diff --git a/README.md b/README.md
index 5328558..13eff3a 100644
--- a/README.md
+++ b/README.md
@@ -1386,6 +1386,8 @@ When `op_callb` is called, the stack frame is like this:
 +----------------------------+
 | old localr(vm_addr)        | <- top[-1]
 +----------------------------+
+| old upvalr(vm_upval)       | <- top[-2]
++----------------------------+
 | local scope(nasal_ref)     |
 | ...                        |
 +----------------------------+ <- local pointer stored in localr
@@ -1403,6 +1405,8 @@ In `op_callb`'s progress, next step the stack frame is:
 +----------------------------+
 | old localr(vm_addr)        |
 +----------------------------+
+| old upvalr(vm_upval)       |
++----------------------------+
 | local scope(nasal_ref)     |
 | ...                        |
 +----------------------------+ <- local pointer stored in localr
@@ -1435,6 +1439,8 @@ but where is the returned `local[1]` sent?
 +----------------------------+
 | old localr(vm_addr)        |
 +----------------------------+
+| old upvalr(vm_upval)       |
++----------------------------+
 | local scope(nasal_ref)     |
 | ...                        |
 +----------------------------+ <- local pointer stored in localr
@@ -1453,6 +1459,8 @@ and the returned `local[1]` in fact is set to the top of the main stack by `op_c
 +----------------------------+
 | old localr(vm_addr)        |
 +----------------------------+
+| old upvalr(vm_upval)       |
++----------------------------+
 | local scope(nasal_ref)     |
 | ...                        |
 +----------------------------+ <- local pointer stored in localr
diff --git a/makefile b/makefile
index c4b8bff..050b851 100644
--- a/makefile
+++ b/makefile
@@ -16,6 +16,7 @@ test:nasal
 	@ ./nasal -op -e -d test/calc.nas
 	@ ./nasal -op -e test/choice.nas
 	@ ./nasal -op -e test/class.nas
+	@ ./nasal -op -d -o test/coroutine.nas
 	@ ./nasal -op -e test/diff.nas
 	-@ ./nasal -op -d test/exception.nas
 	@ ./nasal -op -t -d test/fib.nas
diff --git a/nasal_builtin.h b/nasal_builtin.h
index d5489fc..2c23853 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -1343,6 +1343,8 @@ nasal_ref builtin_cocreate(nasal_ref* local,nasal_gc& gc)
     // +-----------------+
     // | old localr      | <- top[-1]
     // +-----------------+
+    // | old upvalr      | <- top[-2]
+    // +-----------------+
     // | local scope     |
     // | ...             |
     // +-----------------+ <- local pointer stored in localr
@@ -1361,11 +1363,13 @@ nasal_ref builtin_cocreate(nasal_ref* local,nasal_gc& gc)
     coroutine.localr=coroutine.top+1;
     coroutine.top=coroutine.localr+func.func().lsize;
     coroutine.localr[0]=func.func().local[0];
-    coroutine.top[0]={vm_addr,(nasal_ref*)nullptr};
+    coroutine.top[0]=nil; // old upvalr
     coroutine.top++;
-    coroutine.top[0]={vm_ret,(uint32_t)0};
+    coroutine.top[0]={vm_addr,(nasal_ref*)nullptr}; // old localr
+    coroutine.top++;
+    coroutine.top[0]={vm_ret,(uint32_t)0}; // old pc, set to zero to make op_ret recognizing this as coroutine function
 
-    coroutine.funcr=func;
+    coroutine.funcr=func; // make sure the coroutine function can use correct upvalues
     coroutine.status=nasal_co::suspended;
     
     return co;
diff --git a/nasal_gc.h b/nasal_gc.h
index 51cd3c8..758ea0c 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -218,6 +218,7 @@ struct nasal_co
     nasal_ref* localr;
     nasal_ref* memr;
     nasal_ref  funcr;
+    nasal_ref  upvalr;
 
     uint32_t   status;
     nasal_co():
@@ -227,6 +228,7 @@ struct nasal_co
         localr(nullptr),
         memr(nullptr),
         funcr({vm_nil,(double)0}),
+        upvalr({vm_nil,(double)0}),
         status(nasal_co::suspended)
     {
         for(uint32_t i=0;i<depth;++i)
@@ -454,13 +456,13 @@ struct nasal_gc
     {
         nasal_ref  stack[stack_depth];
         uint32_t   pc;
+        nasal_ref* top;
         nasal_ref* localr;
         nasal_ref* memr;
-        nasal_ref* canary;
         nasal_ref  funcr;
-        nasal_ref* top;
+        nasal_ref  upvalr;
+        nasal_ref* canary;
     } main_ctx;
-    nasal_ref*              global;                  // global values pointer(this should not be changed)
     
