library ieee;
use ieee.std_logic_1164.all;

entity if01 is
  port (a : std_logic;
        b : std_logic;
        sel : std_logic;
        s : out std_logic);
end if01;

architecture behav of if01 is
begin
  s <= a when sel = '0'
       else b when sel = '1';
end behav;