ASCII uses a fixed number of bits to represent each character.
Explain one reason why ASCII uses 7 bits to represent each character.
Mark scheme
Award up to two marks for a linked explanation such as:
- 7 bits give 128 different codes/combinations (1) which is enough to represent all the characters on a keyboard (1)
- 7 bits allow 128 unique characters (1) so uppercase and lowercase letters, digits and punctuation can all be represented (1)
- 6 bits give only 64 combinations (1) which is not enough for the number of characters on a keyboard (1)
- A keyboard needs to represent more than 64 different characters (1) so 7 bits are needed to give the 128 combinations required (1)
- A fixed number of 7 bits gives 128 possible codes (1) which is enough for the characters ASCII represents, so more bits are not needed (1)
For both marks, the expansion must follow/associate with the statement.
Do not accept 'because ASCII is 7-bit' or 'because 7 bits is enough' as these repeat the question without giving the number of combinations or saying what they allow.
Do not accept '7 bits give 256 characters'; 256 combinations come from 8 bits (extended ASCII).
Allow 'ASCII has 128 characters/codes' for the 128 point.
Example answers and how they were marked
7 bits give 128 different codes, which is enough for every letter, digit and punctuation symbol on a keyboard.
Why: Gives the point that 7 bits give 128 codes (1) and links it to the reason that this is enough for all the characters on a keyboard (1).
7 bits give 128 different combinations.
Why: Gives the point that 7 bits give 128 combinations (1) but does not say what this number of combinations allows, so the linked reason (1) is not earned.
Because ASCII uses 7 bits so it can store 256 characters.
Why: Repeats the question and gives the wrong number of characters (256 is 8-bit extended ASCII), so there is no creditworthy point.