--mux2
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
use IEEE.std_logic_arith.all;
entity mux2 is
 port(a,b,en:in std_logic;
        y:out std_logic);
 end mux2;
architecture behav of mux2 is
begin
y<=b when en='1' else a;
end behav;