diff --git a/include/pocketpy/interpreter/frame.h b/include/pocketpy/interpreter/frame.h
index 118bf363..ce06e2c5 100644
--- a/include/pocketpy/interpreter/frame.h
+++ b/include/pocketpy/interpreter/frame.h
@@ -37,7 +37,7 @@ typedef struct Frame {
     struct Frame* f_back;   // TODO: set this
     const Bytecode* ip;
     const CodeObject* co;
-    PyObject* module_;
+    PyObject* module;
     PyObject* function;     // a function object or NULL (global scope)
     PyVar* p0;              // unwinding base
     PyVar* locals;          // locals base
@@ -46,7 +46,7 @@ typedef struct Frame {
 } Frame;
 
 
-Frame* Frame__new(Frame* f_back, const CodeObject* co, PyObject* module_, PyObject* function, PyVar* p0, PyVar* locals, const CodeObject* locals_co);
+Frame* Frame__new(const CodeObject* co, PyObject* module, PyObject* function, PyVar* p0, PyVar* locals, const CodeObject* locals_co);
 void Frame__delete(Frame* self);
 
 PK_INLINE int Frame__ip(const Frame* self){
@@ -64,11 +64,11 @@ PK_INLINE int Frame__iblock(const Frame* self){
 }
 
 PK_INLINE pk_NameDict* Frame__f_globals(Frame* self){
-    return self->module_->dict;
+    return self->module->dict;
 }
 
 PK_INLINE PyVar* Frame__f_globals_try_get(Frame* self, StrName name){
-    return pk_NameDict__try_get(self->module_->dict, name);
+    return pk_NameDict__try_get(self->module->dict, name);
 }
 
 PyVar* Frame__f_closure_try_get(Frame* self, StrName name);
diff --git a/include/pocketpy/interpreter/vm.h b/include/pocketpy/interpreter/vm.h
index 6ae7eb2b..903d5347 100644
--- a/include/pocketpy/interpreter/vm.h
+++ b/include/pocketpy/interpreter/vm.h
@@ -58,8 +58,11 @@ typedef struct pk_VM {
     
     // singleton objects
     PyVar True, False, None, NotImplemented, Ellipsis;
+    // last error
+    py_Error* last_error;
+    // last retval
+    PyVar last_retval;
 
-    PyObject* __last_exception;
     PyObject* __curr_class;
     PyObject* __cached_object_new;
     FuncDecl_ __dynamic_func_decl;
@@ -74,6 +77,18 @@ typedef struct pk_VM {
 void pk_VM__ctor(pk_VM* self);
 void pk_VM__dtor(pk_VM* self);
 
+void pk_VM__push_frame(pk_VM* self, Frame* frame);
+void pk_VM__pop_frame(pk_VM* self);
+
+typedef enum pk_FrameResult{
+    RES_ERROR,
+    RES_CALL,
+    RES_YIELD,
+    RES_RETURN
+} pk_FrameResult;
+
+pk_FrameResult pk_VM__run_top_frame(pk_VM* self);
+
 Type pk_VM__new_type(pk_VM* self, const char* name, Type base, PyObject* module, bool subclass_enabled);
 PyObject* pk_VM__new_module(pk_VM* self, const char* name, const char* package);
 
diff --git a/include/pocketpy/objects/object.h b/include/pocketpy/objects/object.h
index 10cd94d0..a906df06 100644
--- a/include/pocketpy/objects/object.h
+++ b/include/pocketpy/objects/object.h
@@ -22,7 +22,7 @@ static_assert(sizeof(PyObject) <= 16, "!(sizeof(PyObject) <= 16)");
 #define PK_OBJ_SIZEOF(T)            (sizeof(T) + 16)
 
 PyObject* PyObject__new(Type type, int size);
-void PyObject__delete(PyObject* self);
+void PyObject__delete(pk_VM* vm, PyObject* self);
 
 PK_INLINE PyVar PyVar__fromobj(PyObject* obj){
     PyVar retval = {
diff --git a/include/pocketpy/pocketpy.h b/include/pocketpy/pocketpy.h
index 1d2b451f..4531bb89 100644
--- a/include/pocketpy/pocketpy.h
+++ b/include/pocketpy/pocketpy.h
@@ -12,7 +12,7 @@ typedef struct py_Error py_Error;
 
 typedef int (*py_CFunction)(const PyVar*, int);
 
