A LevelComputer ScienceCambridge (CIE)Exam Questions4. Processor FundamentalsBit Manipulation

Bit Manipulation (Cambridge (CIE) A Level Computer Science): Exam Questions

Exam code: 9618

39 mins13 questions
1
6 marks

The instruction set also includes these bit manipulation instructions:

Instruction

Explanation

Opcode

Operand

AND

#n/Bn/&n

BitwiseAND operation of the contents ofACC with the operand

AND

<address>

Bitwise AND operation of the contents of ACC with the contents of <address>

XOR

#n/Bn/&n

Bitwise XOR operation of the contents ofACC with the operand

XOR

<address>

Bitwise XOR operation of the contents of ACC with the contents of <address>

OR

#n/Bn/&n

Bitwise OR operation of the contents ofACC with the operand

OR

<address>

Bitwise OR operation of the contents of ACC with the contents of <address>

LSL

#n

Bits in ACC are shifted logically n places to the left. Zeros are introduced on the right-hand end

LSR

#n

Bits in ACC are shifted logically n places to the right. Zeros are introduced on the left-hand end

<address> can be an absolute or a symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001010
& denotes a hexadecimal number, e.g. &4A

The current content of the ACC is shown:

ACC

1

0

0

1

1

0

1

0

The table has three sets of instructions. The binary number 10011010 is reloaded into the ACC before each set of instructions is run.

Complete the table by writing the content of the ACC after each set of instructions has run.

Instructions

ACC content

1

LSL #2

2

ADD #5
AND #30

3

OR B11110010
INC ACC

(3)

(ii) Explain how bit manipulation can be used to test whether the binary number stored in the ACC represents an odd denary number.

Write the bit manipulation instruction that will be used.

(3)

How did you do?

2
3 marks

The instruction set also includes these bit manipulation instructions:

Instruction

Explanation

Opcode

Operand

AND

#n/Bn/&n

Bitwise AND operation of the contents of ACC with the operand

AND

<address>

Bitwise AND operation of the contents of ACC with the contents of <address>

XOR

#n/Bn/&n

Bitwise XOR operation of the contents of ACC with the operand

XOR

<address>

Bitwise XOR operation of the contents of ACC with the contents of <address>

OR

#n/Bn/&n

Bitwise OR operation of the contents of ACC with the operand

OR

<address>

Bitwise OR operation of the contents of ACC with the contents of <address>

<address> can be an absolute or a symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001010
& denotes a hexadecimal number, e.g. &4A

Explain how bit manipulation can be used to clear the data in an 8-bit register.

Write the bit manipulation instruction that will be used.

How did you do?

3
2 marks

Describe the difference between a right logical binary shift and a right arithmetic binary shift.

How did you do?

4
3 marks

The processor includes these bit manipulation instructions:

Instruction

Explanation

Opcode

Operand

AND

#n/Bn/&n

Bitwise AND operation of the contents of ACC with the operand

AND

<address>

Bitwise AND operation of the contents of ACC with the contents of <address>

XOR

#n/Bn/&n

Bitwise XOR operation of the contents of ACC with the operand

XOR

<address>

Bitwise XOR operation of the contents of ACC with the contents of <address>

OR

#n/Bn/&n

Bitwise OR operation of the contents of ACC with the operand

OR

<address>

Bitwise OR operation of the contents of ACC with the contents of <address>

<address> can be an absolute or a symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001010
& denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address

Data

30

01110101

31

11111111

32

00000000

33

11001100

34

10101010

The current content of the ACC is shown:

1

0

0

1

1

0

1

0

Complete the table by writing the content of the ACC after each program has run.

The binary number 10011010 is reloaded into the ACC before each program is run.

Program number

Code

ACC content

1

AND 31

2

XOR B01001111

3

OR #30

How did you do?

5
3 marks

The processor includes these bit manipulation instructions:

Instruction

Explanation

Opcode

Operand

AND

#n/Bn/&n

Bitwise AND operation of the contents of ACC with the operand

AND

<address>

Bitwise AND operation of the contents of ACC with the contents of <address>

XOR

#n/Bn/&n

Bitwise XOR operation of the contents of ACC with the operand

XOR

<address>

Bitwise XOR operation of the contents of ACC with the contents of <address>

OR

#n/Bn/&n

Bitwise OR operation of the contents of ACC with the operand

OR

<address>

Bitwise OR operation of the contents of ACC with the contents of <address>

<address> can be an absolute or a symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001010
& denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address

Data

25

11000110

26

11100001

27

10000001

28

11001101

29

00001111

The current content of the ACC is shown:

0

1

0

0

0

1

1

0

Complete the table by writing the content of the ACC after each program has run.

The binary number 01000110 is reloaded into the ACC before each program is run.

Program number

Code

ACC content

1

XOR 29

2

AND #29

3

OR B11111111

How did you do?

