VHDL code for half adder

1. VHDL code for half adder using Dataflow modelling:

library ieee;
use ieee.std_logic_1164.all;

entity half_adder is
  port (a, b: in std_logic;
    sum, carry_out: out std_logic); 
  end half_adder;

architecture dataflow of half_adder is 
  begin
  sum <= a xor b;
  carry_out <= a and b; 
end dataflow;

2. VHDL code for half adder using Structural modelling:

library ieee;
use ieee.std_logic_1164.all;

entity half_adder is 					-- Entity declaration for half adder 
  port (a, b: in std_logic;
    sum, carry_out: out std_logic);
end half_adder;

architecture structure of half_adder is 	-- Architecture body for half adder

  component xor_gate					-- xor component declaration
    port (i1, i2: in std_logic; 
      o1: out std_logic);
  end component;

  component and_gate					-- and component declaration
    port (i1, i2: in std_logic;
      o1: out std_logic);
  end component; 

begin
   u1: xor_gate port map (i1 => a, i2 => b, o1 => sum);
   u2: and_gate port map (i1 => a, i2 => b, o1 => carry_out);
-- We can also use Positional Association
--	=> u1: xor_gate port map (a, b, sum); 
--	=> u2: and_gate port map (a, b, carry_out);
end structure;

3. VHDL code for half adder using Behavioral modelling:

library ieee;
use ieee.std_logic_1164.all;

entity half_adder is
  port (a, b: in std_logic;
    sum, carry_out: out std_logic); 
  end half_adder;

architecture behavior of half_adder is 
  begin
    ha: process (a, b)
    begin
      if a = ‘1’ then
        sum <= not b;
        carry_out <= b;
      else
        sum <= b;
        carry_out <= ‘0’; 
      end if;
    end process ha;

end behavior;