Oops, my mistake, as Jonah Cohen pointed out...it DOES in fact lock the calc. For some reason I thought I'd tried it and it didn't... But pulling a battery and putting it back in fixes it, without clearing the ram. Quite interesting. (F376C9) > interrupt (which I think is an NMI, but can't say for sure...but if not, > shouldn't DI \ HALT lock the calc?). Which is why it sits at the HALT until