What would cause single step debugging through a module under the control of the ISD51 monitor to suddenly start stepping at 2 line intervals, and refusing to breakpoint at many locations? (The program suddenly takes on this behaviour, having started out fine.)
well the answer is: if you have soft timers, where the tick timer is at a high priority than the ISD interrupt, the latter gets lost (regularly) - this manifests itself as single steps of more than 1 instruction.