Objectives
• Understand the use of binary codes to represent
characters
• Understand the term ‘character set’
• Explain the relationship between the number of bits
per character in a character set, and the number of
characters that can be represented using:
• ASCII
• Extended ASCII
• Unicode
ASCII and Unicode
Unit 2 Data representation
Starter
• A computers memory and storage only hold binary
1s and 0s
• How might it be possible to store letters with only binary?
ASCII and Unicode
Unit 2 Data representation
Representing text characters
• If a computer understands only 1s and 0s, what
happens when the ‘M’ key is pressed on the
keyboard?
0 1 0 0 1 1 0 1
ASCII and Unicode
Unit 2 Data representation
Representing characters
in binary
• Every character on the keyboard is represented by a
binary value
• Uppercase letters (capitals) have different values from lowercase
characters
• Punctuation symbols have their own character code
• How many characters are there on a standard
keyboard?
• How many bits would be required to represent this many
combinations?
ASCII and Unicode
Unit 2 Data representation
Characters in binary
• A keyboard needs to contain
• 26 lowercase letters
• 26 uppercase letters
• 10 numbers
• (around) 36 other characters
• There are around 98 unique characters that are
available on a keyboard
• 6 bits give 64 different combinations – this isn’t enough
• 7 bits give 128 different combinations which can represent
128 different characters
ASCII and Unicode
Unit 2 Data representation
Character sets
• A character set is a set of letters, symbols and digits
that can be represented by a computer
• There are two major character sets in use today
• ASCII
• Unicode
ASCII and Unicode
Unit 2 Data representation
The ASCII character set
• ASCII (American Standard Code for Information
Interchange) has become the standard code, used
worldwide
• It was originally developed in the 1960s for representing the
English alphabet
• It encodes 128 characters into 7-bit binary codes
• Characters include numbers 0 to 9, uppercase and
lowercase letters A-Z, a-z, punctuation symbols and
the space character
ASCII and Unicode
Unit 2 Data representation
The ASCII character set
• What happens if you press ALT+65 on a keyboard?
• What character is represented by 0100000 (32)?
• What is the ASCII character for the number 7? Is this
the same as the binary value for 7?
• Why not? What is happening? What does this mean?
ASCII and Unicode
Unit 2 Data representation
ASCII groups and sequences
• Character codes are commonly grouped and run in
sequence
• Numeric characters 0 to 9 run consecutively from 48 to 57 on
the ASCII table
• A-Z characters are from 65-90 or
01000001 to 01011010
• What range does lowercase characters a-z use?
• If you know Capital A is 65 or 01000001, what is Capital E?
ASCII and Unicode
Unit 2 Data representation
ASCII character set
• ASCII character 32 (010 0000) represents a space
• The ASCII character code for ‘7’ is 55
• 55 (011 0111) is the ASCII character code that represents the
character ‘7’
• In programming this is very different to the integer 7 which is
represented by 0000 0111 (7)
• Lowercase characters a-z use 97-122
• If A is 65 (0100 0001) then E is 69 (0100 0101)
ASCII and Unicode
Unit 2 Data representation
7- and 8-bit ASCII
• Numerous different codes for representing
characters have been created, but ASCII is
commonly used on PCs
• Originally only seven bits were used, but now an
eighth bit is used allowing for many more characters
such as ©, ® etc.
• How many different characters can be encoded using
7 bits, 8 bits or 16 bits?
