Representing Characters (AQA GCSE Computer Science)
Revision Note
Character Sets
Computers represent all data in binary, including characters that are input using a keyboard
1 binary digit (bit) would allow us to represent only two possible characters, for example 1=A and 0=B
Using more bits allows more characters to be represented, 2 bits or 22 = 4 and so on
What is a character set?
A character set is a defined list of characters that can be understood by a computer
Each character is given a unique binary code
Character sets are ordered logically, the code for ‘B’ is one more than the code for ‘A’
A character set provides a standard for computers to communicate and send/receive information
Without a character set, one system might interpret 01000001 differently from another
The number of characters that can be represented is determined by the number of bits used by the character set
Two common character sets are:
American Standard Code for Information Interchange (ASCII)
Universal Character Encoding (UNICODE)
ASCII
What is ASCII?
ASCII is a character set and was an accepted standard for information interchange
ASCII uses 7 bits, providing 27 unique codes (128) or a maximum of 128 characters it can represent
ASCII only represents basic characters needed for English, limiting its use for other languages
Limitations of ASCII
ASCII has a limited number of characters which means it can only represent the English alphabet, numbers and some special characters
A, B, C, ………, Z
a, b, c ,.............,z
0, 1, 2,........, 9
!, @, #, …..
ASCII cannot represent characters from languages other than English
ASCII does not include modern symbols or emojis common in today's digital communication
UNICODE
What is UNICODE?
UNICODE is a character set and was created as a solution to the limitations of ASCII
UNICODE uses a minimum of 16 bits, providing 216 unique codes (65,536) or a minimum of 65,536 characters it can represent
UNICODE can represent characters from all the major languages around the world
Examiner Tip
Exam questions often ask you to compare ASCII & UNICODE, for example the number of bits, number of characters and what they store
ASCII vs UNICODE
| ASCII | UNICODE |
|---|---|---|
Number of bits | 7-bits | 16-bits |
Number of characters | 128 characters | 65,536 characters |
Uses | Used to represent characters in the English language. | Used to represent characters across the world. |
Benefits | It uses a lot less storage space than UNICODE. | It can represent more characters than ASCII. It can support all common characters across the world. It can represent special characters such as emoji's. |
Drawbacks | It can only represent 128 characters. It cannot store special characters such as emoji's. | It uses a lot more storage space than ASCII. |
Worked Example
The computer stores text using the ASCII character set.
Part of the ASCII character set is shown:
Character | ASCII Denary Code |
E | 69 |
F | 70 |
G | 71 |
H | 72 |
Identify the character that will be represented by the ASCII denary code 76 [1]
L (must be a capital)
Identify a second character set [1]
UNICODE
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