ASCII and Unicode: Character Encoding

Computers process only binary — every character, whether a letter, digit, or punctuation mark, must be stored as a number. Character encoding is the system that assigns a unique number (a code point) to each character, allowing software to consistently translate between binary values and readable text.

Without an agreed standard, text created on one system would display as gibberish on another. A file of English text is just a sequence of bytes; the encoding standard tells the software which character each byte represents.

Two encoding standards are required by the AQA GCSE spec:

Standard	Bits per character	Characters covered
ASCII (7-bit)	7 (stored in 8)	128 characters (0–127)
Unicode	Variable	Over 140,000 characters

A character encoding is a lookup table: a number maps to a character, and that number is what the computer stores in binary.

ASCII (American Standard Code for Information Interchange) uses 7 bits to represent each character, giving $2^7=128$ unique code points numbered 0–127.

The 128 positions are divided into three groups:

Range (denary)	Contents
0–31	Control characters — non-printable: newline, tab, backspace
32–126	Printable characters — letters, digits, punctuation, space
127	Delete (control character)

Key code points to know:

Character	ASCII code (denary)
Space	32
0	48
9	57
A	65
Z	90
a	97
z	122

In practice, ASCII is stored in 8-bit bytes with the leading bit set to 0. The standard itself is 7-bit — this distinction appears in mark schemes.

Exam questions provide an ASCII table and ask you to convert in both directions. The key reference points from the previous slide are enough to answer most questions without memorising every value.

Worked example 1 — encode Cat as ASCII code points (denary):

Character	ASCII code
C	67
a	97
t	116

Worked example 2 — decode the ASCII sequence 72, 101, 120 to text:

Code	Character
72	H
101	e
120	x

Result: Hex

ASCII stores one character per byte. A 10-character string in ASCII occupies exactly 10 bytes — no more, no less.

The ASCII table has structured ranges that allow quick calculation from first principles.

Digits ('0'–'9', codes 48–57): The character '0' = 48, '1' = 49, up to '9' = 57.

The character '5' has ASCII code 53, not 5. The digit and its code are different. To find the code: 48 + (digit value).

Uppercase letters ('A'–'Z', codes 65–90): 'A' = 65. The code for any uppercase letter = 65 + (alphabetical position − 1). 'E' is the 5th letter: 65 + 4 = 69.

Lowercase letters ('a'–'z', codes 97–122): 'a' = 97. The gap between upper and lowercase is always 32.

Character	Code	Difference
A	65	—
a	97	+32
M	77	—
m	109	+32

To convert uppercase to lowercase in code: add 32. To convert lowercase to uppercase: subtract 32.

ASCII was designed for English-language computing. Its 128 characters cover the Latin alphabet, Arabic numerals, and common punctuation — sufficient for American English but inadequate for almost everything else.

Characters ASCII cannot represent:

• Accented letters: é, ñ, ü, ç
• Non-Latin scripts: Arabic (ع), Mandarin (文), Hindi (अ), Korean (한)
• Mathematical symbols: ∑, π, ≠, ∞
• Currency symbols: €, £, ¥ (£ exists in extended ASCII, but not the core 7-bit set)

As computing became global, different regions created incompatible extensions. The same byte value (e.g. 130) represented different characters in different extended ASCII variants, causing encoding conflicts when files were transferred between systems.

A single, universal standard was needed — one large enough to cover every writing system in regular use.

Unicode assigns a unique code point to every character across all the world's writing systems. The current standard defines over 140,000 characters spanning 150+ scripts, plus emoji, mathematical symbols, and historical scripts.

Key facts for GCSE:

• Unicode uses the same code points as ASCII for characters 0–127. This is deliberate backward compatibility — any software that reads ASCII can read the first 128 Unicode characters without modification.
• Unicode requires more bits than 7-bit ASCII to represent its full range of characters. This means Unicode files can be larger than ASCII files when they contain characters outside the 0–127 range.

Property	ASCII	Unicode
Characters covered	128	140,000+
Bit depth	7 bits per character	More bits than ASCII
Compatible with ASCII?	—	Yes — codes 0–127 are identical
Used for	English-language systems	Multilingual, global use

The GCSE requirement is: Unicode covers far more characters than ASCII; Unicode uses the same codes as ASCII for values 0–127; Unicode requires more bits per character. You do not need to know specific encoding names such as UTF-8 or UTF-16 for AQA 8525.

1. Confusing a digit's value with its ASCII code

The character '7' has ASCII code 55 (= 48 + 7), not 7. Any question asking for the ASCII code of a digit requires the 48 offset.

2. Stating ASCII uses 8 bits per character

ASCII is a 7-bit standard. It is stored in an 8-bit byte with a leading zero, but the encoding itself is 7-bit. Mark schemes distinguish between these.

3. Thinking Unicode replaces ASCII code points 0–127

Unicode was designed to extend ASCII, not replace it. Code points 0–127 are identical in both standards.

4. Claiming Unicode always uses more storage

Unicode uses the same codes as ASCII for characters 0–127, and in those cases uses no more storage than ASCII. Unicode files are only larger when they contain characters outside the ASCII range, because those characters require more bits to represent.

5. Not using the table provided in the exam

Almost every GCSE paper that tests character encoding supplies the ASCII table. Use it — do not attempt to recall every code point. The marks are for applying the table correctly, not for memorisation.