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.