6
1 mark

The processor includes these bit manipulation instructions:

Instruction

Explanation

Opcode

Operand

AND

#n / Bn / &n

Bitwise AND operation of the contents of ACC with the operand

AND

<address>

Bitwise AND operation of the contents of ACC with the contents of <address>

XOR

#n / Bn / &n

Bitwise XOR operation of the contents of ACC with the operand

XOR

<address>

Bitwise XOR operation of the contents of ACC with the contents of <address>

OR

#n / Bn / &n

Bitwise OR operation of the contents of ACC with the operand

OR

<address>

Bitwise OR operation of the contents of ACC with the contents of <address>

LSL

#n

Bits in ACC are shifted logically n places to the left. Zeros are introduced on the right-hand end

LSR

#n

Bits in ACC are shifted logically n places to the right. Zeros are introduced on the left-hand end

<address> can be an absolute or a symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001010
& denotes a hexadecimal number, e.g. &4A

The current contents of memory are shown:

Address

Data

100

00001101

101

10111110

102

11110011

103

00110111

104

00000000

The current content of the ACC is shown:

1

1

1

1

1

1

1

1

Complete the table by writing the content of the ACC after each instruction has run.

The binary number 11111111 is reloaded into the ACC before each instruction is run.

Instruction number

Instruction

ACC content

1

LSL #2

2

XOR 100

3

AND 103

How did you do?

7
3 marks

The following table shows another part of the instruction set for the same processor.

Instruction

Explanation

Opcode

Operand

AND

Bn

Bitwise AND operation of the contents of ACC with the operand

XOR

Bn

Bitwise XOR operation of the contents of ACC with the operand

LSR

#n

Bits in ACC are shifted logically n places to the right. Zeros are introduced on the left hand end

# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B01001101

(i) The current contents of the ACC are:

0

1

0

0

1

1

1

1

Show the contents of the ACC after the execution of the following instruction.

AND B10100101

(1)

(ii) The current contents of the ACC are:

0

0

0

1

0

1

1

1

Show the contents of the ACC after the execution of the following instruction.

LSR #3

(1)

(iii) The current contents of the ACC are:

1

1

1

1

0

1

1

1

Show the contents of the ACC after the execution of the following instruction.

XOR B00100101

(1)

How did you do?

8
2 marks

A processor has one general purpose register, the Accumulator (ACC).

The ACC holds the unsigned binary integer 00010110.

A 2-place left logical shift is carried out on the contents of the ACC.

(a) Show the contents of the ACC after the shift.

(b) Give the denary value held in the ACC after the shift.

How did you do?

9
3 marks

An 8-bit register holds the two's complement binary integer 11001010.

(a) Show the contents of the register after a 2-place right logical shift.

(b) Show the contents of the register after a 2-place right arithmetic shift.

(c) Explain why a right arithmetic shift is used instead of a right logical shift when the register holds a negative two's complement integer.

How did you do?

10
4 marks

An 8-bit register holds an unsigned binary integer.

The register currently holds 01101001.

A programmer wants to multiply the value in the register by 4. The programmer carries out a 2-place left logical shift on the register.

(a) Show the contents of the register after the shift.

(b) Give the denary value held in the register before the shift and after the shift.

(c) Explain why the shift has not multiplied the value by 4.

How did you do?

11
2 marks

The table shows part of the instruction set for a processor. The processor has one general purpose register, the Accumulator (ACC).

Instruction

Explanation

Opcode

Operand

AND

Bn

Bitwise AND operation of the contents of ACC with the operand

OR

Bn

Bitwise OR operation of the contents of ACC with the operand

B denotes a binary number, for example B01001010.

The binary value 01101101 is loaded into the ACC before each instruction is executed.

Complete the table to show the contents of the ACC after each instruction has been executed.

Instruction

ACC contents after the instruction

AND B00001111

OR B10000000

How did you do?

12
3 marks

An automated greenhouse is controlled by an 8-bit control register. Each bit controls one device. A bit set to 1 switches its device on. A bit set to 0 switches its device off.

The table shows which device each bit controls.

Place value

128

64

32

16

8

4

2

1

Device

alarm

pump

fan

heater

lamp

vent

mister

door

The control register currently holds 01001010.

The fan must be switched on. No other device may change state.

(a) Give the 8-bit mask that must be used.

(b) State the bitwise operation that must be used with this mask.

(c) Show the contents of the control register after the operation has been carried out.

How did you do?

13
4 marks

A building security system uses an 8-bit status register. Each bit reports one sensor. A bit is set to 1 when its sensor has been triggered and is 0 when it has not.

The table shows which sensor each bit reports.

Place value

128

64

32

16

8

4

2

1

Sensor

front door

window

motion

smoke

heat

glass

gate

panic button

The system must raise a full alarm only when the window sensor and the motion sensor have both been triggered.

Explain how bit masking is used to carry out this test. Your answer must include the 8-bit mask, the bitwise operation used, and how the result of the operation is interpreted.

Use the status register contents 01001010 as an example in your answer.

How did you do?