Shift operators in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a left shift by 2 bits on the signal data_in.

VHDL

data_out <= data_in [1] 2;

Drag options to blanks, or click blank then click option'

Asrl

Bsll

Crol

Dror

Attempts:

3 left

💡 Hint

Common Mistakes

Using srl which shifts bits to the right.

Using rotate operators rol or ror instead of shift.

The sll operator shifts bits to the left, filling with zeros on the right.

2fill in blank

medium

Complete the code to perform a right shift by 3 bits on the signal data_in.

VHDL

data_out <= data_in [1] 3;

Drag options to blanks, or click blank then click option'

Asll

Bror

Crol

Dsrl

Attempts:

3 left

💡 Hint

Common Mistakes

Using sll which shifts bits left.

Using rotate operators rol or ror instead of shift.

The srl operator shifts bits to the right, filling with zeros on the left.

3fill in blank

hard

Fix the error in the code to rotate the signal data_in left by 1 bit.

VHDL

data_out <= data_in [1] 1;

Drag options to blanks, or click blank then click option'

Arol

Bsrl

Cror

Dsll

Attempts:

3 left

💡 Hint

Common Mistakes

Using sll which shifts bits but does not wrap.

Using ror which rotates right instead of left.

The rol operator rotates bits to the left, wrapping the leftmost bit to the right end.

4fill in blank

hard

Fill both blanks to shift data_in right logically by 4 bits and assign to data_out.

VHDL

data_out <= data_in [1] [2];

Drag options to blanks, or click blank then click option'

Asrl

Bsll

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using sll which shifts left.

Using wrong number of bits like 1.

Use srl to shift right logically and 4 as the number of bits.

5fill in blank

hard

Fill both blanks to rotate data_in right by 2 bits and assign to data_out.

VHDL

data_out <= data_in [1] [2];

Drag options to blanks, or click blank then click option'

Asrl

Bror

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using srl which shifts but does not rotate.

Using wrong number of bits like 3.

Use ror to rotate right and 2 as the number of bits.

Practice

1. What does the shift_left operator do in VHDL?

easy

A. Inverts all bits in the vector

B. Moves bits to the left, filling with zeros on the right

C. Rotates bits without changing their order

D. Moves bits to the right, filling with zeros on the left

Solution

1. Step 1: Understand the shift_left operator

   The shift_left operator moves all bits in a vector to the left by a specified number of positions.

2. Step 2: Identify what fills the empty positions

   When bits move left, zeros fill the rightmost positions to keep the vector size constant.

3. Final Answer:

   Moves bits to the left, filling with zeros on the right -> Option B

4. Quick Check:

   shift_left = left shift with zero fill [OK]

Hint: Left shift moves bits left, adds zeros on right [OK]

Common Mistakes:

• Confusing shift_left with shift_right
• Thinking bits rotate instead of filling zeros
• Assuming bits are inverted

2. Which of the following is the correct syntax to shift a 8-bit std_logic_vector data 3 bits to the right in VHDL?

easy

A. data <= shift_right(data, 3);

B. data <= shift_left(data, 3);

C. data <= data >> 3;

D. data <= shift(data, -3);

Solution

1. Step 1: Identify the correct shift function for right shift

   In VHDL, shift_right is used to shift bits to the right.

2. Step 2: Check the syntax for shifting

   The syntax is variable <= shift_right(variable, number_of_bits); so data <= shift_right(data, 3); is correct.

3. Final Answer:

   data <= shift_right(data, 3); -> Option A

4. Quick Check:

   shift_right(data, 3) is correct syntax [OK]

Hint: Use shift_right(variable, count) to shift right [OK]

Common Mistakes:

• Using shift_left instead of shift_right
• Using C-style operators like >> which are invalid in VHDL
• Using negative shift counts

3. Given the VHDL code:

signal data : std_logic_vector(7 downto 0) := "00011011";
signal result : std_logic_vector(7 downto 0);

result <= shift_left(data, 2);

What is the value of result after the shift?

medium

A. "01101100"

B. "11011000"

C. "00011000"

D. "01100000"

Solution

1. Step 1: Write the original bits and shift left by 2

   Original data: 00011011
   Shift left by 2 moves bits left 2 places: 01100000 (last two bits become zeros)

2. Step 2: Confirm the shifted bits and zero fill

   Bits shifted left: 0 0 0 1 1 0 1 1
   After shift left 2: 0 1 1 0 0 0 0 0

3. Final Answer:

   "01100000" -> Option D

4. Quick Check:

   shift_left("00011011", 2) = "01100000" [OK]

Hint: Shift left moves bits left, add zeros on right [OK]

Common Mistakes:

• Forgetting to add zeros on right
• Shifting bits right instead of left
• Miscounting bit positions

4. Consider this VHDL snippet:

signal data : std_logic_vector(7 downto 0) := "10101010";
signal result : std_logic_vector(7 downto 0);

result <= shift_right(data, 3);

But the output is unexpected. What is the likely error?

medium

A. Using shift_left instead of shift_right

B. Assigning to signal instead of variable

C. Using shift_right on std_logic_vector without numeric_std library

D. Incorrect bit width of result signal

Solution

1. Step 1: Understand shift_right requirements

   The shift_right function requires the numeric_std library for std_logic_vector operations.

2. Step 2: Identify missing library causing unexpected output

   If numeric_std is not included, shift_right may not work properly on std_logic_vector, causing wrong results.

3. Final Answer:

   Using shift_right on std_logic_vector without numeric_std library -> Option C

4. Quick Check:

   numeric_std needed for shift_right on std_logic_vector [OK]

Hint: Include numeric_std for shift_right on std_logic_vector [OK]

Common Mistakes:

• Forgetting to include numeric_std library
• Confusing shift_left and shift_right
• Assuming signals behave like variables

5. You want to multiply an 8-bit unsigned number num by 8 using shift operators in VHDL. Which expression correctly does this?

hard

A. result <= shift_left(num, 3);

B. result <= shift_right(num, 3);

C. result <= shift_left(num, 2);

D. result <= shift_right(num, 2);

Solution

1. Step 1: Understand multiplication by powers of two

   Multiplying by 8 is the same as multiplying by 2^3, which is shifting left by 3 bits.

2. Step 2: Choose the correct shift operator and count

   Use shift_left(num, 3) to multiply by 8 correctly.

3. Final Answer:

   result <= shift_left(num, 3); -> Option A

4. Quick Check:

   Multiply by 8 = shift_left by 3 [OK]

Hint: Multiply by 2^n with shift_left by n [OK]

Common Mistakes:

• Using shift_right instead of shift_left
• Shifting by wrong number of bits
• Confusing unsigned and signed types