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-- Example of a four bit adder

library  ieee;

use  ieee.std_logic_1164.all;

-- definition of a full adder

entity FULLADDER is

     port (a, b, c: in std_logic;

           sum, carry: out std_logic);

end FULLADDER;

architecture fulladder_behav of FULLADDER is

begin

   sum <= (a xor b) xor c ;

   carry <= (a and b) or (c and (a xor b));

end fulladder_behav;

-- 4-bit adder

library  ieee;

use  ieee.std_logic_1164.all;

entity FOURBITADD is

     port (a, b: in std_logic_vector(3 downto 0);

           Cin : in std_logic;

           sum: out std_logic_vector (3 downto 0);

           Cout, V: out std_logic);

end FOURBITADD;

architecture fouradder_structure of FOURBITADD is

     signal c: std_logic_vector (4 downto 0);

     component FULLADDER

          port(a, b, c: in std_logic;

               sum, carry: out std_logic);

     end component;

begin

   FA0: FULLADDER

        port map (a(0), b(0), Cin, sum(0), c(1));

   FA1: FULLADDER

        port map (a(1), b(1), C(1), sum(1), c(2));

   FA2: FULLADDER

        port map (a(2), b(2), C(2), sum(2), c(3));

   FA3: FULLADDER

        port map (a(3), b(3), C(3), sum(3), c(4));

   V <= c(3) xor c(4);

   Cout <= c(4);

end fouradder_structure;