CS19: Character Encoding
How computers represent text using ASCII and Unicode character encoding, key codes to remember, and converting between characters and codes.
📋 What is Character Encoding?
Character encoding is a system that assigns a unique numeric code to each character. Computers can only process numbers, so every letter, digit, and symbol must be represented by a number. The two main encoding systems are ASCII and Unicode.
Without character encoding, the binary pattern 01000001 could mean anything. Encoding standards ensure that 01000001 is always interpreted as the letter 'A'.
🔠 ASCII
ASCII (American Standard Code for Information Interchange) is a 7-bit character encoding system that represents 128 different characters. It was designed for English-language text and includes letters, digits, punctuation, and control characters.
ASCII Key Facts:
Uses 7 bits per character
Can represent 2^7 = 128 different characters
Characters 0-31: Control characters (non-printable)
Characters 32-126: Printable characters
Character 127: DEL (delete)
In practice, ASCII is stored in 1 byte (8 bits) with the leading bit set to 0.
Uses 7 bits per character
Can represent 2^7 = 128 different characters
Characters 0-31: Control characters (non-printable)
Characters 32-126: Printable characters
Character 127: DEL (delete)
In practice, ASCII is stored in 1 byte (8 bits) with the leading bit set to 0.
Essential ASCII Codes to Remember
| Character | ASCII Code (Decimal) | Binary |
|---|---|---|
| 'A' | 65 | 01000001 |
| 'B' | 66 | 01000010 |
| 'Z' | 90 | 01011010 |
| 'a' | 97 | 01100001 |
| 'b' | 98 | 01100010 |
| 'z' | 122 | 01111010 |
| '0' | 48 | 00110000 |
| '1' | 49 | 00110001 |
| '9' | 57 | 00111001 |
| ' ' (space) | 32 | 00100000 |
Pattern to remember:
Uppercase letters: 'A' = 65 to 'Z' = 90 (26 letters)
Lowercase letters: 'a' = 97 to 'z' = 122 (26 letters)
Digits: '0' = 48 to '9' = 57 (10 digits)
The difference between 'a' and 'A' is always 32 (add 32 to convert upper to lower)
Uppercase letters: 'A' = 65 to 'Z' = 90 (26 letters)
Lowercase letters: 'a' = 97 to 'z' = 122 (26 letters)
Digits: '0' = 48 to '9' = 57 (10 digits)
The difference between 'a' and 'A' is always 32 (add 32 to convert upper to lower)
Finding Character Codes
What is the ASCII code for 'D'?
'A' = 65, 'B' = 66, 'C' = 67, 'D' = 68
What is the ASCII code for 'f'?
'a' = 97, 'b' = 98, 'c' = 99, 'd' = 100, 'e' = 101, 'f' = 102
Converting Characters to Codes in Pseudo-code
code ← ASC('A') // code = 65
char ← CHR(65) // char = 'A'
// Convert uppercase to lowercase
upper ← 'G'
lower ← CHR(ASC(upper) + 32) // lower = 'g'
ASCII Character Groups
| Code Range | Characters | Count |
|---|---|---|
| 0-31 | Control codes (non-printable: newline, tab, bell) | 32 |
| 32-47 | Symbols and space | 16 |
| 48-57 | Digits 0-9 | 10 |
| 65-90 | Uppercase letters A-Z | 26 |
| 97-122 | Lowercase letters a-z | 26 |
🌍 Unicode
Unicode is a character encoding standard that supports characters from all languages in the world, plus symbols, emojis, and historical scripts. It can represent over 1 million different characters.
