Shift operators in VHDL

Introduction

Shift operators move bits left or right in a binary number. This helps change values quickly, like multiplying or dividing by 2.

When you want to multiply a number by 2 or powers of 2 quickly.

When you want to divide a number by 2 or powers of 2 without using division.

When you need to move bits to align data in hardware design.

When implementing digital circuits that require bit manipulation.

When optimizing arithmetic operations in hardware description.

Syntax

VHDL

signal_name <= shift_left(signal_name, number_of_positions);
signal_name <= shift_right(signal_name, number_of_positions);

shift_left moves bits to the left, filling with zeros on the right.

shift_right moves bits to the right, filling with zeros on the left.

Examples

This shifts bits in input_signal two places to the left.

VHDL

result <= shift_left(input_signal, 2);

This shifts bits in input_signal three places to the right.

VHDL

result <= shift_right(input_signal, 3);

Shifting by zero means the signal stays the same.

VHDL

result <= shift_left(input_signal, 0);

Sample Program

This VHDL code shifts an 8-bit unsigned input left by 2 bits and right by 3 bits, showing how bits move and zeros fill in.

VHDL

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity shift_example is
    port(
        input_signal : in unsigned(7 downto 0);
        output_left  : out unsigned(7 downto 0);
        output_right : out unsigned(7 downto 0)
    );
end shift_example;

architecture behavior of shift_example is
begin
    output_left <= shift_left(input_signal, 2);
    output_right <= shift_right(input_signal, 3);
end behavior;

Important Notes

Shift operators fill empty bit positions with zeros, not with the original bit values.

Shifting left by 1 is like multiplying by 2, shifting right by 1 is like dividing by 2 (ignoring remainder).

Use unsigned or std_logic_vector types with shift operators carefully.

Summary

Shift operators move bits left or right, filling with zeros.

They help multiply or divide numbers by powers of two quickly.

In VHDL, use shift_left and shift_right functions for this.

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