ASCII — Text, Sound and Images | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

ASCII/Unicode, sound sampling and pixel/colour depth appear regularly as 2 to 4 mark questions.

ASCII (American Standard Code for Information Interchange) was the first widely-used character set, dating from the 1960s.

Feature	Standard ASCII	Extended ASCII
Bits per character	7 bits	8 bits
Number of characters	2⁷ = 128	2⁸ = 256
Covers	English letters (A-Z and a-z), digits 0-9, basic punctuation, control codes	Standard ASCII plus extra symbols like ©, ±, accented letters

Limitations of ASCII

Even extended ASCII has serious limits:

Cannot represent characters from non-Latin alphabets (no Chinese, Arabic, Hindi, Cyrillic, Greek, etc.)
Cannot represent modern symbols such as emoji
Different "extended" versions disagreed about which extra symbols belonged where, causing compatibility problems

Part of the ASCII table:

Character	Denary code
P	80
Q	81
R	82
S	83

If you are given P = 80 and the question asks for the code for W, count forward four letters: Q = 81, R = 82, S = 83, T = 84, U = 85, V = 86, W = 87. The same trick works for finding a character from its code: ASCII 78 must be 80 − 2 = N.