ASCII & Unicode
Computer Science: OCR GCSE, topic 1.2.25
American Standard Code for Information Interchange
The American Standard Code for Information Interchange (ASCII) character set is the most common character set.
7-bit
- Initially, each character in ASCII was represented by a seven-bit binary code.
- That means there was a maximum of 128 characters.
- This was enough to include all commonly used letters and symbols in the English language.
- When each letter is represented by seven bits, an 8-bit system can use the extra bit as a check digit.
8-bit
- An extended ASCII uses all 8 bits.
- The additional bit allows for an extra 128 characters to be represented.
- With that extension, most Western languages can use the same character set.
- Note: In your exam, the representation of ASCII will use 8 bits.
Unicode
Unicode is a character set which was released because of the need to standardise character sets internationally.
Unicode
- Unicode aims to represent every possible character in the world.
- The most common form of Unicode is UTF-8 and uses between eight and 32 bit binary codes to represent each character.
Compatability
- The first 256 characters in Unicode are identical to extended ASCII, which makes it backwards compatible with documents encoded using older character sets.
- Characters may not be recognised or displayed correctly if the computer it is being read on is using a different character set from the computer it was created on.
Types of characters
- Unicode represents characters from all major alphabets of the world.
- Unicode is also used to represent emojis!
Logical Ordering
Both ASCII and Unicode store characters in a logical, numerical order.
ASCII - example
- Denary
- 'A' = 65
- 'B' = 66
- 'a' = 97
- 'b' = 98
- Hex
- 'A' = 41
- 'B' = 42
- 'a' = 61
- 'b' = 62
Unicode - examples
- 'A' = U+0391
- 'B' = U+0392
- 'a' = U+0061
- 'b' = U+0062
Notes
- The codes for uppercase letters are different from the codes for lowercase letters.
- The character for 'B' will be one more than the character code for 'A', and so on.
Test yourself on ASCII & Unicode
Why are the first 256 characters in Unicode identical to extended ASCII?
What characters are supported by Unicode?
Which of these can be represented using Unicode?