process(clock)
-- Declare the format and values of the array of constants
type t_my_table is array(0 to 11) of natural range 0 to 255; -- Or use std_logic_vector(7 downto 0) rather than natural range 0 to 255 if you prefer
constant my_table: t_my_table := (55, 10, 18, 0, 0, 55, 31, 250, 0, 0, 1, 12);
begin
if rising_edge(clock) then
data_out <= my_table(address); -- Assuming address and data_out to be type natural. If something else, then add type conversions as appropriate
end if;
end process;