diff --git a/CMakeLists.txt b/CMakeLists.txt
index e6c3521..64d7e3c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,6 +10,10 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 set(CMAKE_CXX_FLAGS_RELEASE_INIT "-Wshadow -Wall")
 
 add_compile_options(-fPIC)
+# MSVC needs this command option to really enable utf-8 output
+if(MSVC)
+    add_compile_options(/utf-8)
+endif()
 
 # generate release executables
 set(CMAKE_BUILD_TYPE "Release")
diff --git a/makefile b/makefile
index 698e9c2..17a35e8 100644
--- a/makefile
+++ b/makefile
@@ -112,7 +112,12 @@ build/nasal_type.o:\
 	src/nasal_type.h src/nasal_type.cpp | build
 	$(CXX) $(CXXFLAGS) src/nasal_type.cpp -o build/nasal_type.o
 
-build/nasal_gc.o: src/nasal.h src/nasal_type.h src/nasal_gc.h src/nasal_gc.cpp | build
+build/nasal_gc.o:\
+	src/nasal.h\
+	src/util/util.h\
+	src/nasal_type.h\
+	src/nasal_gc.h\
+	src/nasal_gc.cpp | build
 	$(CXX) $(CXXFLAGS) src/nasal_gc.cpp -o build/nasal_gc.o
 
 build/nasal_import.o: \
diff --git a/src/ast_dumper.cpp b/src/ast_dumper.cpp
index 03fbb71..c570019 100644
--- a/src/ast_dumper.cpp
+++ b/src/ast_dumper.cpp
@@ -185,6 +185,7 @@ bool ast_dumper::visit_binary_operator(binary_operator* node) {
         case binary_operator::binary_type::leq: std::cout << "<="; break;
         case binary_operator::binary_type::condition_and: std::cout << "and"; break;
         case binary_operator::binary_type::condition_or: std::cout << "or"; break;
+        case binary_operator::binary_type::null_chain: std::cout << "??"; break;
     }
     std::cout << "\"" << format_location(node);
     push_indent();
@@ -241,6 +242,13 @@ bool ast_dumper::visit_call_hash(call_hash* node) {
     return true;
 }
 
+bool ast_dumper::visit_null_access(null_access* node) {
+    dump_indent();
+    std::cout << "null_access " << node->get_field();
+    std::cout << format_location(node);
+    return true;
+}
+
 bool ast_dumper::visit_call_vector(call_vector* node) {
     dump_indent();
     std::cout << "call_vector";
diff --git a/src/ast_dumper.h b/src/ast_dumper.h
index 3855509..1586d0b 100644
--- a/src/ast_dumper.h
+++ b/src/ast_dumper.h
@@ -45,7 +45,7 @@ private:
 
     std::string format_location(expr* node) {
         std::stringstream ss;
-        ss << " → [";
+        ss << " ⇒ [";
         node->get_location().dump_begin(ss);
         ss << "]\n";
         return ss.str();
@@ -70,6 +70,7 @@ public:
     bool visit_unary_operator(unary_operator*) override;
     bool visit_call_expr(call_expr*) override;
     bool visit_call_hash(call_hash*) override;
+    bool visit_null_access(null_access*) override;
     bool visit_call_vector(call_vector*) override;
     bool visit_call_function(call_function*) override;
     bool visit_slice_vector(slice_vector*) override;
@@ -90,10 +91,10 @@ public:
 
 public:
     void dump(code_block* root) {
-        util::windows_code_page_manager wcpm;
-        wcpm.set_utf8_output();
+        util::windows_code_page_manager wm;
+        wm.set_utf8_output();
         root->accept(this);
-        wcpm.restore_code_page();
+        wm.restore_code_page();
     }
 };
 
diff --git a/src/ast_visitor.cpp b/src/ast_visitor.cpp
index f438b4b..dacd80b 100644
--- a/src/ast_visitor.cpp
+++ b/src/ast_visitor.cpp
@@ -116,6 +116,10 @@ bool ast_visitor::visit_call_hash(call_hash* node) {
     return true;
 }
 
+bool ast_visitor::visit_null_access(null_access* node) {
+    return true;
+}
+
 bool ast_visitor::visit_call_vector(call_vector* node) {
     for(auto i : node->get_slices()) {
         i->accept(this);
diff --git a/src/ast_visitor.h b/src/ast_visitor.h
index 4235541..e6a5abd 100644
--- a/src/ast_visitor.h
+++ b/src/ast_visitor.h
@@ -26,6 +26,7 @@ public:
     virtual bool visit_unary_operator(unary_operator*);
     virtual bool visit_call_expr(call_expr*);
     virtual bool visit_call_hash(call_hash*);
+    virtual bool visit_null_access(null_access*);
     virtual bool visit_call_vector(call_vector*);
     virtual bool visit_call_function(call_function*);
     virtual bool visit_slice_vector(slice_vector*);
diff --git a/src/cli/cli.cpp b/src/cli/cli.cpp
index fa0caf5..8605fa1 100644
--- a/src/cli/cli.cpp
+++ b/src/cli/cli.cpp
@@ -23,4 +23,40 @@ cli_config parse(const std::vector<std::string>& args) {
     return result;
 }
 
