Represent text with ASCII and Unicode
Explain how character sets map text to binary and compare the range and storage implications of ASCII and Unicode.
Exam scope and assessment notes
This sequence follows Cambridge IGCSE Computer Science 0984, 2026–2028 syllabus Version 4. It covers subject content sections 1.1–1.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding. Cambridge states that 0984 has the same 2026–2028 subject content as 0478, with 9–1 grading. In June and November, Cambridge lists 0984 in Administrative zone 3 and for centres in Oman and the UAE. Confirm availability for your centre and exam series with Cambridge International. The separately published 2029–2031 syllabus changes to Python 3 only and removes pseudocode; this 2026–2028 route does not cover those changes.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a character set do?
What you will learn
- Explain that characters are stored as binary codes
- Compare ASCII and Unicode character ranges
- Relate character-set size to bits used per character
A character set assigns codes
A character set maps each supported character to a number, which is stored in binary. ASCII represents a limited set of mainly English letters, digits and punctuation.
Apply and check the rule
Unicode represents a much wider range, including characters used in many languages and symbols such as emoji. Supporting a larger set requires more bits overall than a small ASCII set; the exact storage depends on the encoding used.
Pause and explain
Which is the best reason to choose Unicode for multilingual text?
Worked example
A school wants to store Vietnamese text and emoji in messages. Why might ASCII be insufficient?
Show the worked solution
- ASCII contains a relatively small character repertoire.
- Vietnamese characters and emoji include symbols outside that limited set.
- Unicode provides codes for a much wider range of characters.
Answer ASCII is too limited for many international characters and emoji; Unicode supports a wider repertoire, using more bits overall than a small ASCII set.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why a computer stores a letter such as A as a binary value.
Give me a hint
Separate the visible glyph from its stored code.
Compare my reasoning
- A character set assigns A a code value.
- The value is converted to binary for storage and processing.
- Software uses the same character set to display the character.
The character set maps A to a code; the code is stored and processed in binary and later interpreted to display A.
Look for these in your work
- Code mapping stated
- Binary storage explained
- Interpretation connected
A text file contains letters from several writing systems. Compare ASCII with Unicode for this file.
Give me a hint
Focus on character range and storage.
Compare my reasoning
- ASCII supports a limited repertoire, mostly basic English characters.
- Unicode supports a much wider range of writing systems and symbols.
- Unicode typically needs more bits overall to encode that wider range.
Unicode is more suitable because it supports the required characters; its wider repertoire has a storage cost compared with a small ASCII character set.
Look for these in your work
- ASCII limit identified
- Unicode range identified
- Storage trade-off stated
An engineer proposes replacing Unicode with ASCII to reduce storage in a multilingual messaging app. Give one benefit and one serious drawback.
Give me a hint
Evaluate the storage trade-off against required characters.
Compare my reasoning
- ASCII can use a smaller repertoire and may reduce storage for supported characters.
- It cannot represent many characters required by multilingual users.
- The storage saving is not useful if the text is corrupted or replaced.
ASCII may reduce storage for its limited repertoire, but it cannot represent many multilingual characters and symbols. Unicode is needed to preserve the intended text.
Look for these in your work
- Benefit given
- Coverage drawback given
- User impact explained
Common mistakes
- Saying ASCII and Unicode are compression methods
- Claiming every Unicode character always takes exactly the same number of bits
What can you explain now?
Why can Unicode store more kinds of character than ASCII?
Compare with the explanation
It provides a larger character repertoire
A wider range needs more possible code values and more bits overall.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why does a wider character repertoire generally require more bits than a small one?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.