Cambridge IGCSE Computer Science · Syllabus 0478 · Data Representation
ASCII
What is ASCII?
ASCII, the American Standard Code for Information Interchange, is a character set that represents each character with a small number of bits, covering the English letters, digits, punctuation and control characters.
This definition is part of the Data Representation chapter in Cambridge IGCSE Computer Science.
ASCII in context
Text has to be converted to binary before a computer can process it. That conversion is done using a character set: an agreed table that gives every character a unique character code, which is then stored as a binary number. ASCII is one such character set. Unicode is another, and it allows for a far greater range of characters and symbols than ASCII, including different languages and emojis — but it requires more bits per character than ASCII.
Common mistakes with ASCII
- M10. "ASCII stores the visual shape of a letter." Why it fails A character set stores a code number. The shape comes from a font, which is separate data. Correct model Character → character code → binary. The font is applied only when the character is drawn. Exam-safe "A character set maps each character to a code number which is stored in binary; the shape displayed is determined separately by the font." Test yourselfHideQ. A document's font is changed from one typeface to another. Do the stored character codes change?A. No. Only the shapes drawn on screen change.
- M11. "Unicode is a font." Why it fails A font is a set of shapes for drawing characters. Unicode is a character set: it assigns code numbers. Correct model Unicode allows for a greater range of characters and symbols than ASCII, including different languages and emojis, and requires more bits per character. Exam-safe "Unicode is a character set, not a font; it assigns a code to a far greater range of characters than ASCII." Test yourselfHideQ. Give one advantage and one disadvantage of Unicode over ASCII.A. Advantage: a greater range of characters and symbols, including different languages and emojis. Disadvantage: it requires more bits per character, so files are larger.