Are soundness and completeness necessary for decidability? Is it necessary for a logical system to be both sound and complete in order to be decidable? 
 A: If you take, say, the classical semantics of propositional logic as granted, then it's very easy to define a formal system that is neither sound nor complete, yet perfectly decidable.
For example, consider this system:


*

*A well-formed formula is a logical axiom iff it contains exactly one negation sign.

*There are no rules of inference.


This is clearly decidable: a formula is provable exactly if it is an axiom, which is easy to check. But it is not sound (because it proves $A\land\neg A$), and it is not complete (because $\neg A\lor\neg\neg A$ is not provable, yet it is valid -- remember that we're assuming the usual semantics).

A thornier question is: Given a deductive system, can we find a semantics that it is sound and complete with respect to? The main problem here is to delineate exactly what would qualify as a "semantics". If we are too strict here, we might find ourselves excluding even tested and true ideas such as Kripke frames. On the other hand if we're too liberal the question could easily become trivial, because one could just slap the name "semantics" onto the formal system itself or something that simulates it.
