3.2b Representing text — Edexcel 4CP0

Character Encoding

Text is represented in binary using a character encoding system — a standard that assigns a unique binary code to every character (letters, digits, punctuation, symbols). Each character has a unique code point (numeric value).

ASCII

ASCII (American Standard Code for Information Interchange) uses 7 bits to represent each character, giving 2⁷ = 128 possible characters (code points 0–127). An extended 8-bit version adds a further 128 characters (code points 128–255).

Character	ASCII Code (denary)	Binary
'A'	65	1000001
'B'	66	1000010
'a'	97	1100001
'0'	48	0110000
' ' (space)	32	0100000

Note: uppercase letters have smaller code values than lowercase. 'A'=65, 'Z'=90, 'a'=97, 'z'=122.

Limitations of ASCII

Only 128 characters — cannot represent accented characters (é, ü, ñ), non-Latin scripts (Arabic, Chinese, Hindi), emoji
Designed primarily for English-language computing
Insufficient for a globally connected world

Unicode

Unicode is a universal character encoding standard designed to represent every character in every language in the world. It includes over 1 million possible code points.

Common Unicode encoding formats:

UTF-8: Uses 1–4 bytes per character; backward-compatible with ASCII; most widely used on the web
UTF-16: Uses 2 or 4 bytes per character; common in operating systems
UTF-32: Uses 4 bytes for every character; simpler but uses more memory

ASCII vs Unicode

Feature	ASCII	Unicode (UTF-8)
Bits per character	7 (or 8 extended)	8–32 (variable)
Number of characters	128 (or 256)	Over 1 million
Languages supported	English primarily	All world languages
File size	Smaller (1 byte per char)	Larger (variable bytes)
Backward compatible?	—	Yes (first 128 = ASCII)

📝 Exam Tip: You may be asked to use an ASCII table to convert characters to binary and back. Remember: 'A'=65, 'a'=97. The difference between uppercase and lowercase is exactly 32 (or bit 5 being set). This is a useful pattern to know.

⚠️ Common Mistakes

Saying ASCII uses 8 bits — standard ASCII uses 7 bits (128 codes); 8-bit extended ASCII is a variant
Confusing the character '0' with the number 0 — the character '0' has ASCII code 48, not 0
Saying Unicode "replaces" ASCII — UTF-8 Unicode is backward-compatible with ASCII