     /* runtime context */
     uint32_t                pc;                      // program counter
@@ -468,20 +470,15 @@ struct nasal_gc
     nasal_ref*              localr;                  // local scope register
     nasal_ref*              memr;                    // used for mem_call
     nasal_ref               funcr;                   // function register
+    nasal_ref               upvalr;                  // upvalue register
     nasal_ref*              canary;                  // avoid stackoverflow
     nasal_ref*              stack;                   // stack pointer
-    nasal_co*               coroutine;               // running coroutin
+    nasal_co*               coroutine;               // running coroutine
 
     /* constants and memory pool */
     std::vector<nasal_ref>  strs;                    // reserved address for const vm_str
     std::vector<nasal_val*> memory;                  // gc memory
     std::queue<nasal_val*>  free_list[vm_type_size]; // gc free list
-    /* upvalue is a temporary space to store upvalues */
-    /* if no new functions generated in local scope   */
-    /* upvalue will pushback(nil)                     */
-    /* if new functions generated in local scope      */
-    /* they will share the same upvalue stored here   */
-    std::vector<nasal_ref>  upvalue;
 
     /* values for analysis */
     uint64_t                size[vm_type_size];
@@ -502,19 +499,19 @@ void nasal_gc::mark()
 {
     std::queue<nasal_ref> bfs;
     
-    for(auto& i:upvalue)
-        bfs.push(i);
     if(!coroutine)
     {
         for(nasal_ref* i=stack;i<=top;++i)
             bfs.push(*i);
         bfs.push(funcr);
+        bfs.push(upvalr);
     }
     else
     {
         for(nasal_ref* i=main_ctx.stack;i<=main_ctx.top;++i)
             bfs.push(*i);
         bfs.push(main_ctx.funcr);
+        bfs.push(main_ctx.upvalr);
     }
     
     while(!bfs.empty())
@@ -545,6 +542,7 @@ void nasal_gc::mark()
                 break;
             case vm_co:
                 bfs.push(tmp.co().funcr);
+                bfs.push(tmp.co().upvalr);
                 for(nasal_ref* i=tmp.co().stack;i<=tmp.co().top;++i)
                     bfs.push(*i);
                 break;
@@ -587,8 +585,7 @@ void nasal_gc::init(const std::vector<std::string>& s)
             nasal_val* tmp=new nasal_val(i);
             memory.push_back(tmp);
             free_list[i].push(tmp);
-        }
-    global=main_ctx.stack;    
+        } 
     stack=main_ctx.stack;
     top=main_ctx.stack;
     coroutine=nullptr;
@@ -606,7 +603,6 @@ void nasal_gc::clear()
     for(auto i:memory)
         delete i;
     memory.clear();
-    upvalue.clear();
     for(uint8_t i=0;i<vm_type_size;++i)
         while(!free_list[i].empty())
             free_list[i].pop();
@@ -681,6 +677,7 @@ void nasal_gc::ctxchg(nasal_co& context)
     main_ctx.localr=localr;
     main_ctx.memr=memr;
     main_ctx.funcr=funcr;
+    main_ctx.upvalr=upvalr;
     main_ctx.canary=canary;
 
     pc=context.pc;
@@ -688,11 +685,11 @@ void nasal_gc::ctxchg(nasal_co& context)
     localr=context.localr;
     memr=context.memr;
     funcr=context.funcr;
+    upvalr=context.upvalr;
     canary=context.canary;
     stack=context.stack;
     coroutine=&context;
 
