dec2to4.vhd

library ieee;
use ieee.std_logic_1164.all;

entity dec2to4 is
  port (
    input  : in  std_logic_vector(1 downto 0);
    output : out std_logic_vector(3 downto 0));
end dec2to4;

-- Implement the decoder using a with-select statement

architecture WITH_SELECT of dec2to4 is
begin
    -- no process for concurrent statements
    with input select
        output <= "0001" when "00",
        "0010"           when "01",
        "0100"           when "10",
        "1000"           when "11",
        "XXXX"           when others;
end WITH_SELECT;

-- Implement the decoder using a when-else statement

architecture WHEN_ELSE of dec2to4 is
begin
    output <= "0001" when input = "00" else
              "0010" when input = "01" else
              "0100" when input = "10" else
              "1000" when input = "11" else
              "XXXX"; 
end WHEN_ELSE;

-- Implement the decoder using an if statement

architecture IF_STATEMENT of dec2to4 is
begin
    -- adding all inputs to the sensitivity list
    process(input)
    begin
        -- Vivado simulator marks the executable lines with an empty red circle
        -- on the left hand margin of the Text Editor
        if (input = "00") then
            output <= "0001";
        elsif (input = "01") then
            output <= "0010";
        elsif (input = "10") then
            output <= "0100";
        elsif (input = "11") then
            output <= "1000";   
        end if;           
    end process;
end IF_STATEMENT;

-- Implement the decoder using a case statement

architecture CASE_STATEMENT of dec2to4 is
begin
    process(input)
    begin
        case input is 
            when "00" =>
                output <= "0001";
            when "01" =>
                output <= "0010";
            when "10" =>
                output <= "0100";
            when "11" =>
                output <= "1000";
            -- assign output to undefined if input is any other combination
            when others =>
                output <= "XXXX";
        end case;
    end process;
end CASE_STATEMENT;