Character encoding
Computers store everything as numbers, including text. A character set is an agreed mapping between characters and integer codes. The two you must know are ASCII and Unicode.
ASCII (American Standard Code for Information Interchange)
ASCII uses 7 bits, allowing 2⁷ = 128 distinct characters. The 8th bit is typically padded with 0 to fit a byte. Codes 0-31 are control characters (newline, tab, escape); 32-126 are printable; 127 is DEL.
Memorise these milestones:
| Character | ASCII |
|---|---|
| Space | 32 |
| '0' | 48 |
| '9' | 57 |
| 'A' | 65 |
| 'Z' | 90 |
| 'a' | 97 |
| 'z' | 122 |
The gap of 32 between an uppercase and lowercase letter is exam gold — you can convert between cases with a single subtraction.
Extended ASCII
Some systems use the 8th bit too, giving 256 codes. The extra 128 codes were used differently in different "code pages" (Western European, Cyrillic, etc.) — confusing and incompatible. Replaced by Unicode for any serious internationalisation.
Unicode
Unicode is a much bigger character set covering every writing system in the world — Latin, Greek, Cyrillic, Arabic, CJK, emoji, mathematical symbols. The most common encoding is UTF-8:
- 1 byte for ASCII characters (compatible).
- 2-4 bytes for other characters.
Other Unicode encodings exist (UTF-16, UTF-32) but UTF-8 dominates the web.
For GCSE you mainly need the principles:
- Unicode supports many more characters than ASCII (over a million code points in theory).
- Each character may take more than one byte to store.
- More storage is the trade-off for international support.
Comparing ASCII and Unicode
| Property | ASCII | Unicode (UTF-8) |
|---|---|---|
| Bits per character | 7 (8 with pad) | Variable: 8 to 32 |
| Distinct characters | 128 | ~1.1 million |
| Languages | English/US | All major writing systems |
| Storage cost (English text) | 1 B/char | 1 B/char |
| Storage cost (Chinese, emoji) | impossible | 3-4 B/char |
✦Worked example— Worked example — ASCII arithmetic
Convert the lowercase letter 'h' to uppercase using ASCII codes. ASC('h') = ASC('a') + 7 = 97 + 7 = 104. Subtract 32: 104 - 32 = 72. CHR(72) = 'H'. ✓
✦Worked example— Worked example — encoding a word
Encode the word "Hi" in ASCII.
- 'H' → 72 → 01001000
- 'i' → 105 → 01101001 Concatenated: 01001000 01101001 (2 bytes total).
✦Worked example— Worked example — Unicode storage
A 100-character English document in UTF-8 takes ≈ 100 bytes. The same document with 100 Chinese characters takes 100 × 3 = ≈ 300 bytes. ASCII could store the English version equally cheaply but could not store the Chinese version at all.
⚠Common mistakes— Pitfalls
- Forgetting case difference is 32. Critical for case-conversion questions.
- Confusing characters with digits. ASC('5') = 53, not 5. Subtract 48 to convert digit characters to integers.
- Treating ASCII as 8-bit. It's 7-bit. The 8th bit is a pad or used for parity in some systems.
- Assuming one Unicode character = one byte. UTF-8 uses variable-length encoding.
- Mixing up character and code value. 'A' is the character; 65 is the code.
Why does ASCII still matter?
It's the foundation. The first 128 Unicode code points are identical to ASCII. So plain English text is interpreted identically by any modern system. You can almost always assume A=65, '0'=48 without checking.
➜Try this— Quick check
What is the ASCII code for 'M'? Hint: 'A' = 65 and M is the 13th letter (12 places after A). 65 + 12 = 77.
What is the binary representation of 'M' as a 7-bit code? 77 = 64 + 8 + 4 + 1 = 1001101.
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