4位全加器VHDL描述



转自http://www.seas.upenn.edu/~ese171/vhdl/vhdl_primer.html#_Toc526061350

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;