Make a first sketch of the coroutine-esque structure I'm going to experiment with here.

Processors/68000Mk2/Implementation/68000Mk2Implementation.hpp

@@ -12,6 +12,57 @@
 namespace CPU {
 namespace MC68000Mk2 {
 
+template <class BusHandler, bool dtack_is_implicit, bool permit_overrun, bool signal_will_perform>
+void Processor<BusHandler, dtack_is_implicit, permit_overrun, signal_will_perform>::run_for(HalfCycles duration) {
+	// Accumulate the newly paid-in cycles. If this instance remains in deficit, exit.
+	time_remaining += duration;
+	if(time_remaining <= HalfCycles(0)) return;
+
+	// Check whether all remaining time has been expended; if so then exit, having set this line up as
+	// the next resumption point.
+#define ConsiderExit()	if(time_remaining <= HalfCycles(0)) { state = __LINE__; return; } [[fallthrough]]; case __LINE__:
+
+	// Subtracts `n` half-cycles from `time_remaining`; if permit_overrun is false, also ConsiderExit()
+#define Spend(n)		cycles_owed -= HalfCycles(n); if constexpr (!permit_overrun) ConsiderExit()
+
+	// Performs ConsiderExit() only if permit_overrun is true
+#define CheckOverrun()	if constexpr (permit_overrun) ConsiderExit()
+
+	//
+	// So structure is, in general:
+	//
+	//	case Action:
+	//		do_something();
+	//		Spend(20);
+	//		do_something_else();
+	//		Spend(10);
+	//		do_a_third_thing();
+	//		Spend(30);
+	//		CheckOverrun();
+	//
+	//		state = next_action;
+	//		break;
+	//
+	// Additional notes:
+	//
+	//	Action and all equivalents should be negative values, since the
+	//	switch-for-computed-goto-for-a-coroutine structure uses __LINE__ for
+	//	its invented entry- and exit-points, meaning that negative numbers are
+	//	the easiest group that is safely definitely never going to collide.
+
+	// Otherwise continue for all time, until back in debt.
+	// Formatting is slightly obtuse here to make this look more like a coroutine.
+	while(true) { switch(state) {
+
+
+	}}
+
+#undef CheckOverrun
+#undef Spend
+#undef ConsiderExit
+
+}
+
 }
 }
 

Processors/68000Mk2/Implementation/68000Mk2Storage.hpp

@@ -13,7 +13,12 @@ namespace CPU {
 namespace MC68000Mk2 {
 
 struct ProcessorBase {
+	enum State: int {
+		Reset = -1,
+	};
 
+	HalfCycles time_remaining;
+	int state = State::Reset;
 };
 
 }