objectives
By the end you'll be able to…
- Explain what a character set is and why we need one.
- Describe ASCII and Extended ASCII: how many bits, how many characters, what it can/can't store.
- Describe Unicode and why it was created.
- Compare ASCII and Unicode on bits, characters, uses, benefits and drawbacks.
character sets
What is a character set?
Computers can only store binary. So every letter you type on a keyboard has to be converted into a unique binary number first.
A character set is a defined list of characters, where every character has been given a unique binary code. Without a character set, one computer might store 01000001 and another might read it as something completely different.
- Each character has a unique binary code.
- Codes are ordered logically — the code for 'B' is one more than the code for 'A'.
- A character set is a standard so systems can share text reliably.
- The number of characters a set can store depends on the number of bits used: n bits → 2n characters.
Two character sets you must know at GCSE:
- ASCII — American Standard Code for Information Interchange
- Unicode — Universal Character Encoding
ASCII
ASCII
ASCII was one of the earliest character sets and is still widely used. It uses 7 bits per character, giving 27 = 128 possible characters.
What ASCII can store
- A–Z (uppercase), a–z (lowercase)
- 0–9 (digits)
- Common punctuation:
!, @, #, $, %, &, ?, . - Non-printable control codes (e.g. tab, newline)
Extended ASCII
Extended ASCII uses 8 bits per character (28 = 256 characters). This adds room for maths symbols and characters like ©.
The exam expects 8-bit Extended ASCII for all binary representations of characters. So if you're asked to write 'A' in binary, use 01000001 (8 bits), not 1000001 (7 bits).
The letters A–Z in ASCII
| Letter | ASCII | Binary | Letter | ASCII | Binary |
|---|---|---|---|---|---|
| A | 65 | 01000001 | a | 97 | 01100001 |
| B | 66 | 01000010 | b | 98 | 01100010 |
| C | 67 | 01000011 | c | 99 | 01100011 |
| D | 68 | 01000100 | d | 100 | 01100100 |
| E | 69 | 01000101 | e | 101 | 01100101 |
| … | … | … | … | … | … |
| Z | 90 | 01011010 | z | 122 | 01111010 |
Just remember A = 65 and a = 97. Every other letter is one more (B = 66, C = 67…). The difference between an uppercase and lowercase letter is always 32.
explorer
🖱️ ASCII explorer
Play with the two tools below to see how any character becomes binary — and how any binary number becomes a character.
Character → ASCII
Denary → Character
The computer stores text using ASCII. Part of the set is: E=69, F=70, G=71, H=72. Identify the character represented by 76.
Counting up: 72=H, 73=I, 74=J, 75=K, 76=L ✅ (must be capital — lowercase would be much higher).
Unicode
Unicode
ASCII was designed for English. It can't represent Greek, Arabic, Chinese, Japanese, emoji, or thousands of modern symbols. Unicode was created to fix that.
- Unicode uses a minimum of 16 bits per character.
- 216 = 65,536 unique codes.
- Can represent characters from all major languages around the world.
- Can store special symbols like emoji 😀.
Unicode is a superset — the first 128 codes match ASCII exactly. So Unicode is backwards compatible with ASCII.
compare
ASCII vs Unicode
This exact comparison shows up in the exam almost every year.
| ASCII | Unicode | |
|---|---|---|
| Number of bits | 7 bits (Extended: 8) | 16 bits (minimum) |
| Number of characters | 128 (256 extended) | 65,536 |
| Uses | English text | All world languages + emoji |
| Benefit | Uses less storage | Represents all common characters |
| Drawback | Only 128 characters, no emoji | Uses more storage per character |
Flip cards
practice
Exam-style practice
Q1. A computer stores text using ASCII. Part of the set: E=69, F=70, G=71, H=72. Identify the character with code 75.
Q2. Identify a character set other than ASCII.
Q3. Give one benefit and one drawback of Unicode compared to ASCII.
Q4. A text file contains 500 characters using Extended ASCII. What is the file size in bytes?
// end of topic 4 — characters