-    upvalue.push_back(nil);
     coroutine->status=nasal_co::running;
 }
 void nasal_gc::ctxreserve()
@@ -705,6 +702,7 @@ void nasal_gc::ctxreserve()
     coroutine->localr=localr;
     coroutine->memr=memr;
     coroutine->funcr=funcr;
+    coroutine->upvalr=upvalr;
     coroutine->canary=canary;
 
     pc=main_ctx.pc;
@@ -712,10 +710,9 @@ void nasal_gc::ctxreserve()
     localr=main_ctx.localr;
     memr=main_ctx.memr;
     funcr=main_ctx.funcr;
+    upvalr=main_ctx.upvalr;
     canary=main_ctx.canary;
     stack=main_ctx.stack;
     coroutine=nullptr;
-
-    upvalue.pop_back();
 }
 #endif
\ No newline at end of file
diff --git a/nasal_vm.h b/nasal_vm.h
index 34d52aa..39b41fe 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -6,10 +6,11 @@ class nasal_vm
 protected:
     /* values of nasal_vm */
     uint32_t&                pc;       // program counter
-    nasal_ref*&              global;   // global scope register
+    nasal_ref*               global;   // global scope register
     nasal_ref*&              localr;   // local scope register
     nasal_ref*&              memr;     // used for mem_call
     nasal_ref&               funcr;    // function register
+    nasal_ref&               upvalr;   // upvalue register
     nasal_ref*&              canary;   // avoid stackoverflow
     nasal_ref*&              top;      // stack top
     /* constant */
@@ -120,10 +121,11 @@ protected:
 public:
     nasal_vm():
         pc(gc.pc),
-        global(gc.global),
+        global(gc.main_ctx.stack),
         localr(gc.localr),
         memr(gc.memr),
         funcr(gc.funcr),
+        upvalr(gc.upvalr),
         canary(gc.canary),
         top(gc.top){}
     void run(
@@ -147,9 +149,11 @@ void nasal_vm::init(
     files_size=filenames.size();
     /* set canary and program counter */
     canary=gc.stack+nasal_gc::stack_depth-1; // gc.stack[nasal_gc::stack_depth-1]
-    memr=nullptr;
-    localr=nullptr;
     pc=0;
+    localr=nullptr;
+    memr=nullptr;
+    funcr=nil;
+    upvalr=nil;
 }
 void nasal_vm::valinfo(nasal_ref& val)
 {
@@ -169,6 +173,7 @@ void nasal_vm::valinfo(nasal_ref& val)
             printf("| str  | <0x" PRTHEX64 "> %.16s%s\n",(uint64_t)p,tmp.c_str(),tmp.length()>16?"...":"");
         }break;
         case vm_func: printf("| func | <0x" PRTHEX64 "> entry:0x%x\n",(uint64_t)p,val.func().entry);break;
+        case vm_upval:printf("| upval| <0x" PRTHEX64 "> [%u val]\n",(uint64_t)p,val.upval().size);break;
         case vm_vec:  printf("| vec  | <0x" PRTHEX64 "> [%zu val]\n",(uint64_t)p,val.vec().size());break;
         case vm_hash: printf("| hash | <0x" PRTHEX64 "> {%zu val}\n",(uint64_t)p,val.hash().size());break;
         case vm_obj:  printf("| obj  | <0x" PRTHEX64 "> obj:0x" PRTHEX64 "\n",(uint64_t)p,(uint64_t)val.obj().ptr);break;
@@ -216,7 +221,7 @@ void nasal_vm::bytecodeinfo(const char* header,const uint32_t p)
         case op_callb:
             printf("0x%x <%s@0x" PRTHEX64 ">",c.num,builtin[c.num].name,(uint64_t)builtin[c.num].func);break;
         case op_upval: case op_mupval: case op_loadu:
-            printf(" (0x%x[0x%x])",(c.num>>16)&0xffff,c.num&0xffff);break;
+            printf("0x%x[0x%x]",(c.num>>16)&0xffff,c.num&0xffff);break;
         case op_happ:  case op_pstr:
         case op_lnkc:
         case op_callh: case op_mcallh:
@@ -318,14 +323,25 @@ void nasal_vm::upval_state()
 }
 void nasal_vm::detail()
 {
-    printf("maddr:\n  (0x" PRTHEX64 ")\n",(uint64_t)memr);
-    printf("localr:\n  (0x" PRTHEX64 ")\n",(uint64_t)localr);
+    printf("registers(%s):\n",gc.coroutine?"coroutine":"main");
+    printf("  [ pc     ]    | pc   | 0x%.x\n",pc);
+    printf("  [ global ]    | addr | 0x" PRTHEX64 "\n",(uint64_t)global);
+    printf("  [ localr ]    | addr | 0x" PRTHEX64 "\n",(uint64_t)localr);
+    printf("  [ memr   ]    | addr | 0x" PRTHEX64 "\n",(uint64_t)memr);
     if(funcr.type==vm_nil)
-        printf("funcr:\n  (nil)\n");
+        printf("  [ funcr  ]    | nil  |\n");
     else
-        printf("funcr:\n  (<0x" PRTHEX64 "> entry:0x%x)\n",
+        printf("  [ funcr  ]    | func | <0x" PRTHEX64 "> entry:0x%x\n",
             (uint64_t)funcr.value.gcobj,
             funcr.func().entry);
+    if(upvalr.type==vm_nil)
+        printf("  [ upvalr ]    | nil  |\n");
+    else
+        printf("  [ upvalr ]    | upval| <0x" PRTHEX64 "> [%u val]\n",
+            (uint64_t)upvalr.value.gcobj,
+            upvalr.upval().size);
+    printf("  [ canary ]    | addr | 0x" PRTHEX64 "\n",(uint64_t)canary);
+    printf("  [ top    ]    | addr | 0x" PRTHEX64 "\n",(uint64_t)top);
     global_state();
     local_state();
     upval_state();
@@ -436,11 +452,11 @@ inline void nasal_vm::opr_newf()
     if(localr)
     {
         func.upvalue=funcr.func().upvalue;
-        nasal_ref upval=(gc.upvalue.back().type==vm_nil)?gc.alloc(vm_upval):gc.upvalue.back();
+        nasal_ref upval=(upvalr.type==vm_nil)?gc.alloc(vm_upval):upvalr;
         upval.upval().size=funcr.func().lsize;
         upval.upval().stk=localr;
         func.upvalue.push_back(upval);
-        gc.upvalue.back()=upval;
+        upvalr=upval;
     }
 }
 inline void nasal_vm::opr_happ()
@@ -739,7 +755,7 @@ inline void nasal_vm::opr_callfv()
     nasal_ref tmp=local[-1];
     local[-1]=funcr;
     funcr=tmp;
-    if(top-argc+func.lsize+2>=canary) // top-argc+lsize(local) +1(old pc) +1(old localr)
+    if(top-argc+func.lsize+3>=canary) // top-argc+lsize(local) +1(old pc) +1(old localr) +1(old upvalr)
         die("stack overflow");
 
