A computer uses 7-bit ASCII to store text.
Describe why 7 bits are needed to represent each character, rather than 6 bits.
Mark scheme
Award up to two marks for a linked description such as:
- 6 bits give only 64 different codes (1) which is fewer than the number of letters, digits and symbols that must be represented (1)
- 7 bits give 128 different codes (1) which is enough to represent every letter, digit and symbol that is needed (1)
- Adding a bit doubles the number of different codes from 64 to 128 (1) so that every character needed can have its own code (1)
- There are more than 64 characters that must be represented (letters, digits and symbols) (1) so 6 bits cannot give each one a different code (1)
A statement of the number of codes without a development can earn one mark only; the development must be linked to the statement for the second mark.
Do not accept responses that only say computers use binary (1s and 0s).
Do not accept that 7 bits are needed because 7 is a bigger number than 6, with no reference to the number of codes or characters.
Do not accept responses about 8 bits, extended ASCII or Unicode.
Example answers and how they were marked
6 bits can only give 64 different codes, which is not enough for all the letters, digits and symbols that have to be represented.
Why: Gives the number of codes 6 bits can make (1) linked to the number of characters that must be represented (1).
7 bits give 128 different codes.
Why: States the number of codes available from 7 bits (1) but gives no linked development, so the second mark is not earned (0).
Because computers only use binary and 7 is a bigger number than 6.
Why: Says only that computers use binary and compares the numbers, which the mark scheme tells markers not to accept; no creditworthy point about codes or characters.