Understand how computers encode characters using 7-bit ASCII

The idea

Computers store characters as binary codes so that text can be processed, compared and transmitted consistently. In 7-bit ASCII each character is given a unique 7-bit pattern, 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; the digit 0 is code 48, which is 00110000 in 8 bits when a leading zero is added. A computer looks up the code for a character when it stores it, and looks up the character for a code when it displays or prints it. Because the code length is fixed at 7 bits, every character takes the same space, and codes can be sorted into a standard order. The 128 codes cover control codes and printable characters such as letters, digits and punctuation, which is enough for basic English text but not for the full range of world languages.

What earns the marks

• 7-bit ASCII uses exactly 7 bits per character, giving 2⁷ = 128 distinct codes, numbered 0 to 127.
• Each character maps to one unique code, so the same code always represents the same character.
• The capital letter A has code 65, written as 1000001 in 7 bits.
• The digit 0 has code 48, written as 0110000 in 7 bits or 00110000 when padded to 8 bits.
• A computer encodes text by replacing each character with its code and decodes text by replacing each code with its character.
• 7-bit ASCII covers 128 codes including control codes and printable English characters, so it cannot represent all world languages.

Common mistakes and how to put them right

• Thinking ASCII uses 8 bits per character.

  Put it right: 7-bit ASCII uses 7 bits, giving 128 codes, although an eighth bit may be added as a leading zero when storing in a byte.

• Confusing the number of codes with the number of bits.

  Put it right: 7 bits give 2⁷ = 128 codes, not 7 codes.

• Assuming the code for A is 1 because A is the first letter.

  Put it right: The code for A is 65, and codes 0 to 64 are used by other characters and control codes.

• Believing 7-bit ASCII can represent any character in any language.

  Put it right: Its 128 codes cover basic English characters and control codes only.

Exam tips

• Learn the key values: 2⁷ = 128 codes, A = 65 = 1000001, a = 97, and 0 = 48.
• When converting a character to binary, write exactly 7 bits and pad with leading zeros if needed.
• Check whether a question asks for the number of bits, the number of codes or a specific code value before answering.