As has been discussed before, the Stamp 2sx is basically just a glorified Ubicom SX microcontroller. The interesting thing about that is that the SX doesn't include SIN or COS either, so something about the extra cicuitry included with the Stamp allows them to implement these things in very minimal program space (the ATN function doesn't seem to take up much more space than a simple +). How exactly this works, I have no idea
.