AQA 7517 · Paper 2 · 4.5.4a

ASCII &
Unicode

Character encoding · Text representation · Section 4.5

WHAT YOU'LL LEARN

ASCII (7-bit, 128 chars) · Unicode (UTF-8/16/32) · Why Unicode needed · Code points

AQA SPEC LINK

4.5.4 — Text: ASCII, Unicode, UTF-8, UTF-16, UTF-32

ASCII

ASCII — American Standard Code for Information Interchange

ASCII uses 7 bits to encode characters, giving 2⁷ = 128 possible characters. Originally designed for English text.

Char	Decimal	Binary
A	65	1000001
B	66	1000010
a	97	1100001
0	48	0110000
SPACE	32	0100000

Key fact: 'A' is 65, 'a' is 97 (32 difference). Digits 0–9 are 48–57.

ASCII Limitations

Why ASCII Wasn't Enough

Only 128 characters — covers basic English, digits, punctuation

No accented characters (é, ü, ñ) for other European languages

No support for Chinese, Arabic, Hindi, Japanese, Korean, etc.

No emoji — modern communication uses thousands of emoji

Extended ASCII (8-bit, 256 chars) helped but still insufficient for global use

Unicode

Unicode — Universal Character Standard

Unicode assigns a unique code point (U+XXXX) to every character in every language. Over 143,000 characters including all scripts, symbols, and emoji.

A = U+0041 (same as ASCII)
é = U+00E9
中 = U+4E2D
😊 = U+1F60A
Arabic ع = U+0639

UTF Encodings

UTF-8, UTF-16, UTF-32

UTF-8 (variable: 1–4 bytes per character)

ASCII characters use 1 byte (backward compatible). Most common on the web. Efficient for English text.

UTF-16 (2 or 4 bytes per character)

Used internally by Windows and Java. Good for CJK (Chinese/Japanese/Korean) text.

UTF-32 (fixed: 4 bytes per character)

Simple fixed-width. Uses most memory. Direct code point lookup.

UTF-8 Detail

Why UTF-8 is So Widely Used

Backward compatible with ASCII — all ASCII files are valid UTF-8

Variable width — common characters (A-Z, 0-9) use just 1 byte; rare characters use more

Space efficient for English text; over 90% of web pages use UTF-8

Self-synchronising — easy to find start of next character even if you start in the middle

Comparison

ASCII vs Unicode Comparison

Feature	ASCII	Unicode (UTF-8)
Bits per char	7 bits	8–32 bits (variable)
Characters	128	143,000+
Languages	English only	All world scripts
Emoji	No	Yes
Memory use	Less	More (for non-ASCII)

Text in Programs

Strings in Memory

Each character in a string is stored as its code point (e.g. ASCII or UTF-8 bytes)

"Hi" in ASCII: 72 (H), 105 (i) → stored as bytes 0x48 0x69

AQA pseudocode: ASC("A") returns 65; CHR(65) returns "A"

Case conversion trick: lowercase = uppercase code + 32 (e.g. 'A'=65, 'a'=97)

AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style

(a) ASCII uses 7 bits. State how many different characters ASCII can represent. [1]
(b) The ASCII code for 'R' is 82. What is the ASCII code for 'r'? Explain. [2]
(c) Give TWO reasons why Unicode was developed to replace ASCII. [2]
(d) State ONE advantage of UTF-8 over UTF-32 when storing English text. [1]

[6 marks]

1 mark

(a) 128 (2⁷)

2 marks

(b) 114. Lower case letters have codes 32 higher than their upper case equivalent (82 + 32 = 114)

2 marks

(c) ASCII only supports English / ASCII has only 128 characters, insufficient for international languages, emoji, special symbols

1 mark

(d) UTF-8 uses only 1 byte per ASCII character vs 4 bytes in UTF-32 — uses less storage for English text

Summary

Key Points to Remember

ASCII — 7-bit, 128 characters, English only. 'A'=65, 'a'=97, '0'=48

Unicode — code points for 143,000+ chars; covers all world languages + emoji

UTF-8 — variable width (1–4 bytes); backwards compatible with ASCII; most common

UTF-16 — 2 or 4 bytes; UTF-32 — fixed 4 bytes, simple but memory-heavy

Lowercase = uppercase + 32 in ASCII; use ASC() and CHR() in AQA pseudocode

🎉 Lesson complete — move to the quiz!