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Exam Questions Edexcel GCSE Computer Science (1CP2)

Denary to Negative Binary Conversion Exam Questions

6 Edexcel GCSE Computer Science (1CP2) exam-style questions on Denary to Negative Binary Conversion (2.1 Binary), 12 marks in all. Answer them on paper or print them, then open the mark schemes and example answers to see how each is marked.

Revise Denary to Negative Binary Conversion first

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1Convert2 marks

Computer systems manipulate bit patterns.

Convert the denary number -75 to 8-bit binary using two's complement.

[2 marks]
Mark scheme

Award one mark for each correct nibble in the correct order:

  • 1011 (1)
  • 0101 (1)

0 marks if the response is not 8-bit binary.

Example answers and how they were marked
Full marks2/2

1011 0101

Why: 75 is 01001011; flip the bits (10110100) and add 1: both nibbles are correct (2).

Some marks1/2

1011 0100

Why: Flips the bits of 75 but doesn't add 1. The first nibble is correct (1); the second is not.

No marks0/2

1100 1011

Why: Uses sign and magnitude (a 1 in front of 75) instead of two's complement. Neither nibble is correct.

2Convert2 marks

Binary patterns can represent numbers.

Convert the 8-bit binary two's complement pattern 1111 1001 to denary.

[2 marks]
Mark scheme

Award one mark for each of:

  • − (negative sign) (1)
  • 7 (1)

The correct answer is −7

Example answers and how they were marked
Full marks2/2

−7

Why: The left-hand bit is worth −128: −128 + 121 = -7. Sign (1) and value (1).

Some marks1/2

7

Why: The value is right (1) but the minus sign is missing.

No marks0/2

249

Why: Reads it as an unsigned number, so neither the sign nor the value is right.

3Convert2 marks

Computer systems manipulate bit patterns.

Convert the denary number -48 to 8-bit binary using two's complement.

[2 marks]
Mark scheme

Award one mark for each correct nibble in the correct order:

  • 1101 (1)
  • 0000 (1)

0 marks if the response is not 8-bit binary.

Example answers and how they were marked
Full marks2/2

1101 0000

Why: 48 is 00110000; flip the bits (11001111) and add 1: both nibbles are correct (2).

Some marks1/2

1011 0000

Why: Uses sign and magnitude (a 1 in front of 48) instead of two's complement. The second nibble is correct (1); the first is not.

No marks0/2

1100 1111

Why: Flips the bits of 48 but doesn't add 1. Neither nibble is correct.

4Convert2 marks

Computer systems manipulate bit patterns.

Convert the 8-bit binary two's complement pattern 1000 0111 to denary.

[2 marks]
Mark scheme

Award one mark for each of:

  • − (negative sign) (1)
  • 121 (1)

The correct answer is −121

Example answers and how they were marked
Full marks2/2

−121

Why: The left-hand bit is worth −128: −128 + 7 = -121. Sign (1) and value (1).

Some marks1/2

121

Why: The value is right (1) but the minus sign is missing.

No marks0/2

135

Why: Reads it as an unsigned number, so neither the sign nor the value is right.

5Convert2 marks

Binary, denary and hexadecimal patterns represent numbers.

Convert the denary number -46 to 8-bit binary using two's complement.

[2 marks]
Mark scheme

Award one mark for each correct nibble in the correct order:

  • 1101 (1)
  • 0010 (1)

0 marks if the response is not 8-bit binary.

Example answers and how they were marked
Full marks2/2

1101 0010

Why: 46 is 00101110; flip the bits (11010001) and add 1: both nibbles are correct (2).

Some marks1/2

1101 0001

Why: Flips the bits of 46 but doesn't add 1. The first nibble is correct (1); the second is not.

No marks0/2

1010 1110

Why: Uses sign and magnitude (a 1 in front of 46) instead of two's complement. Neither nibble is correct.

6Convert2 marks

Computer systems manipulate bit patterns.

Convert the 8-bit binary two's complement pattern 1100 1011 to denary.

[2 marks]
Mark scheme

Award one mark for each of:

  • − (negative sign) (1)
  • 53 (1)

The correct answer is −53

Example answers and how they were marked
Full marks2/2

−53

Why: The left-hand bit is worth −128: −128 + 75 = -53. Sign (1) and value (1).

Some marks1/2

53

Why: The value is right (1) but the minus sign is missing.

No marks0/2

203

Why: Reads it as an unsigned number, so neither the sign nor the value is right.

Keep practising Denary to Negative Binary Conversion

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