diff --git a/src/d_dehacked.cpp b/src/d_dehacked.cpp
index 44e675ed2..89ac12401 100644
--- a/src/d_dehacked.cpp
+++ b/src/d_dehacked.cpp
@@ -749,6 +749,7 @@ static void (*MBFCodePointerFactories[])(FunctionCallEmitter&, int, int) =
 
 void SetDehParams(FState *state, int codepointer)
 {
+	static uint8_t regts[] = { REGT_POINTER, REGT_POINTER, REGT_POINTER };
 	int value1 = state->GetMisc1();
 	int value2 = state->GetMisc2();
 	if (!(value1|value2)) return;
@@ -788,6 +789,7 @@ void SetDehParams(FState *state, int codepointer)
 		buildit.Emit(OP_RET, RET_FINAL, REGT_NIL, 0);
 		// Attach it to the state.
 		VMScriptFunction *sfunc = new VMScriptFunction;
+		sfunc->RegTypes = regts;	// These functions are built after running the script compiler so they don't get this info.
 		buildit.MakeFunction(sfunc);
 		sfunc->NumArgs = numargs;
 		sfunc->ImplicitArgs = numargs;
diff --git a/src/events.cpp b/src/events.cpp
index 8dd9baa96..6ae49f04c 100755
--- a/src/events.cpp
+++ b/src/events.cpp
@@ -688,7 +688,7 @@ DEFINE_EMPTY_HANDLER(DStaticEventHandler, WorldLineDamaged);
 DEFINE_EMPTY_HANDLER(DStaticEventHandler, WorldLightning)
 DEFINE_EMPTY_HANDLER(DStaticEventHandler, WorldTick)
 
-DEFINE_EMPTY_HANDLER(DStaticEventHandler, RenderFrame)
+//DEFINE_EMPTY_HANDLER(DStaticEventHandler, RenderFrame)
 DEFINE_EMPTY_HANDLER(DStaticEventHandler, RenderOverlay)
 
 DEFINE_EMPTY_HANDLER(DStaticEventHandler, PlayerEntered)
@@ -972,6 +972,7 @@ static FRenderEvent E_SetupRenderEvent()
 
 void DStaticEventHandler::RenderFrame()
 {
+	/* This is intentionally and permanently disabled.
 	IFVIRTUAL(DStaticEventHandler, RenderFrame)
 	{
 		// don't create excessive DObjects if not going to be processed anyway
@@ -981,6 +982,7 @@ void DStaticEventHandler::RenderFrame()
 		VMValue params[2] = { (DStaticEventHandler*)this, &e };
 		VMCall(func, params, 2, nullptr, 0);
 	}
+	*/
 }
 
 void DStaticEventHandler::RenderOverlay(EHudState state)
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index 1e4d412a2..ff48ab79d 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -4689,14 +4689,6 @@ void AActor::Tick ()
 	}
 }
 
-DEFINE_ACTION_FUNCTION(AActor, Tick)
-{
-	PARAM_SELF_PROLOGUE(AActor);
-	self->Tick();
-	return 0;
-}
-
-
 //==========================================================================
 //
 // AActor :: CheckNoDelay
@@ -8282,13 +8274,6 @@ DEFINE_ACTION_FUNCTION(AActor, SetDamage)
 	return 0;
 }
 
-DEFINE_ACTION_FUNCTION(AActor, GetDefaultByType)
-{
-	PARAM_PROLOGUE;
-	PARAM_CLASS(cls, AActor);
-	ACTION_RETURN_OBJECT(cls == nullptr? nullptr : GetDefaultByType(cls));
-}
-
 // This combines all 3 variations of the internal function
 DEFINE_ACTION_FUNCTION(AActor, VelFromAngle)
 {
diff --git a/src/p_sectors.cpp b/src/p_sectors.cpp
index 1a95f051d..1ed2c78d6 100644
--- a/src/p_sectors.cpp
+++ b/src/p_sectors.cpp
@@ -1899,13 +1899,6 @@ DEFINE_ACTION_FUNCTION(_Sector, NextLowestFloorAt)
 	 ACTION_RETURN_INT(self->GetLightLevel());
  }
 