     uint32_t psize=func.psize-1; // parameter size is func->psize-1, 1 is reserved for "me"
@@ -763,11 +779,12 @@ inline void nasal_vm::opr_callfv()
     if(func.dynpara>=0)
         local[psize+1]=dynamic;
 
-    top[0]={vm_addr,localr};
+    top[0]=upvalr;
+    (++top)[0]={vm_addr,localr};
     (++top)[0]={vm_ret,pc};
     pc=func.entry-1;
     localr=local;
-    gc.upvalue.push_back(nil);
+    upvalr=nil;
 }
 inline void nasal_vm::opr_callfh()
 {
@@ -779,7 +796,7 @@ inline void nasal_vm::opr_callfh()
     nasal_ref tmp=top[-1];
     top[-1]=funcr;
     funcr=tmp;
-    if(top+func.lsize+1>=canary) // top -1(hash) +lsize(local) +1(old pc) +1(old localr)
+    if(top+func.lsize+2>=canary) // top -1(hash) +lsize(local) +1(old pc) +1(old localr) +1(old upvalr)
         die("stack overflow");
     if(func.dynpara>=0)
         die("callfh: special call cannot use dynamic argument");
@@ -797,11 +814,12 @@ inline void nasal_vm::opr_callfh()
             die("callfh: lack argument(s): \""+i.first+"\"");
     }
 
