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CAIE 9618 · Paper 1 · Topic 1.1.4

Character
Encoding

ASCII · Extended ASCII · Unicode · UTF-8 · UTF-16 · UTF-32

CSZone Cambridge International AS & A Level Computer Science 9618
Learning Objectives

By the end of this video you will be able to...

Describe ASCII and explain its limitations
Explain why Unicode was introduced and its advantages over ASCII
Distinguish between UTF-8, UTF-16, and UTF-32 encoding schemes
Calculate the number of bits used to represent text in different encodings
ASCII

American Standard Code for Information Interchange

STANDARD ASCII (7-BIT)
Uses 7 bits per character
Represents 128 characters (2⁷)
Includes: uppercase A–Z, lowercase a–z, digits 0–9, punctuation, control characters
EXTENDED ASCII (8-BIT)
Uses 8 bits per character
Represents 256 characters (2⁸)
Extra 128 characters for accented letters, special symbols
Example: 'A' = 65₁₀ = 0100 0001 · 'a' = 97₁₀ · '0' = 48₁₀
Unicode

A Universal Character Standard

"Unicode assigns a unique code point to every character in every written language — over 1.1 million possible code points, covering 149,000+ characters."
Supports every world language: Arabic, Chinese, Tamil, Devanagari, emoji, mathematical symbols...
Backward compatible — the first 128 Unicode code points are identical to ASCII
The Unicode Consortium maintains and expands the standard
Unicode Encoding Schemes

UTF-8 vs UTF-16 vs UTF-32

FormatBits per characterRange coveredBest for
UTF-8 8–32 bits (variable) Full Unicode Web content, ASCII-heavy text
UTF-16 16 or 32 bits (variable) Full Unicode Windows OS, Java, many APIs
UTF-32 32 bits (fixed) Full Unicode Internal processing, databases
UTF-8 key feature: ASCII characters use only 1 byte (8 bits), making files backward compatible and storage-efficient for English text
ASCII vs Unicode

Why Unicode Replaced ASCII

ASCII LIMITATIONS
Only 128/256 characters — cannot represent most languages
No standard for non-Latin scripts (Chinese, Arabic, etc.)
Different extended ASCII tables caused incompatibility
UNICODE ADVANTAGES
Over 1 million code points — covers all world languages
Single universal standard — no compatibility issues
Supports emoji, mathematical notation, historical scripts
⚠ Disadvantage: Unicode files can use more storage than ASCII (especially UTF-16 and UTF-32)
Exam Practice

Cambridge-style questions

Question 1
Give two reasons why Unicode was introduced to replace ASCII.
2 marks
✓ MARK SCHEME (any two)
1 mark
ASCII cannot represent characters from all languages/scripts
1 mark
ASCII has a limited character set (only 128/256 characters)
1 mark
Different extended ASCII versions caused incompatibility between systems
1 mark
Unicode provides a universal/global standard for all characters
Exam Practice
Question 2
A document contains 500 characters, all within the ASCII range. Calculate the storage required in bytes for this document using (a) ASCII and (b) UTF-32.
3 marks
✓ MARK SCHEME
1 mark
(a) ASCII: 500 × 1 byte = 500 bytes
1 mark
(b) UTF-32: 500 × 4 bytes = 2000 bytes
1 mark
UTF-32 uses 4 times more storage (fixed 32 bits per character)
Common Mistakes

Don't lose easy marks

1
Saying ASCII uses 8 bits — standard ASCII uses 7 bits (128 characters). Extended ASCII uses 8 bits. Know which is being asked about.
2
Confusing code point with encoding — a Unicode code point (like U+0041 for 'A') is the abstract number. UTF-8/16/32 are the physical ways that code point is stored in bits.
3
Saying UTF-8 always uses 8 bits — UTF-8 is variable width: common characters use 1 byte, rare characters use up to 4 bytes. Only UTF-32 is fixed at 32 bits.
4
Not giving the storage disadvantage of Unicode when asked to compare — always mention that Unicode files can be larger than ASCII files, particularly with UTF-16 and UTF-32.
Topic Summary — 1.1.4

What You Need to Know

ASCII
Standard: 7 bits, 128 characters
Extended: 8 bits, 256 characters
'A'=65, 'a'=97, '0'=48
Limited to mostly English/Latin characters
ASCII LIMITATIONS
Cannot represent all world languages
Incompatible extended ASCII versions
No support for emoji or special symbols
UNICODE & UTF
1.1M+ code points, all world languages
UTF-8: variable (1–4 bytes), best for web
UTF-16: variable (2–4 bytes), Windows/Java
UTF-32: fixed 4 bytes, simple but large
Disadvantage: more storage than ASCII
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Image Representation
Pixels · Resolution · Colour Depth · File Size
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