- DEFINE_ACTION_FUNCTION(_Sector, ClearSpecial)
- {
-	 PARAM_SELF_STRUCT_PROLOGUE(sector_t);
-	 self->ClearSpecial();
-	 return 0;
- }
-
  DEFINE_ACTION_FUNCTION(_Sector, PortalBlocksView)
  {
 	 PARAM_SELF_STRUCT_PROLOGUE(sector_t);
diff --git a/src/scripting/backend/codegen.cpp b/src/scripting/backend/codegen.cpp
index f58fd7719..aba8102c1 100644
--- a/src/scripting/backend/codegen.cpp
+++ b/src/scripting/backend/codegen.cpp
@@ -259,7 +259,7 @@ void ExpEmit::Reuse(VMFunctionBuilder *build)
 //
 //==========================================================================
 
-static PSymbol *FindBuiltinFunction(FName funcname, VMNativeFunction::NativeCallType func)
+static PSymbol *FindBuiltinFunction(FName funcname, VMNativeFunction::NativeCallType func, const uint8_t *reginfo)
 {
 	PSymbol *sym = Namespaces.GlobalNamespace->Symbols.FindSymbol(funcname, false);
 	if (sym == nullptr)
@@ -267,6 +267,7 @@ static PSymbol *FindBuiltinFunction(FName funcname, VMNativeFunction::NativeCall
 		PSymbolVMFunction *symfunc = Create<PSymbolVMFunction>(funcname);
 		VMNativeFunction *calldec = new VMNativeFunction(func, funcname);
 		calldec->PrintableName = funcname.GetChars();
+		calldec->RegTypes = reginfo;
 		symfunc->Function = calldec;
 		sym = symfunc;
 		Namespaces.GlobalNamespace->Symbols.AddSymbol(sym);
@@ -5487,7 +5488,8 @@ ExpEmit FxRandom::Emit(VMFunctionBuilder *build)
 {
 	// Call DecoRandom to generate a random number.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom, BuiltinRandom);
+	static const uint8_t reginfo[] = { REGT_POINTER, REGT_INT, REGT_INT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom, BuiltinRandom, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -5595,7 +5597,8 @@ ExpEmit FxRandomPick::Emit(VMFunctionBuilder *build)
 
 	// Call BuiltinRandom to generate a random number.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom, BuiltinRandom);
+	static const uint8_t reginfo[] = { REGT_POINTER, REGT_INT, REGT_INT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom, BuiltinRandom, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -5714,7 +5717,8 @@ ExpEmit FxFRandom::Emit(VMFunctionBuilder *build)
 {
 	// Call the BuiltinFRandom function to generate a floating point random number..
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinFRandom, BuiltinFRandom);
+	static uint8_t reginfo[] = { REGT_POINTER, REGT_FLOAT, REGT_FLOAT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinFRandom, BuiltinFRandom, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -5793,7 +5797,8 @@ ExpEmit FxRandom2::Emit(VMFunctionBuilder *build)
 {
 	// Call the BuiltinRandom function to generate the random number.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom2, BuiltinRandom2);
+	static uint8_t reginfo[] = { REGT_POINTER, REGT_INT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandom2, BuiltinRandom2, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -5865,7 +5870,8 @@ ExpEmit FxRandomSeed::Emit(VMFunctionBuilder *build)
 {
 	// Call DecoRandom to generate a random number.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandomSeed, BuiltinRandomSeed);
+	static uint8_t reginfo[] = { REGT_POINTER, REGT_INT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinRandomSeed, BuiltinRandomSeed, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -8567,7 +8573,8 @@ ExpEmit FxActionSpecialCall::Emit(VMFunctionBuilder *build)
 	}
 	// Call the BuiltinCallLineSpecial function to perform the desired special.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinCallLineSpecial, BuiltinCallLineSpecial);
+	static uint8_t reginfo[] = { REGT_INT, REGT_POINTER, REGT_INT, REGT_INT, REGT_INT, REGT_INT, REGT_INT };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinCallLineSpecial, BuiltinCallLineSpecial, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -10776,7 +10783,8 @@ ExpEmit FxClassTypeCast::Emit(VMFunctionBuilder *build)
 
