rhettinger, helper functions used by the instruction code would also be moved into instructions.h (e.g. call_function, do_call_core, cmp_outcome, etc). Done properly, there should be very little need to move between instructions.h and ceval.c to understand what is happening.

The contract between the two files has to be very explicit. The only dependencies instructions.h would have on ceval.c would be to receive the evaluator macros (for manipulating the stack & locals, flow control, triggering error condition, etc). A strict (non-leaky) abstraction interface is necessary for being able to re-use the instruction code in an alternate evaluator.