AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Understand what a character set is and be able to describe the following…
Full specification wording
Character encoding: Understand what a character set is and be able to describe the following character encoding methods:
A character set is the agreed list of characters a computer can represent, with each character mapped to a unique binary code.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 1 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
A character set is the agreed list of characters a computer can represent, with each character mapped to a unique binary code. The encoding method is the rule that turns a character into that code and back again. Learners must be able to describe the methods the specification names. For example, ASCII uses 7-bit codes covering 128 characters, enough for English letters, digits and common punctuation, while Extended ASCII uses 8 bits for 256 codes. Unicode uses more bits and assigns a code point to characters from many writing systems, so it can represent far more symbols than ASCII. A description should state the bit width, the number of characters representable and the purpose, then explain that a program must use the same encoding to decode text correctly.
What earns the marks
- Defines a character set as the collection of characters that a system can represent, each linked to a unique binary code.
- States that the encoding method is the agreed rule mapping each character to its binary code and back.
- Describes ASCII as using 7-bit codes giving 128 possible characters, sufficient for basic English text and punctuation.
- Describes Extended ASCII as using 8 bits giving 256 codes, adding further symbols beyond standard ASCII.
- Describes Unicode as using a larger code space with code points covering characters from many languages and symbol sets.
- Explains that text encoded one way must be decoded with the same character set, otherwise characters are misread.
Common mistakes and how to put them right
Confusing a character set with a font or keyboard layout.
Put it right by explaining it is the list of representable characters and their binary codes.
Claiming ASCII uses 8 bits for 128 characters.
Put it right by stating standard ASCII is 7-bit with 128 codes, while Extended ASCII is 8-bit with 256.
Saying Unicode is simply 'more ASCII' without purpose.
Put it right by explaining it assigns code points across many writing systems and symbol sets.
Exam tips
- Learn the bit width and character count for ASCII and Extended ASCII, since these are frequently tested facts.
- When asked to describe a method, give the bit width, the number of characters and the purpose in one sentence.
- Link encoding to decoding: state that the same character set must be used to interpret the stored binary correctly.
Try the lesson
The start of the Character encoding: Understand what a character set is and be able to describe the following character encoding methods: lesson, made from checked teaching notes for this specification point.
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Which description best explains what a character set is?
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