Structural implementation of the N-bit adder :
library IEEE;
use IEEE.std_logic_1164.all;

entity adderN is
     generic(N : integer := 16);                             -- The width of the adder is determined by generic N
         port (a : in std_logic_vector(N downto 1);
                  b : in std_logic_vector(N downto 1);
                  cin : in std_logic;
                  sum : out std_logic_vector(N downto 1);
                  cout : out std_logic);
end adderN;

architecture structural of adderN is
component adder
      port (a : in std_logic;
               b : in std_logic;
               cin : in std_logic;
               sum : out std_logic;
               cout : out std_logic);
end component;
signal carry : std_logic_vector(0 to N);
    begin
            carry(0) <= cin;
            cout <= carry(N);
-- instantiate a single-bit adder N times
        gen: for I in 1 to N generate
                    add: adder port map(a => a(I), b => b(I), cin => carry(I - 1), sum => sum(I), cout => carry(I));
                 end generate;
end structural;

Behavioral implementation of the N-bit adder :
library IEEE;
use IEEE.std_logic_1164.all;

entity adderN is
     generic(N : integer := 16);                 -- The width of the adder is determined by generic N
         port (a : in std_logic_vector(N downto 1);
                  b : in std_logic_vector(N downto 1);
                  cin : in std_logic;
                  sum : out std_logic_vector(N downto 1);
                  cout : out std_logic);
end adderN;

architecture behavioral of adderN is
begin
     p1: process(a, b, cin)
                variable vsum : std_logic_vector(N downto 1);
                variable carry : std_logic;
                begin
                       carry := cin;
                            for i in 1 to N loop
                               vsum(i) := (a(i) xor b(i)) xor carry;
                               carry := (a(i) and b(i)) or (carry and (a(i) or b(i)));
                            end loop;
                       sum <= vsum;
                      cout <= carry;
     end process p1;
end behavioral;


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