library IEEE;
use IEEE.STD_LOGIC_1164.all;
entity encoder is
port(
a : in STD_LOGIC_VECTOR(3 downto 0);
b : out STD_LOGIC_VECTOR(1 downto 0)
);
end encoder;
architecture bhv of encoder is
begin
process(a)
begin
case a is
when "1000" => b <= "00"; 
when "0100" => b <= "01"; 
when "0010" => b <= "10"; 
when "0001" => b <= "11"; 
when others => b <= "ZZ";
end case;
end process;
end bhv;