diff --git a/include/picrin.h b/include/picrin.h
index 03209454..7f9015e4 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -13,6 +13,8 @@ struct pic_env {
 
 typedef struct {
   pic_value *sp;
+  pic_value *stbase, *stend;
+
   struct pic_env *global_env;
 } pic_state;
 
diff --git a/src/state.c b/src/state.c
index c1e08699..8666b8f8 100644
--- a/src/state.c
+++ b/src/state.c
@@ -20,7 +20,11 @@ pic_open()
   pic_state *pic;
 
   pic = (pic_state *)malloc(sizeof(pic_state));
-  pic->sp = (pic_value *)malloc(sizeof(pic_value) * 1024);
+
+  /* prepare VM stack */
+  pic->stbase = pic->sp = (pic_value *)malloc(sizeof(pic_value) * 1024);
+  pic->stend = pic->stbase + 1024;
+
   pic->global_env = pic_new_empty_env();
 
   return pic;