Lesson Objectives
• Explain the use of binary
codes to represent characters
• Explain the term “Character
set”
• Describe with examples (for
examples ASCII and Unicode)
the relationship between the
number of bits per character
in a character set and the
number of characters which can
be represented.
Lesson Outcomes
• All students will identify
different character sets
• MOST students will score at
least 4/6 on the exam
questions
• SOME students will attempt
the extension activity
When you type a letter on
your keyboard, for example
the letter "K," have you
ever wondered what has to
happen in order to display
the letter on your screen?
In order to display the
letter you just typed on the
screen, the computer must
decode the binary to
determine that the letter is
a "K" and then must display
the image of a letter "K."
When you type a "K," the "K"
button sends a special
electrical signal consisting
of 0's and 1's to the CPU of
the computer. The letter is
encoded in binary.
Look at the keyboard in
front of you…
• How many different
characters/letters/symbols are you able
to type on your keyboard?
• Don’t forget upper case!
• You should be able to find around 100.
• How many BITS would we need to represent
this number of characters?
128
64
1
32
1
We need 7
16
1
8
1
4
1
bits. 1111111 = 127
2
1
1
1
Representing Characters
• Computers can only work with
binary data.
• Binary data can easily
represent numbers.
• If we make each number
represent a letter, or
character, then we can store
characters.
• One common code (ASCII) uses
seven bits to store each
character.
ASCII
• ASCII = American Standard
Code for Information
Interchange
• In total 127 codes (95
printable, 32 non-printable)
plus the null character
00000000 used as a control
character but with no
associated symbol.
This is just part of the
ASCII table…
• MISS LEVITT in denary is…
109 105 115 115 108 101 118 105 116 116
• MISS LEVITT in binary is…
• 01101101
• 01101001
• 01110011
• 01110011
• 00100000
• 01101100
• 01100101
• 01110110
• 01101001
• 01110100
• 01110100
Work out your names in DENARY and BINARY
Write a coded message
• Use the ASCII table to write
a coded message to a friend.
• Use the DECIMIL/DENARY
conversion to write your
message
• Swap with a friend and decode it!
What if we need more
characters?
• 128 characters are not enough to
encode all languages.
• There are other systems that use
more than 7 bits.
• EBCDIC is the Extended Binary
Coded Decimal Interchange Code –
this is used in IBM mainframes.
• It uses 8 bits so can encode 256
different characters.
• Unicode is more common these
days.
• Unicode comes in different
variants, but can encode more
than 107,000 characters, covering
90 languages.
• For example,
Հայերեն हिन्दी ‫עברית‬ಕನ್ನಡ ქართული कश्मीरी
)‫كشميري‬Кыргызча മലയാളം मराठी
• Unicode is a 16 bit system
(2 x 8 bits)
• E.g 01011011 11101101
What you need to know…
1. These ways of representing
letters and characters are knows
as “Character sets”
2. Two of the most common “Character
sets” are Unicode and ASCII
3. The number of characters which
need to be represented effects
the number of bits required.
1. ASCII – 7 bits for less than 127
characters.
2. EBCDIC or Extended ASCII – 8 bits
for 256 characters
3. Unicode – 16 bits for over 65,00
characters
Explain how ASCII is
used to represent text
in a computer system [3]
• There are 3 marks therefore 3
points must be made. You could
include
– Each character is given a numeric
code
– This code is stored in binary
– Each character takes up 7 bits
– The binary code is then
translated in the CPU
– Some codes are reserved for
characters and keyboard shortcuts
State what is meant by
the term “character set”
[1]
• For one mark a simple
definition is needed
• “A character set is all the
characters which have to be
recognised by the computer
system.”
Unicode is also used to
represent text in a
computer system. Explain
the difference between
the character set of
Unicode and ASCII [2]
• You need to directly compare two
contrasting features of Unicode and ASCII
• Eg. “ACCII has 7 or 8 bits per character
whereas Unicode uses 16 bits”
• OR
• “ASCII can represent only 127 characters
whereas Unicode has a much larger
character set (around 50,000)
Scores:
• C - 3/6 on the exam questions
• B - 4/6 on the exam questions
• A/A* 5+ on the exam questions
Extension
• Correct the teacher –
identify the errors in the
passage of text.