Unicode Key Facts:
Supports virtually every written language
Includes emojis, mathematical symbols, and technical symbols
Commonly stored using UTF-8 (variable length: 1-4 bytes)
Or UTF-16 (2 or 4 bytes per character)
The first 128 Unicode characters are the same as ASCII
Supports virtually every written language
Includes emojis, mathematical symbols, and technical symbols
Commonly stored using UTF-8 (variable length: 1-4 bytes)
Or UTF-16 (2 or 4 bytes per character)
The first 128 Unicode characters are the same as ASCII
Why Unicode Was Created
- ASCII only supports 128 characters - enough for English but not for other languages
- Extended ASCII (8-bit) added 128 more characters but still couldn't cover all languages
- Different countries used different extended ASCII sets, causing compatibility problems
- Unicode was created as a universal standard to encode all the world's writing systems
Unicode Examples Beyond ASCII
| Character | Description | Unicode Code Point |
|---|---|---|
| e-acute | Latin small letter e with acute | U+00E9 |
| n-tilde | Latin small letter n with tilde | U+00F1 |
| Chinese "middle" | Chinese character | U+4E2D |
| Grinning face | Emoji | U+1F600 |
| Pound sign | Currency symbol | U+00A3 |
| Euro sign | Currency symbol | U+20AC |
⚖️ ASCII vs Unicode Comparison
| Feature | ASCII | Unicode |
|---|---|---|
| Number of characters | 128 | Over 1 million |
| Bits per character | 7 bits (stored in 1 byte) | Variable (8-32 bits depending on encoding) |
| Languages supported | English only | All world languages |
| Emojis supported | No | Yes |
| File size | Smaller (1 byte per character) | Larger (2+ bytes for non-ASCII chars) |
| Compatibility | Universal for English | Backwards compatible with ASCII |
| When to use | English-only text, small file size needed | Multi-language text, modern applications |
✅ Advantages of Unicode Over ASCII
Unicode is the modern standard because it solves ASCII's biggest limitation: only supporting English. With Unicode, any text in any language can be represented, stored, and transmitted correctly.
- Universal support: Represents characters from every language, not just English
- Consistency: One standard worldwide, eliminating incompatible regional encodings
- Emojis and symbols: Supports modern communication needs
- Backwards compatible: The first 128 codes are identical to ASCII
- Future-proof: Can be extended to include new characters as needed
Unicode Backwards Compatibility
Unicode code points 0-127 are the same as ASCII:
- Unicode U+0041 = 'A' (same as ASCII 65)
- Unicode U+0061 = 'a' (same as ASCII 97)
- Unicode U+0030 = '0' (same as ASCII 48)
This means any valid ASCII text is also valid Unicode text.
Trade-off: Unicode uses more storage space than ASCII. A plain English text file in Unicode (UTF-16) uses roughly twice the space of the same file in ASCII. However, with modern storage being cheap, this trade-off is usually acceptable.
🔄 Converting Characters to Codes and Back
ASC() and CHR() are the functions used to convert between characters and their numeric codes. ASC converts a character to its code; CHR converts a code to its character.
Character-Code Conversion Examples
// Character to code
OUTPUT ASC('A') // 65
OUTPUT ASC('a') // 97
OUTPUT ASC('0') // 48
OUTPUT ASC(' ') // 32
// Code to character
OUTPUT CHR(65) // A
OUTPUT CHR(97) // a
OUTPUT CHR(48) // 0
Practical Application: Checking if Input is a Letter
INPUT "Enter a character: ", char
code ← ASC(char)
IF (code >= 65 AND code <= 90) OR (code >= 97 AND code <= 122) THEN
OUTPUT char, " is a letter"
ELSE
OUTPUT char, " is not a letter"
ENDIF
Converting a Digit Character to Its Numeric Value
// The character '7' has ASCII code 55, not value 7
digitChar ← '7'
digitValue ← ASC(digitChar) - ASC('0') // 55 - 48 = 7
OUTPUT digitValue // 7 (the number, not the character)
⚠️ Common Mistakes
| Mistake | Why It's Wrong | How to Fix It |
|---|---|---|
| Thinking ASCII can store any character | ASCII only has 128 characters - no foreign languages or emojis | Use Unicode for non-English characters |
| Confusing '5' and 5 | The character '5' has code 53, not value 5 | Use ASC('5') - 48 to get the numeric value |
| Forgetting the +32 offset | 'a' is not code 65 + 1, it is 65 + 32 = 97 | Remember: lowercase = uppercase code + 32 |
| Saying Unicode replaces ASCII | Unicode includes ASCII (first 128 codes are the same) | Unicode extends ASCII - it is backwards compatible |
| Not considering storage size | Unicode can use more bytes per character | Be aware of file size implications |
❓ Practice Questions
Q1: What is the ASCII code for the letter 'M'?
Q2: How many characters can 7-bit ASCII represent?
Q3: Give two advantages of Unicode over ASCII.
Q4: Write pseudo-code that converts a lowercase letter to uppercase.
Q5: Explain why the character '3' and the integer 3 are stored differently in a computer.
✅ Answers
- 'A' = 65, so 'M' is the 13th letter: 65 + 12 = 77.
