Full adder16. Oct '14

Full adder is essentially composed of two half adders.

a b ci co s

Full adder constructed using two XOR, two OR and one AND gate

Full adder can be described in VHDL:

library ieee;
use ieee.std_logic_1164.all;

entity full_adder is
    port (
        a  : in  std_logic;
        b  : in  std_logic;
        ci : in  std_logic;
        s  : out std_logic;
        co : out std_logic);
end;

architecture behavioral of full_adder is
begin
    s <= a xor b xor ci;
    co <= (a and b) or ((a xor b) and ci);
end;

Full adder can also be implemented using two 4:1 muxes:

4:1 4:1 Carry out B Sum Carry in 0 1 A

Full adder using two 4:1 muxes

GCC VHDL KTH GHDL