 	// Call the BuiltinNameToClass function to convert from 'name' to class.
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinNameToClass, BuiltinNameToClass);
+	static uint8_t reginfo[] = { REGT_INT, REGT_POINTER };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinNameToClass, BuiltinNameToClass, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
@@ -10885,7 +10893,8 @@ ExpEmit FxClassPtrCast::Emit(VMFunctionBuilder *build)
 	emitters.AddParameterPointerConst(desttype);
 
 	VMFunction *callfunc;
-	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinClassCast, BuiltinClassCast);
+	static uint8_t reginfo[] = { REGT_POINTER, REGT_POINTER };
+	PSymbol *sym = FindBuiltinFunction(NAME_BuiltinClassCast, BuiltinClassCast, reginfo);
 
 	assert(sym->IsKindOf(RUNTIME_CLASS(PSymbolVMFunction)));
 	assert(((PSymbolVMFunction *)sym)->Function != nullptr);
diff --git a/src/scripting/backend/vmbuilder.cpp b/src/scripting/backend/vmbuilder.cpp
index 1aa5c09f7..f11feab86 100644
--- a/src/scripting/backend/vmbuilder.cpp
+++ b/src/scripting/backend/vmbuilder.cpp
@@ -909,6 +909,7 @@ void FFunctionBuildList::Build()
 		fprintf(dump, "\n*************************************************************************\n%i code bytes\n%i data bytes", codesize * 4, datasize);
 		fclose(dump);
 	}
+	VMFunction::CreateRegUseInfo();
 	FScriptPosition::StrictErrors = false;
 	if (Args->CheckParm("-dumpjit")) DumpJit();
 	mItems.Clear();
diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index 6d62e7081..531af0701 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -943,6 +943,8 @@ void InitThingdef()
 	auto fcp = NewStruct("FCheckPosition", nullptr);
 	fcp->mConstructor = *FindFunction(fcp, "_Constructor")->VMPointer;
 	fcp->mDestructor = *FindFunction(fcp, "_Destructor")->VMPointer;
+	static const uint8_t reguse[] = { REGT_POINTER };
+	fcp->mConstructor->RegTypes = fcp->mDestructor->RegTypes = reguse;
 	fcp->Size = sizeof(FCheckPosition);
 	fcp->Align = alignof(FCheckPosition);
 
@@ -991,11 +993,6 @@ void SynthesizeFlagFields()
 		}
 	}
 }
-DEFINE_ACTION_FUNCTION(DObject, GameType)
-{
-	PARAM_PROLOGUE;
-	ACTION_RETURN_INT(gameinfo.gametype);
-}
 
 DEFINE_ACTION_FUNCTION(DObject, BAM)
 {
diff --git a/src/scripting/vm/vm.h b/src/scripting/vm/vm.h
index ab1fdad71..f1266bd51 100644
--- a/src/scripting/vm/vm.h
+++ b/src/scripting/vm/vm.h
@@ -319,10 +319,11 @@ class VMFunction
 {
 public:
 	bool Unsafe = false;
-	int VarFlags = 0; // [ZZ] this replaces 5+ bool fields
 	uint8_t ImplicitArgs = 0;	// either 0 for static, 1 for method or 3 for action
+	int VarFlags = 0; // [ZZ] this replaces 5+ bool fields
 	unsigned VirtualIndex = ~0u;
 	FName Name;
+	const uint8_t *RegTypes = nullptr;
 	TArray<VMValue> DefaultArgs;
 	FString PrintableName;	// so that the VM can print meaningful info if something in this function goes wrong.
 
@@ -352,8 +353,16 @@ public:
 		}
 		AllFunctions.Clear();
 	}
+	static void CreateRegUseInfo()
+	{
+		for (auto f : AllFunctions)
+		{
+			f->CreateRegUse();
+		}
+	}
 	static TArray<VMFunction *> AllFunctions;
 protected:
+	void CreateRegUse();
 };
 
 class VMNativeFunction : public VMFunction
diff --git a/src/scripting/vm/vmexec.cpp b/src/scripting/vm/vmexec.cpp
index 5b58180a8..3424b81d7 100644
--- a/src/scripting/vm/vmexec.cpp
+++ b/src/scripting/vm/vmexec.cpp
@@ -193,17 +193,20 @@ void VMFillParams(VMValue *params, VMFrame *callee, int numparam)
 	const VMRegisters calleereg(callee);
 