-    top[0]={vm_addr,localr};
+    top[0]=upvalr;
+    (++top)[0]={vm_addr,localr};
     (++top)[0]={vm_ret,pc}; // rewrite top with vm_ret
     pc=func.entry-1;
     localr=local;
-    gc.upvalue.push_back(nil);
+    upvalr=nil;
 }
 inline void nasal_vm::opr_callb()
 {
@@ -939,6 +957,8 @@ inline void nasal_vm::opr_ret()
     // +-----------------+
     // | old localr      | <- top[-2]
     // +-----------------+
+    // | old upvalr      | <- top[-3]
+    // +-----------------+
     // | local scope     |
     // | ...             |
     // +-----------------+ <- local pointer stored in localr
@@ -947,9 +967,11 @@ inline void nasal_vm::opr_ret()
     nasal_ref  ret=top[0];
     nasal_ref* local=localr;
     nasal_ref  func=funcr;
+    nasal_ref  up=upvalr;
 
     pc=top[-1].ret();
     localr=top[-2].addr();
+    upvalr=top[-3];
 
     top=local-1;
     func.func().local[0]=nil;// get func and set 'me' to nil
@@ -957,21 +979,19 @@ inline void nasal_vm::opr_ret()
 
     top[0]=ret; // rewrite func with returned value
 
-    if(gc.upvalue.back().type==vm_upval) // synchronize upvalue
+    if(up.type==vm_upval) // synchronize upvalue
     {
-        auto& upval=gc.upvalue.back().upval();
+        auto& upval=up.upval();
         auto size=func.func().lsize;
         upval.onstk=false;
         for(uint32_t i=0;i<size;++i)
             upval.elems.push_back(local[i]);
     }
-    if(!pc)
+    if(!pc) // cannot use gc.coroutine to judge, because there maybe another function call inside
     {
         gc.coroutine->status=nasal_co::dead;
         gc.ctxreserve();
-        return;
     }
-    gc.upvalue.pop_back();
 }
 void nasal_vm::run(
     const nasal_codegen& gen,
@@ -1101,6 +1121,6 @@ mcalll:  exec_operand(opr_mcalll  ,op_mcalll  ); // +1
 mupval:  exec_operand(opr_mupval  ,op_mupval  ); // +1
 mcallv:  exec_opnodie(opr_mcallv  ,op_mcallv  ); // -0
 mcallh:  exec_opnodie(opr_mcallh  ,op_mcallh  ); // -0
-ret:     exec_opnodie(opr_ret     ,op_ret     ); // -1
+ret:     exec_opnodie(opr_ret     ,op_ret     ); // -2
 }
 #endif
\ No newline at end of file
diff --git a/test/coroutine.nas b/test/coroutine.nas
index 5a39ca3..0858604 100644
--- a/test/coroutine.nas
+++ b/test/coroutine.nas
@@ -2,16 +2,11 @@
 # 2022/5/19
 var fib=func(){
     var (a,b)=(1,1);
-    if(coroutine.running()){
-        coroutine.yield(a);
-        coroutine.yield(b);
-    }
+    coroutine.yield(a);
+    coroutine.yield(b);
     while(1){
         (a,b)=(b,a+b);
-        if(coroutine.running())
-            coroutine.yield(b);
-        else
-            break;
+        coroutine.yield(b);
     }
     return;
 }
@@ -22,4 +17,34 @@ for(var i=0;i<45;i+=1){
     println('coroutine[0]:',res[0]==nil?nil:res[0][0],'\ncoroutine[1]:',res[1]==nil?nil:res[1][0]);
 }
 
-fib();
\ No newline at end of file
+var productor=func(){
+    for(var i=0;;i+=1)
+        coroutine.yield(i);
+}
+var counter=0;
+var consumer=func(){
+    counter+=1;
+    print('[',counter,']: ');
+    for(var i=0;i<5;i+=1){
+        print('[',i,']',coroutine.resume(co)[0],' ');
+    }
+    print('\n');
+}
+
+var co=coroutine.create(productor);
+var tm=maketimestamp();
+tm.stamp();
+while(tm.elapsedMSec()<1000)
+    consumer();
+
+func(){
+    var x=1;
+    var co=coroutine.create(func(){
+        for(var j=0;j<1024;j+=1){
+            coroutine.yield(x,i,j);
+            x+=1;
+        }
+    });
+    for(var i=0;i<256;i+=1)
+        println(coroutine.resume(co));
+}();
\ No newline at end of file