- 7-bit ASCII can represent 2^7 = 128 different characters.
- 1) Unicode supports all world languages while ASCII only supports English. 2) Unicode includes emojis and special symbols that ASCII cannot represent. Also acceptable: backwards compatible with ASCII, one universal standard eliminating regional encodings.
-
INPUT letter code ← ASC(letter) IF code >= 97 AND code <= 122 THEN upper ← CHR(code - 32) OUTPUT upper ELSE OUTPUT "Already uppercase or not a letter" ENDIF - The character '3' is stored as its ASCII code 51 (binary 00110011) and takes 1 byte. The integer 3 is stored as the binary number 00000011 (just the value 3). They look completely different in binary and are processed differently by the computer.
🎯 Exam Tips
- Remember the key ASCII codes: A=65, a=97, 0=48, space=32
- Lowercase = uppercase code + 32
- ASCII = 7 bits = 128 characters; Unicode = millions of characters
- Unicode is backwards compatible with ASCII (first 128 codes match)
- ASC() converts character to code; CHR() converts code to character
- Character '5' is NOT the same as integer 5 - they have different binary representations
⚠️ Common Errors
✗ Thinking ASCII and Unicode store the same number of characters ✓ ASCII uses 7 bits (128 characters) or 8 bits (Extended ASCII, 256 characters). Unicode uses up to 32 bits and can represent over 143,000 characters from all world languages.
✗ Believing Unicode always uses 4 bytes per character ✓ Unicode uses variable-length encoding: UTF-8 uses 1-4 bytes per character. Common ASCII characters still use just 1 byte in UTF-8, making it backward-compatible.
✗ Forgetting that ASCII cannot represent non-English characters ✓ ASCII only covers English letters, digits, and basic symbols. It cannot represent characters from other languages (Chinese, Arabic, etc.) — this is why Unicode was developed.
✗ Confusing character code with the character itself ✓ A character code (e.g. 65 in ASCII) is the numeric representation stored in memory. The character 'A' is how it is displayed. The computer stores 65, not 'A'.
✍️ Model Answer
Full-Mark Response
Explain why Unicode was developed as an alternative to ASCII, and discuss one advantage and one disadvantage of using Unicode. [4 marks]
Unicode was developed because ASCII could only represent 128 characters (or 256 in Extended ASCII), which is insufficient for the world's languages, symbols, and emoji. ASCII only covers English alphabet characters, digits, and basic punctuation. Advantage: Unicode supports over 143,000 characters from all writing systems worldwide, enabling global communication and multilingual documents in a single encoding standard. Disadvantage: Unicode requires more storage space per character than ASCII — up to 4 bytes compared to ASCII's 1 byte. This means text files are larger when using Unicode, though UTF-8 mitigates this by using 1 byte for ASCII characters.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Computational Thinking — 40%): Demonstrate knowledge and understanding of the principles and concepts of computer science, including character encoding: ASCII and Unicode for AQA 8525, OCR J277 & Edexcel 1CP2.
AO2 (Application — 40%): Apply knowledge and understanding of computer science, including character encoding: ASCII and Unicode to analyse problems in computational terms and to design, write and evaluate solutions.
AO3 (Evaluation — 20%): Evaluate the effectiveness, correctness and efficiency of computational solutions, including character encoding: ASCII and Unicode, and make reasoned judgements about trade-offs.
📝 Exam Technique
GCSE Computer Science Exam Tips:
ASCII: 7-bit (128 chars) or 8-bit Extended (256 chars). Unicode: variable length, 143,000+ chars. Know why Unicode replaced ASCII (international support). ASCII advantage: small file size. Unicode advantage: universal character support. UTF-8 is backward-compatible with ASCII. When comparing, always discuss storage size vs character support trade-off.
ASCII: 7-bit (128 chars) or 8-bit Extended (256 chars). Unicode: variable length, 143,000+ chars. Know why Unicode replaced ASCII (international support). ASCII advantage: small file size. Unicode advantage: universal character support. UTF-8 is backward-compatible with ASCII. When comparing, always discuss storage size vs character support trade-off.
📝 Exam Questions by Topic
- AQA Past Papers - Topic Questions -- AQA
- Edexcel Past Papers - Topic Questions -- Edexcel
- OCR Past Papers - Topic Questions -- OCR
- Eduqas Past Papers - Topic Questions -- Eduqas
🎬 Video Resources
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