library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity MUX_2_1 is
Port ( a,b,S : in std_logic;
O : out std_logic;
end MUX_2_1;

architecture Behavioral of MUX_2_1 is
begin
if (S='0') then
    O <= a;
else
    O <= b;
end if;

end Behavioral;