-extern pk_VM* pk_vm;
+extern pk_VM* pk_current_vm;
 
 void py_initialize();
 // void py_switch_vm(const char* name);
diff --git a/src/interpreter/frame.c b/src/interpreter/frame.c
index 28714691..3521b97c 100644
--- a/src/interpreter/frame.c
+++ b/src/interpreter/frame.c
@@ -39,13 +39,13 @@ void UnwindTarget__delete(UnwindTarget* self){
     free(self);
 }
 
-Frame* Frame__new(Frame* f_back, const CodeObject* co, PyObject* module_, PyObject* function, PyVar* p0, PyVar* locals, const CodeObject* locals_co){
+Frame* Frame__new(const CodeObject* co, PyObject* module, PyObject* function, PyVar* p0, PyVar* locals, const CodeObject* locals_co){
     static_assert(sizeof(Frame) <= kPoolFrameBlockSize, "!(sizeof(Frame) <= kPoolFrameBlockSize)");
     Frame* self = PoolFrame_alloc();
-    self->f_back = f_back;
+    self->f_back = NULL;
     self->ip = (Bytecode*)co->codes.data - 1;
     self->co = co;
-    self->module_ = module_;
+    self->module = module;
     self->function = function;
     self->p0 = p0;
     self->locals = locals;
diff --git a/src/interpreter/vm.c b/src/interpreter/vm.c
index 6609cbba..b0089607 100644
--- a/src/interpreter/vm.c
+++ b/src/interpreter/vm.c
@@ -154,6 +154,22 @@ void pk_VM__dtor(pk_VM* self){
     ValueStack__clear(&self->stack);
 }
 
+void pk_VM__push_frame(pk_VM* self, Frame* frame){
+    frame->f_back = self->top_frame;
+    self->top_frame = frame;
+}
+
+void pk_VM__pop_frame(pk_VM* self){
+    assert(self->top_frame);
+    Frame* frame = self->top_frame;
+    self->top_frame = frame->f_back;
+    Frame__delete(frame);
+}
+
+pk_FrameResult pk_VM__run_top_frame(pk_VM* self){
+    return RES_RETURN;
+}
+
 Type pk_VM__new_type(pk_VM* self, const char* name, Type base, PyObject* module, bool subclass_enabled){
     Type type = self->types.count;
     pk_TypeInfo* ti = c11_vector__emplace(&self->types);
@@ -165,7 +181,7 @@ Type pk_VM__new_type(pk_VM* self, const char* name, Type base, PyObject* module,
 
 /****************************************/
 void PyObject__delete(PyObject *self){
-    pk_TypeInfo* ti = c11__getitem(pk_TypeInfo*, &pk_vm->types, self->type);
+    pk_TypeInfo* ti = c11__getitem(pk_TypeInfo*, &pk_current_vm->types, self->type);
     if(ti->dtor) ti->dtor(PyObject__value_ptr(self));
     if(self->dict) pk_NameDict__delete(self->dict);
     if(self->gc_is_large){
diff --git a/src/pocketpy.c b/src/pocketpy.c
index f87bef30..afe4c94c 100644
--- a/src/pocketpy.c
+++ b/src/pocketpy.c
@@ -1,20 +1,40 @@
 #include "pocketpy/pocketpy.h"
 #include "pocketpy/objects/object.h"
 #include "pocketpy/interpreter/vm.h"
+#include <assert.h>
+#include <stdlib.h>
 
-pk_VM* pk_vm;
+pk_VM* pk_current_vm;
 static pk_VM pk_default_vm;
 
 void py_initialize(){
     Pools_initialize();
     pk_StrName__initialize();
-    pk_vm = &pk_default_vm;
+    pk_current_vm = &pk_default_vm;
     pk_VM__ctor(&pk_default_vm);
 }
 
+py_Error* py_exec_simple(const char* source){
+    CodeObject* co = NULL;
+    pk_VM* vm = pk_current_vm;
+    Frame* frame = Frame__new(
+        co,
+        vm->main,
+        NULL,
+        vm->stack.sp,
+        vm->stack.sp,
+        co
+    );
+    pk_VM__push_frame(vm, frame);
+    pk_FrameResult res = pk_VM__run_top_frame(vm);
+    if(res == RES_ERROR) return vm->last_error;
+    if(res == RES_RETURN) return NULL; // vm->last_retval;
+    assert(0);  // unreachable
+}
+
 void py_finalize(){
     pk_VM__dtor(&pk_default_vm);
-    pk_vm = NULL;
+    pk_current_vm = NULL;
     pk_StrName__finalize();
     Pools_finalize();
 }