It really depends on how much you want to spend.You can store the code in ram as an excrypted version of the actual code and decode the instructions internally with some additional hardware which is what dallas do with some of thier controllers and I offer the same sort of thing on my cores with a very fast IDEA engine,and you could make that as secure as you like using AES,3-des or idea or whatever but there are other ways for someone who is determined enough to still find out what your code is. The trouble is real security is not easy to achieve faced with a determined attacker.