entity foo is
  port (
    address : in  bit_vector (5 downto 0);
    e_n     : in  bit;
    data    : out mux_vector (7 downto 0) bus;
    vdd     : in bit;
    vss     : in bit);
end foo;

architecture VBE of foo is

  signal rom_out : bit_vector (7 downto 0);

begin

  write_out : block (e_n = '0')
  begin
    data <= guarded rom_out;
  end block;

  with address (5 downto 0) select
  rom_out <=
          X"00" when others;

end;