diff --git a/include/pocketpy/vm.h b/include/pocketpy/vm.h
index f8ccefa8..9c92c61e 100644
--- a/include/pocketpy/vm.h
+++ b/include/pocketpy/vm.h
@@ -117,6 +117,9 @@ public:
     NameDict _modules;                                 // loaded modules
     std::map<StrName, Str> _lazy_modules;              // lazy loaded modules
 
+    PyObject* _reg[32];     // registers for user purpose, also used by C-API
+    static constexpr int REG_COUNT = sizeof(_reg) / sizeof(void*);
+
     PyObject* None;
     PyObject* True;
     PyObject* False;
diff --git a/src/pocketpy_c.cpp b/src/pocketpy_c.cpp
index bc90d03d..fca6f1ce 100644
--- a/src/pocketpy_c.cpp
+++ b/src/pocketpy_c.cpp
@@ -50,18 +50,20 @@ struct LuaStack: public ValueStackImpl<32>{
         return false; \
     }
 
-class CVM;
-void gc_marker_ex(CVM* vm);
 
 class CVM: public VM {
 public:
-    LuaStack c_data;
+    LuaStack c_data;        // operation stack
     PyObject* error;
 
     CVM(bool use_stdio, bool enable_os) : VM(enable_os) {
-        c_data = new LuaStack();
         error = nullptr;
-        heap._gc_marker_ex = (void (*)(VM*)) gc_marker_ex;
+        heap._gc_marker_ex = [](VM* vm_) {
+            CVM* vm = (CVM*)vm_;
+            for(PyObject* obj: vm->c_data) if(obj!=nullptr) PK_OBJ_MARK(obj);
+            if(vm->error != nullptr) PK_OBJ_MARK(vm->error);
+        };
+
         if (!use_stdio) {
             _stdout = _stderr = [](VM* vm, const Str& s){
                 PK_UNUSED(vm);
@@ -71,10 +73,6 @@ public:
     }
 };
 
-void gc_marker_ex(CVM* vm) {
-    for(PyObject* obj: *vm->c_data) if(obj!=nullptr) PK_OBJ_MARK(obj);
-    if(vm->error != nullptr) PK_OBJ_MARK(vm->error);
-}
 
 //for now I will unpack a tuple automatically, we may not want to handle
 //it this way, not sure
diff --git a/src/vm.cpp b/src/vm.cpp
index fed28252..47284c01 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -4,6 +4,7 @@ namespace pkpy{
 
     VM::VM(bool enable_os) : heap(this), enable_os(enable_os) {
         this->vm = this;
+        for(int i=0; i<REG_COUNT; i++) _reg[i] = nullptr;
         _stdout = [](VM* vm, const Str& s) {
             PK_UNUSED(vm);
             std::cout << s;
@@ -999,6 +1000,9 @@ void ManagedHeap::mark() {
     for(PyObject* obj: vm->s_data) PK_OBJ_MARK(obj);
     if(_gc_marker_ex) _gc_marker_ex(vm);
     if(vm->_last_exception) PK_OBJ_MARK(vm->_last_exception);
+    for(int i=0; i<vm->REG_COUNT; i++){
+        if(vm->_reg[i] != nullptr) PK_OBJ_MARK(vm->_reg[i]);
+    }
 }
 
 Str obj_type_name(VM *vm, Type type){