an analogue signal) is represented in binary
Questions And Complete Solutions
1. 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)
2 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
3 Explain how sign and magnitude is used to represent positive and negative
values
The leftmost bit is known as the most significant bit (MSB) The rightmost bit is known as
the least significant bit (LSB) With Sign and Magnitude, the MSB represents the sign
With Sign and Magnitude 0 indicates positive, and 1 indicates negative 0110 1010 = a
positive value (64 + 32 + 8 + 2 = 106) 1110 1010 = a negative value (64 + 32 + 8 + 2 = -
106)
4 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 001011 = 8 + 2 + 1 = 11 Flipped = 110100
Add 1 = 110101 = -11 (in 2's complement)
5 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
6 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
7 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) If you prefer, you could count the 1s in each
column to get the same result 0 ones = 0 1 one = 1 2 ones = 10 (0, carry 1) 3 ones = 11
(0, carry 1)
8 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
9 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 001010 = 8 + 2 = 10 Left shifted = 010100 = 16 + 4 = 20
(value is doubled) Right shifted = 000101 = 4 + 1 = 5 (value is halves)
10 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)
11 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
12 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)
13 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
14 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
15 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)
16 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
17 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
18 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 e.g. if the colour depth is 4 bits, then each pixel will need 4 bits of
memory to store If the image has a width and height of 10x10, then it will have 100
pixels Therefore, the amount of memory needed would be 100 x 4 = 400 bits (divided by
8 = 50 bytes)
19 Explain how sound (an analogue signal) is represented in binary
