[tom_mellior]

Typically an undefined instruction "can't happen" because the interpreter assumes that it gets its code from a trusted frontend that validated all the invariants assumed by the interpreter. The most important being that, for any instruction that might use DISPATCH, there actually is a next instruction at the following address. This means that every block of code must end in an unconditional branch and that unconditional branches must not dispatch using DISPATCH (which would not make sense anyway).