 	assert(calleefunc != NULL && !(calleefunc->VarFlags & VARF_Native));
-	assert(numparam == calleefunc->NumArgs || ((int)calleefunc->DefaultArgs.Size() == calleefunc->NumArgs));
+	assert(numparam == calleefunc->NumArgs);
 	assert(REGT_INT == 0 && REGT_FLOAT == 1 && REGT_STRING == 2 && REGT_POINTER == 3);
 
 	regd = regf = regs = rega = 0;
-	for (int i = 0; i < calleefunc->NumArgs; ++i)
+	const uint8_t *reginfo = calleefunc->RegTypes;
+	assert(reginfo != nullptr);
+	for (int i = 0; i < calleefunc->NumArgs; ++i, reginfo++)
 	{
-		// get all actual parameters and fill the rest from the defaults.
-		VMValue &p = i < numparam? params[i] : calleefunc->DefaultArgs[i];
-		if (p.Type < REGT_STRING)
+		// copy all parameters to the local registers.
+		VMValue &p = params[i];
+		assert(*reginfo == p.Type);
+		if (*reginfo < REGT_STRING)
 		{
-			if (p.Type == REGT_INT)
+			if (*reginfo == REGT_INT)
 			{
 				calleereg.d[regd++] = p.i;
 			}
@@ -212,13 +215,13 @@ void VMFillParams(VMValue *params, VMFrame *callee, int numparam)
 				calleereg.f[regf++] = p.f;
 			}
 		}
-		else if (p.Type == REGT_STRING)
+		else if (*reginfo == REGT_STRING)
 		{
 			calleereg.s[regs++] = p.s();
 		}
 		else
 		{
-			assert(p.Type == REGT_POINTER);
+			assert(*reginfo == REGT_POINTER);
 			calleereg.a[rega++] = p.a;
 		}
 	}
diff --git a/src/scripting/vm/vmframe.cpp b/src/scripting/vm/vmframe.cpp
index 0a69c8c2b..0f9da7383 100644
--- a/src/scripting/vm/vmframe.cpp
+++ b/src/scripting/vm/vmframe.cpp
@@ -65,6 +65,42 @@ IMPLEMENT_CLASS(VMException, false, false)
 
 TArray<VMFunction *> VMFunction::AllFunctions;
 
+// Creates the register type list for a function.
+// Native functions only need this to assert their parameters in debug mode, script functions use this to load their registers from the VMValues.
+void VMFunction::CreateRegUse()
+{
+#ifdef NDEBUG
+	if (VarFlags & VARF_Native) return;	// we do not need this for native functions in release builds.
+#endif
+	int count = 0;
+	if (!Proto)
+	{
+		if (RegTypes) return;
+		Printf(TEXTCOLOR_ORANGE "Function without prototype needs register info manually set: %s\n", PrintableName.GetChars());
+		return;
+	}
+	assert(Proto->isPrototype());
+	for (auto arg : Proto->ArgumentTypes)
+	{
+		count += arg? arg->GetRegCount() : 1;
+	}
+	uint8_t *regp;
+	RegTypes = regp = (uint8_t*)ClassDataAllocator.Alloc(count);
+	count = 0;
+	for (auto arg : Proto->ArgumentTypes)
+	{
+		if (arg == nullptr)
+		{
+			// Marker for start of varargs.
+			*regp++ = REGT_NIL;
+		}
+		else for (int i = 0; i < arg->GetRegCount(); i++)
+		{
+			*regp++ = arg->GetRegType();
+		}
+	}
+}
+
 VMScriptFunction::VMScriptFunction(FName name)
 {
 	Name = name;
diff --git a/wadsrc/static/zscript/mapdata.txt b/wadsrc/static/zscript/mapdata.txt
index 3440d4b24..9f58c0724 100644
--- a/wadsrc/static/zscript/mapdata.txt
+++ b/wadsrc/static/zscript/mapdata.txt
@@ -34,6 +34,7 @@ struct SectorPortal native play
 struct Vertex native play
 {
 	native readonly Vector2 p;
+	native int Index();
 }
 
 struct Side native play