+std::ostream& help(std::ostream& out) {
+    out
+    << "\n"
+    << "     ,--#-,\n"
+    << "<3  / \\____\\  <3\n"
+    << "    |_|__A_|\n"
+    << "\nnasal <option>\n"
+    << "option:\n"
+    << "   -h,   --help     | get help.\n"
+    << "   -v,   --version  | get version.\n"
+    << "   -r,   --repl     | use repl interpreter.\n"
+    << "\nnasal [option] <file> [argv]\n"
+    << "option:\n"
+    << "   -a,   --ast      | view ast after link/optimize process.\n"
+    << "         --raw-ast  | view ast without after-processing.\n"
+    << "   -c,   --code     | view generated bytecode.\n"
+    << "   -s,   --symbol   | show analysed symbol info.\n"
+    << "   -e,   --exec     | execute directly.\n"
+    << "   -t,   --time     | show execute time.\n"
+    << "   -d,   --detail   | get detail info.\n"
+    << "   -f,   --ref-file | get referenced files.\n"
+    << "   -dbg, --debug    | debug mode.\n"
+    << "         --prof     | show profiling result, "
+    << "available under debug mode.\n"
+    << "         --prof-all | show profiling result of all files, "
+    << "available under debug mode.\n"
+    << "         --limit    | use limited execution mode "
+    << "(readonly api enabled).\n"
+    << "file:\n"
+    << "   <filename>       | execute file.\n"
+    << "argv:\n"
+    << "   <args>           | cmd arguments used in program.\n"
+    << "\n";
+    return out;
+}
+
 }
\ No newline at end of file
diff --git a/src/cli/cli.h b/src/cli/cli.h
index 517e27d..1b070aa 100644
--- a/src/cli/cli.h
+++ b/src/cli/cli.h
@@ -5,6 +5,7 @@
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
+#include <iostream>
 
 namespace nasal::cli {
 
@@ -67,4 +68,6 @@ const std::unordered_map<std::string, option> cli_options = {
 
 cli_config parse(const std::vector<std::string>&);
 
+std::ostream& help(std::ostream&);
+
 }
\ No newline at end of file
diff --git a/src/main.cpp b/src/main.cpp
index f72ae4c..70e344f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -18,45 +18,9 @@
 #include "repl/repl.h"
 #include "cli/cli.h"
 
-#include <unordered_map>
 #include <thread>
 #include <cstdlib>
 
-std::ostream& help(std::ostream& out) {
-    out
-    << "\n"
-    << "     ,--#-,\n"
-    << "<3  / \\____\\  <3\n"
-    << "    |_|__A_|\n"
-    << "\nnasal <option>\n"
-    << "option:\n"
-    << "   -h,   --help     | get help.\n"
-    << "   -v,   --version  | get version.\n"
-    << "   -r,   --repl     | use repl interpreter.\n"
-    << "\nnasal [option] <file> [argv]\n"
-    << "option:\n"
-    << "   -a,   --ast      | view ast after link/optimize process.\n"
-    << "         --raw-ast  | view ast without after-processing.\n"
-    << "   -c,   --code     | view generated bytecode.\n"
-    << "   -s,   --symbol   | show analysed symbol info.\n"
-    << "   -e,   --exec     | execute directly.\n"
-    << "   -t,   --time     | show execute time.\n"
-    << "   -d,   --detail   | get detail info.\n"
-    << "   -f,   --ref-file | get referenced files.\n"
-    << "   -dbg, --debug    | debug mode.\n"
-    << "         --prof     | show profiling result, available in debug mode.\n"
-    << "         --prof-all | show profiling result of all files, "
-    << "available under debug mode.\n"
-    << "         --limit    | use limited execution mode "
-    << "(readonly api enabled).\n"
-    << "file:\n"
-    << "   <filename>       | execute file.\n"
-    << "argv:\n"
-    << "   <args>           | cmd arguments used in program.\n"
-    << "\n";
-    return out;
-}
-
 std::ostream& logo(std::ostream& out) {
     out
     << "\n"
@@ -67,7 +31,8 @@ std::ostream& logo(std::ostream& out) {
     << "  \\_\\ \\/ \\__,_|___/\\__,_|_|\n"
     << "\n"
     << "ver  : " << __nasver__
-    << " " << nasal::util::get_platform() << " " << nasal::util::get_arch()
+    << " " << nasal::util::get_platform()
+    << " " << nasal::util::get_arch()
     << " (" << __DATE__ << " " << __TIME__ << ")\n"
     << "std  : c++ " << __cplusplus << "\n"
     << "core : " << std::thread::hardware_concurrency() << " core(s)\n"
@@ -75,9 +40,9 @@ std::ostream& logo(std::ostream& out) {
     << "repo : https://gitee.com/valkmjolnir/Nasal-Interpreter\n"
     << "wiki : https://wiki.flightgear.org/Nasal_scripting_language\n"
     << "\n"
-    << "presented by fgprc members\n"
-    << "  - http://fgprc.org\n"
-    << "  - http://fgprc.org.cn\n"
+    << "presented by fgprc members:\n"
+    << " - http://fgprc.org\n"
+    << " - http://fgprc.org.cn\n"
     << "\n"
     << "input <nasal -h> to get help .\n\n";
     return out;
@@ -96,7 +61,8 @@ std::ostream& version(std::ostream& out) {
     }
 
