diff --git a/main.cpp b/main.cpp
index c2f791f..d5e3859 100644
--- a/main.cpp
+++ b/main.cpp
@@ -12,7 +12,7 @@ const uint32_t VM_OPTIMIZE =0x100;
 
 void help()
 {
-    std::cout
+    std::clog
     <<"     ,--#-,\n"
     <<"<3  / \\____\\  <3\n"
     <<"    |_|__A_|\n"
@@ -47,7 +47,7 @@ void help()
 
 void logo()
 {
-    std::cout
+    std::clog
     <<"       __                _\n"
     <<"    /\\ \\ \\__ _ ___  __ _| |\n"
     <<"   /  \\/ / _` / __|/ _` | |\n"
@@ -64,7 +64,7 @@ void logo()
 
 void err()
 {
-    std::cout
+    std::cerr
     <<"invalid argument(s).\n"
     <<"use <nasal -h> to get help.\n";
     std::exit(1);
@@ -112,7 +112,7 @@ void execute(const std::string& file,const std::vector<std::string>& argv,const
         auto start=std::chrono::high_resolution_clock::now();
         vm.run(gen,linker,argv,cmd&VM_OPCALLNUM,cmd&VM_DBGINFO);
         auto end=std::chrono::high_resolution_clock::now();
-        std::cout<<"process exited after "<<(end-start).count()*1.0/std::chrono::high_resolution_clock::duration::period::den<<"s.\n";
+        std::clog<<"process exited after "<<(end-start).count()*1.0/std::chrono::high_resolution_clock::duration::period::den<<"s.\n";
     }
     else if(cmd&VM_EXEC)
         vm.run(gen,linker,argv,cmd&VM_OPCALLNUM,cmd&VM_DBGINFO);
diff --git a/nasal_gc.h b/nasal_gc.h
index 102847b..a051106 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -1,7 +1,7 @@
 #ifndef __NASAL_GC_H__
 #define __NASAL_GC_H__
 
-enum vm_type:std::uint32_t{
+enum vm_type:std::uint8_t{
     /* none-gc object */
     vm_none=0,
     vm_cnt,
@@ -76,12 +76,6 @@ struct nasal_ref
     // vm_addr
     nasal_ref(const uint8_t t,nasal_ref* n):type(t){val.addr=n;}
     nasal_ref(const nasal_ref& nr):type(nr.type),val(nr.val){}
-    nasal_ref& operator=(const nasal_ref& nr)
-    {
-        type=nr.type;
-        val=nr.val;
-        return *this;
-    }
     bool operator==(const nasal_ref& nr){return type==nr.type && val.gcobj==nr.val.gcobj;}
     bool operator!=(const nasal_ref& nr){return type!=nr.type || val.gcobj!=nr.val.gcobj;}
     // number and string can be translated to each other
diff --git a/nasal_vm.h b/nasal_vm.h
index f81e1aa..8c603bb 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -154,6 +154,10 @@ void nasal_vm::init(
     funcr=upvalr=nil;
     canary=stack+STACK_DEPTH-1; // stack[STACK_DEPTH-1]
     top=stack;
+
+    /* clear main stack */
+    for(uint32_t i=0;i<STACK_DEPTH;++i)
+        stack[i]=nil;
 }
 void nasal_vm::valinfo(nasal_ref& val)
 {
@@ -325,20 +329,20 @@ void nasal_vm::opcallsort(const uint64_t* arr)
     std::sort(opcall.begin(),opcall.end(),
         [](const op& a,const op& b){return a.second>b.second;}
     );
-    std::cout<<"\noperands call info";
+    std::clog<<"\noperands call info";
     for(auto& i:opcall)
     {
         uint64_t rate=i.second*100/total;
         if(rate)
-            std::cout<<"\n "<<code_table[i.first].name
+            std::clog<<"\n "<<code_table[i.first].name
                      <<" : "<<i.second<<" ("<<rate<<"%)";
         else
         {
-            std::cout<<"\n ...";
+            std::clog<<"\n ...";
             break;
         }
     }
-    std::cout<<"\n total  : "<<total<<'\n';
+    std::clog<<"\n total  : "<<total<<'\n';
 }
 void nasal_vm::die(const std::string& str)
 {
@@ -365,8 +369,7 @@ inline bool nasal_vm::condition(nasal_ref val)
 inline void nasal_vm::opr_intg()
 {
     // global values store on stack
-    for(uint32_t i=0;i<imm[pc];++i)
-        (top++)[0].type=vm_nil;
+    top+=imm[pc];
     --top;// point to the top
 }
 inline void nasal_vm::opr_intl()