library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

entity zx is 
port (a :in std_logic_vector (7 downto 0);
      b :out std_logic);
end zx;

architecture zx of zx is 
begin 
process (a)
begin 
case a(7 downto 6) is
when "11" => b<='1';
when "00" => b<='0';
when "10" => b<='1';
when "01" => b<='0';
end case ;
end process;
end zx;