Character encoding: 7-bit ASCII

The idea

ASCII is a character set that maps characters to binary codes. In 7-bit ASCII each character is represented by a 7-bit binary code, so there are 2⁷ = 128 possible codes, numbered 0 to 127. For example, the capital letter A is code 65, which is 1000001 in 7 bits. Because 7 bits give only 128 codes, 7-bit ASCII can represent a limited range: the English alphabet in upper and lower case, the digits 0–9, punctuation and some control codes. It cannot represent characters from other languages such as accented letters, Greek or Chinese characters, nor most symbols and emoji. When a computer stores ASCII text it usually uses one byte (8 bits) per character, with the extra bit often set to 0, but the character set itself defines only 7 bits.

What earns the marks

• 7-bit ASCII uses 7 bits per character code.
• 2⁷ = 128 distinct codes, numbered 0 to 127.
• Storage often uses one byte per character with the eighth bit unused or set to 0, but the set defines 7 bits.

Common mistakes and how to put them right

• Thinking 7-bit ASCII has 2⁸ = 256 codes.

  Put it right: 7 bits give 2⁷ = 128 codes; 256 would require 8 bits.

• Believing ASCII can store any language's characters.

  Put it right: Its 128 codes cover only a limited set, mainly English letters, digits and punctuation.

• Confusing the number of bits per code with the number of bytes used in storage.

  Put it right: The set defines 7 bits, while a byte is 8 bits and the extra bit is often unused.

Exam tips

• Show the calculation 2⁷ = 128 when asked how many characters 7-bit ASCII can represent.
• Link the limited range of 128 codes to a concrete example, such as being unable to store an accented letter.
• Read the question carefully to see whether it asks about bits per character or bytes used in storage.