Shift operators in Verilog - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the << operator do in Verilog?

The << operator shifts bits to the left by a specified number of positions, filling the right with zeros. It's like moving digits left in a number, making it larger in value.

intermediate

What is the difference between << and <<< in Verilog?

The << operator is a logical left shift that fills with zeros. The <<< operator is an arithmetic left shift, which behaves the same as << but is used for signed numbers to preserve sign.

beginner

How does the >> operator work in Verilog?

The >> operator shifts bits to the right by a specified number of positions, filling the left with zeros for unsigned numbers (logical shift).

intermediate

What is the purpose of the >>> operator in Verilog?

The >>> operator is an arithmetic right shift. It shifts bits right but fills the left with the sign bit (the leftmost bit) to keep the number's sign intact.

beginner

What is the result of 2'b01 << 1?

The 2'b01 shifted left by 1 becomes 2'b10. Shifting moves bits left and adds zero on the right.

What does the Verilog operator << do?

A Shifts bits left and fills right with zeros

B Shifts bits right and fills left with zeros

C Shifts bits right and fills left with sign bit

D Performs bitwise AND

The << operator shifts bits to the left and fills the right side with zeros.

Which operator performs an arithmetic right shift in Verilog?

A >>

B <<<

C >>>

D <<

The >>> operator shifts bits right and fills the left with the sign bit to preserve the number's sign.

What happens when you use the <<< operator on a signed number?

A Logical left shift with zeros filled

B Arithmetic left shift preserving sign

C Logical right shift with zeros filled

D Arithmetic right shift preserving sign

The <<< operator is an arithmetic left shift that preserves the sign of signed numbers.

If you shift 4'b1101 >> 2, what is the result?

A 4'b0011

B 4'b1110

C 4'b0000

D 4'b0010

Shifting right by 2 moves bits right and fills left with zeros, so 1101 becomes 0011.

Which shift operator fills the left bits with zeros regardless of sign?

A >>>

B <<

C <<<

D >>

The >> operator is a logical right shift that fills the left bits with zeros.

Explain the difference between logical and arithmetic shift operators in Verilog.

Think about how the empty bits are filled during shifting.

Describe how the >>> operator works and when you would use it.

Consider signed numbers and keeping their positive or negative meaning.

Practice

1. What does the left shift operator << do to a binary number in Verilog?

easy

A. Moves bits to the left, multiplying the number by powers of 2

B. Moves bits to the right, dividing the number by powers of 2

C. Reverses the bits in the number

D. Sets all bits to zero

Solution

1. Step 1: Understand the left shift operator

   The left shift operator << moves all bits in a binary number to the left by a specified number of positions.

2. Step 2: Effect on the number's value

   Moving bits left multiplies the number by 2 for each position shifted.

3. Final Answer:

   Moves bits to the left, multiplying the number by powers of 2 -> Option A

4. Quick Check:

   Left shift = multiply by 2 [OK]

Hint: Left shift multiplies by 2 powers quickly [OK]

Common Mistakes:

• Confusing left shift with right shift
• Thinking it reverses bits
• Assuming it clears bits

2. Which of the following is the correct syntax to shift a 4-bit register reg [3:0] a; two bits to the right in Verilog?

easy

A. a >>> 2;

B. a >> 2;

C. a << 2;

D. a >> 2 =;

Solution

1. Step 1: Identify the right shift operator syntax

   In Verilog, the right shift operator is >> and is used as variable >> number_of_bits;.

2. Step 2: Check the options for correct syntax

   a >> 2; uses a >> 2; which correctly shifts a two bits to the right. a << 2; is left shift, a >>> 2; is arithmetic right shift (signed), and a >> 2 =; has invalid syntax.

3. Final Answer:

   a >> 2; -> Option B

4. Quick Check:

   Right shift syntax = a >> number [OK]

Hint: Use >> for right shift, << for left shift [OK]

Common Mistakes:

• Using left shift operator instead of right shift
• Confusing logical and arithmetic shifts
• Writing invalid syntax like assignment inside shift

3. Given reg [7:0] x = 8'b00011010; what is the value of x << 3 in binary?

medium

A. 8'b11010000

B. 8'b11010000000

C. 8'b00011010

D. 8'b00000000

Solution

1. Step 1: Understand the initial value and shift

   The initial value is 00011010 (decimal 26). Left shifting by 3 moves bits 3 places left, adding zeros on the right.

2. Step 2: Perform the left shift

   Shifting left by 3: 00011010 becomes 11010000 (the leftmost bits shifted out are lost, and zeros fill from right).

3. Final Answer:

   8'b11010000 -> Option A

4. Quick Check:

   Left shift 3 bits = 11010000 [OK]

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

Common Mistakes:

• Expecting longer bit width after shift
• Not losing bits shifted out
• Confusing left and right shift effects

4. What is wrong with this Verilog code snippet?

reg [3:0] a = 4'b1010;
reg [3:0] b;
b = a >> 5;

medium

A. Right shift operator should be << instead of >>

B. Syntax error due to missing semicolon

C. Shifting by more bits than width causes all zeros

D. Cannot assign shifted value to a register

Solution

1. Step 1: Analyze the shift amount vs register width

   The register a is 4 bits wide. Shifting right by 5 bits exceeds its width.

2. Step 2: Effect of shifting more than width

   Shifting more bits than the width results in all bits shifted out, so the result is zero.

3. Final Answer:

   Shifting by more bits than width causes all zeros -> Option C

4. Quick Check:

   Shift > width = zero result [OK]

Hint: Shift count > width clears all bits [OK]

Common Mistakes:

• Thinking syntax error occurs
• Confusing left and right shift operators
• Believing assignment to register is invalid

5. You want to multiply a 6-bit unsigned number num by 8 using shift operators in Verilog. Which expression correctly does this without overflow?

hard

A. num >>> 3

B. num >> 3

C. num << 2

D. num << 3

Solution

1. Step 1: Understand multiplication by powers of 2 using shifts

   Multiplying by 8 equals multiplying by 2^3, so shifting left by 3 bits achieves this.

2. Step 2: Check for overflow considerations

   Since num is 6 bits, shifting left by 3 bits results in a 9-bit value. To avoid overflow, the result should be stored in a wider register. The expression num << 3 correctly multiplies by 8.

3. Final Answer:

   num << 3 -> Option D

4. Quick Check:

   Multiply by 8 = shift left 3 [OK]

Hint: Multiply by 8 = shift left 3 bits [OK]

Common Mistakes:

• Using right shift instead of left shift
• Using arithmetic right shift (>>>)
• Shifting by wrong number of bits