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9 questions · 18 marks
Computers manipulate binary patterns.
Convert the binary pattern 1010 0100 to denary.
Award one mark for:
164
Why: Correct: 128 + 32 + 4 = 164 (1).
37
Why: Reads the place values from the wrong end (the left-hand bit as 1), so the value is wrong.
3
Why: Counts the 1s instead of adding their place values.
Computers represent all data using binary.
Explain one reason why computers use binary to represent data.
Award up to two marks for a linked explanation such as:
For both marks, the expansion must follow/associate with the statement.
Do not accept 'computers only understand 0s and 1s' as a reason on its own without saying what the two states are or why binary matches them.
Do not accept 'computers are digital' or 'that is how computers work' without expansion.
Do not accept 'binary is easier to read/write' or 'binary is easier to understand' as a reason for using binary.
Do not award the same point twice if it is repeated in different words, e.g. two answers that both say circuits/components are on or off.
Computers use binary because their electronic circuits have two states, on and off. Binary also has only two values, 0 and 1, so these can represent the states of the circuit.
Why: Links the two circuit states (1) to binary's two values representing them (1).
Computers use binary because their circuits are either on or off.
Why: Identifies the two circuit states (1) but does not link them to binary's two values, so the second mark is not awarded.
Computers use binary because it is easier to read than denary.
Why: 'Easier to read' is a reason for using hexadecimal, not a correct reason for using binary; the mark scheme tells markers not to accept it, so no mark.
Computers manipulate binary patterns.
Convert the denary number 184 to 8-bit binary.
Award one mark for each correct nibble in the correct order:
0 marks if the response is not 8-bit binary.
1011 1000
Why: 184 = 128 + 32 + 16 + 8: both nibbles are correct (2).
1011 1001
Why: Converts 185 instead (a slip in adding up the place values). The first nibble is correct (1); the second is not.
0001 1101
Why: The bits are written in reverse, smallest place value first. Neither nibble is correct.
Computers manipulate binary patterns.
Convert the denary number 90 to hexadecimal.
Award one mark for each correct digit in the correct position:
Answer: 5A
5A
Why: 90 ÷ 16 = 5 remainder 10, so the digits are 5 and A: both digits are correct (2).
510
Why: One digit is correct (1), but a digit over 9 is written in denary instead of as a letter.
A5
Why: The digits are the wrong way round, so neither is in the correct position.
Binary patterns can represent numbers.
Convert the 8-bit binary two's complement pattern 1111 1001 to denary.
Award one mark for each of:
The correct answer is −7
−7
Why: The left-hand bit is worth −128: −128 + 121 = -7. Sign (1) and value (1).
7
Why: The value is right (1) but the minus sign is missing.
249
Why: Reads it as an unsigned number, so neither the sign nor the value is right.
Computer systems manipulate bit patterns.
Convert the 8-bit binary two's complement pattern 1000 0111 to denary.
Award one mark for each of:
The correct answer is −121
−121
Why: The left-hand bit is worth −128: −128 + 7 = -121. Sign (1) and value (1).
121
Why: The value is right (1) but the minus sign is missing.
135
Why: Reads it as an unsigned number, so neither the sign nor the value is right.
Computer systems manipulate bit patterns.
Convert the 8-bit binary two's complement pattern 1100 1011 to denary.
Award one mark for each of:
The correct answer is −53
−53
Why: The left-hand bit is worth −128: −128 + 75 = -53. Sign (1) and value (1).
53
Why: The value is right (1) but the minus sign is missing.
203
Why: Reads it as an unsigned number, so neither the sign nor the value is right.
Computers manipulate binary patterns.
Complete the table to show the result of the 8-bit binary addition.
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 1 | 0 | 1 | 0 | 1 |
| 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 |
Award one mark for each correct nibble in the correct order:
0 marks if the response is not 8-bit binary.
1110 0100
Why: 149 + 79 = 228, carrying where a column adds to 2: both nibbles are correct (2).
1110 0101
Why: A slip in the last bit. The first nibble is correct (1); the second is not.
1101 1111
Why: Writes 1 in every column with a 1 and never carries. Neither nibble is correct.
Computers manipulate binary patterns.
Complete the table to show the 8-bit result of the binary addition.
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 1 | 1 | 1 | 0 | 0 |
Award one mark for each correct nibble in the correct order:
0 marks if the response is not 8-bit binary.
Ignore a carry shown beyond the eighth bit.
0111 0101
Why: 185 + 188 = 373, which needs 9 bits; the 8 bits that are kept are both correct (2).
0000 0101
Why: Forgets to carry the 1s. The second nibble is correct (1); the first is not.
1011 1101
Why: Never carries. Neither nibble is correct.
State the name of the error that occurs when this addition is carried out.
Award one mark for:
Overflow
Why: The result needs more bits than are available (1).
Underflow
Why: A near miss: the result is too big for 8 bits, not too small.
A syntax error
Why: A syntax error is a mistake in a program's code, not in arithmetic.
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