A computer system uses a 4-bit code to represent the characters typed on its keyboard.
Explain one limitation of using 4 bits to represent characters.
Mark scheme
Award up to two marks for a linked explanation such as:
- Only 16 different characters can be represented (1) so there are not enough characters codes for every letter of the alphabet (1)
- 4 bits give only 16 different combinations (1) which is not enough for the uppercase letters, lowercase letters, digits and symbols on a keyboard (1)
- More than 16 characters are needed on a keyboard (1) so some letters, digits or symbols cannot be represented (1)
- The number of character codes available is limited to 16 (1) because 4 bits give 2 to the power of 4 combinations (1)
For both marks, the expansion must follow/associate with the statement.
Calculating that 4 bits give 2 to the power of 4 = 16 combinations can earn either the point or the linked reason, provided the other half of the answer says what this means (too few characters/codes).
Allow an expansion that names only the letters of the alphabet, e.g. there are not enough character codes for the 26 letters (1).
Do not accept '4 bits can only store 4 characters' (the number of bits is not the number of characters).
Do not accept 'it can only store 16 bits' (16 is the number of character codes/combinations, not the number of bits).
Do not accept 'not enough memory/storage' (the limitation is the number of character codes, not the amount of storage).
Do not accept 'it is limited/smaller' without saying how many characters/codes are available or what cannot be represented.
Example answers and how they were marked
Only 16 different characters can be represented, so there are not enough character codes for all 26 letters.
Why: Gives the point that only 16 characters can be represented (1) and links it to the reason that this is fewer than the letters needed (1).
Only 16 different characters can be represented.
Why: Gives the point that only 16 characters can be represented (1) but does not explain what cannot be represented.
It can only store 4 characters because there are 4 bits.
Why: Misconception that the number of bits equals the number of characters, which the mark scheme says not to accept; no creditworthy point.