ASCII and Unicode
Unit 2 Data representation
Character codes
• A 7-bit character code (like ASCII) has
128 different characters that can be encoded
• An 8-bit character code (like extended ASCII) has
256 different characters that can be encoded
• A 16-bit character code has 65 536 different characters that
can be encoded
ASCII and Unicode
Unit 2 Data representation
Using the eighth bit
• Sometimes it is useful to be able to type special
characters like á, à, ®
• Here are the codes for some of them:
© Alt+0169
® Alt+0174
á Alt+0225
à Alt+0224
â Alt+0226
ä Alt+0228
• Try out these different character codes
ASCII and Unicode
Unit 2 Data representation
Worksheet 3
• Complete Task 1 and Task 2 on Worksheet 3
ASCII and Unicode
Unit 2 Data representation
Programming with text
and numbers
• The ASCII code for ‘7’ is 011 0111
• The binary code for the digit 7 is 0000 0111
• When you write a program in Python, for example,
you have to specify whether a variable is text or
integer
• You cannot do arithmetic with characters
• If the character represents a number it must first be converted
to an integer before any arithmetic can be carried out
ASCII and Unicode
Unit 2 Data representation
Working with string input
• In Python, a string will be surrounded by
speech marks, whilst an integer won’t be
age = 15
name = "Sam"
• If text is entered to a program, it will be as a string
unless it is converted
• The + symbol will concatenate (join) two strings together
• The function int(s) will convert the string named s into
an integer
ASCII and Unicode
Unit 2 Data representation
Working with string input
• Look at the following program
firstNum = input("Enter first number: ")
secondNum = input("Enter second number: ")
print(firstNum + secondNum)
sum = int(firstNum) + int(secondNum)
print(sum)
• What is the output if the user enters "3" and "17"?
ASCII and Unicode
Unit 2 Data representation
Working with string input
• Look at the following program
firstNum = input("Enter first number: ")
secondNum = input("Enter second number: ")
print(firstNum + secondNum)
sum = int(firstNum) + int(secondNum)
print(sum)
• What is the output if the user enters "3" and "17"?
Enter first number: 3
Enter second number: 17
317
20
ASCII and Unicode
Unit 2 Data representation
ASCII representation
of numbers
• Try typing ALT + 55
• What is the binary representation of the ASCII
character 7? Is this the same as the binary value for 7?
• Why not? What does this mean?
ASCII and Unicode
Unit 2 Data representation
Converting ASCII to pure binary
• Clearly, we cannot do arithmetic with ASCII characters
• Programming languages deal with the input of
numbers in different ways
• In some languages, variables have to be declared as
type char, string, integer, real etc. at the beginning of
the program
• In other languages such as Python, all data is input as string, and
if it is to be regarded as an integer, it has to be converted using
an inbuilt function
e.g. xString = input (“Enter an integer: ”)
x = int(xString)
ASCII and Unicode
Unit 2 Data representation
Using different alphabets
• To represent other characters for different
languages, a new code allowing for many more
characters is needed
• Unicode was developed to use 16 bits 65 536
possible combinations
• The 32 bit version gives 4 294 967 296 (over 4 billion)
possible combinations
ASCII and Unicode
Unit 2 Data representation
Unicode
• In Japanese, konnichiwa is used as a greeting
meaning ‘good day’
• In Unicode this is written as three 16-bit characters
• How many bytes does the English ‘good day’ require
in ASCII?
• How many bytes does the Japanese require in Unicode?
ASCII and Unicode
Unit 2 Data representation
Unicode
• ‘good day’ requires 8 bytes to store
• 今日は requires 6 bytes to store
(3 characters × 2 bytes)
• Unicode is also used to
store emoji
• ‘e’ is Japanese for picture
• ‘moji’ is Japanese for
character or alphabet
Smiling face with sunglasses
Unicode: 1F60E
ASCII and Unicode
Unit 2 Data representation
Worksheet 3
• Complete Task 3 on Worksheet 3
ASCII and Unicode
Unit 2 Data representation
Plenary
• Work in a pair to answer the following questions
• How many bits are in extended ASCII?
• How many characters does this allow for?
• How many bytes are in Unicode?
• If ‘f’ has the ASCII code 102, what is the ASCII code for ‘g’?
• How many bytes are needed to store “Hello everyone.”?
ASCII and Unicode
Unit 2 Data representation
Plenary
• How many bits are in extended ASCII? 8 bits
• How many characters does this allow for? 256
• How many bytes are in Unicode?
16-bit has 2 bytes, 32-bit has 4 bytes
• If ‘f’ has the ASCII code 102, what is the ASCII code for ‘g’?
103
• How many bytes are needed to store “Hello everyone.”?
15 letters (remember space and the full stop)
ASCII and Unicode
Unit 2 Data representation
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