Cambridge O Level Computer Science · Syllabus 2210 · 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 O Level 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.