AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Character encoding: convert character codes to characters.
Converting a character code back to a character is the reverse of encoding: the computer takes a stored numeric value and looks it up in the character set to find which character it represents.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 4 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
Converting a character code back to a character is the reverse of encoding: the computer takes a stored numeric value and looks it up in the character set to find which character it represents. To do this, identify the character set in use (for example ASCII or Unicode), then find the given code in that set's table and read off the matching character. For instance, in ASCII the decimal value 65 maps to the capital letter A, and 97 maps to lowercase a. The same numeric value can map to different characters in different character sets, so the set must be known. A reliable method is: take the code, confirm the character set, locate the value in the table, and record the character. This skill explains how stored text is turned back into readable characters.
What earns the marks
- Identifies that a character code is looked up in a character set to find the matching character.
- States the correct character for a given code in a stated character set, e.g. 65 is A in ASCII.
- Recognises that the resulting character depends on which character set is used.
- Handles codes given in decimal, binary or hexadecimal by converting to the table value first.
- Explains that decoding reverses the encoding process to recover readable text.
Common mistakes and how to put them right
Assuming a code always gives the same character regardless of character set.
Put it right: Is that the character set must be known because the same value can map differently.
Mixing up uppercase and lowercase results.
Put it right: Is that in ASCII 65 is uppercase A while 97 is lowercase a.
Failing to convert binary or hexadecimal codes to the table value before lookup.
Put it right: Is to convert to the matching base first, then find the character.
Exam tips
- State the character set before naming the character for a code.
- Convert binary or hexadecimal codes to decimal first if that helps you use the table.
- Use anchor values such as 65 = A and 97 = a in ASCII to check your answer.
Try the lesson
The start of the Character encoding: convert character codes to characters. lesson, made from checked teaching notes for this specification point.
Have a go first
A computer uses 7-bit ASCII, in which the decimal code 65 represents the character A, 66 represents B, 67 represents C and so on in alphabetical order. What character does the decimal code 68 represent?
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