     out << "nasal version " << __nasver__;
-    out << " " << nasal::util::get_platform() << " " << nasal::util::get_arch();
+    out << " " << nasal::util::get_platform();
+    out << " " << nasal::util::get_arch();
     out << " (" << __DATE__ << " " << __TIME__ << ")\n";
     return out;
 }
@@ -195,7 +161,7 @@ i32 main(i32 argc, const char* argv[]) {
     // run directly or show help
     if (argc==2) {
         if (config.has(nasal::cli::option::cli_help)) {
-            std::clog << help;
+            std::clog << nasal::cli::help;
         } else if (config.has(nasal::cli::option::cli_version)) {
             std::clog << version;
         } else if (config.has(nasal::cli::option::cli_repl_mode)) {
diff --git a/src/nasal_ast.cpp b/src/nasal_ast.cpp
index b1c27e1..2ab1d62 100644
--- a/src/nasal_ast.cpp
+++ b/src/nasal_ast.cpp
@@ -173,6 +173,10 @@ void call_hash::accept(ast_visitor* visitor) {
     visitor->visit_call_hash(this);
 }
 
+void null_access::accept(ast_visitor* visitor) {
+    visitor->visit_null_access(this);
+}
+
 call_vector::~call_vector() {
     for(auto i : calls) {
         delete i;
diff --git a/src/nasal_ast.h b/src/nasal_ast.h
index 3913ff9..3607e1b 100644
--- a/src/nasal_ast.h
+++ b/src/nasal_ast.h
@@ -23,6 +23,7 @@ enum class expr_type {
     ast_pair,        // pair of key and value in hashmap
     ast_call,        // mark a sub-tree of calling an identifier
     ast_callh,       // id.name
+    ast_null_access, // id?.name
     ast_callv,       // id[index]
     ast_callf,       // id()
     ast_subvec,      // id[index:index]
@@ -302,7 +303,8 @@ public:
         bitwise_xor,
         bitwise_and,
         condition_and,
-        condition_or
+        condition_or,
+        null_chain
     };
 
 private:
@@ -389,6 +391,19 @@ public:
     void accept(ast_visitor*) override;
 };
 
+class null_access: public call {
+private:
+    std::string field;
+
+public:
+    null_access(const span& location, const std::string& name):
+        call(location, expr_type::ast_null_access),
+        field(name) {}
+    ~null_access() override = default;
+    const std::string& get_field() const {return field;}
+    void accept(ast_visitor*) override;
+};
+
 class call_vector: public call {
 private:
     std::vector<slice_vector*> calls;
diff --git a/src/nasal_codegen.cpp b/src/nasal_codegen.cpp
index 2a0b3cc..3873733 100644
--- a/src/nasal_codegen.cpp
+++ b/src/nasal_codegen.cpp
@@ -339,6 +339,8 @@ void codegen::call_gen(call_expr* node) {
                 call_vector_gen(reinterpret_cast<call_vector*>(i)); break;
             case expr_type::ast_callf:
                 call_func_gen(reinterpret_cast<call_function*>(i)); break;
+            case expr_type::ast_null_access:
+                null_access_gen(reinterpret_cast<null_access*>(i)); break;
             default: break;
         }
     }
@@ -382,6 +384,24 @@ void codegen::call_hash_gen(call_hash* node) {
     emit(op_callh, const_string_map.at(node->get_field()), node->get_location());
 }
 
+void codegen::null_access_gen(null_access* node) {
+    regist_string(node->get_field());
+
+    emit(op_dup, 0, node->get_location());
+    emit(op_pnil, 0, node->get_location());
+    emit(op_eq, 0, node->get_location());
+
+    const auto jmp_false_point = code.size();
+    emit(op_jf, 0, node->get_location());
+
+    const auto jmp_direct_point = code.size();
+    emit(op_jmp, 0, node->get_location());
+
+    code[jmp_false_point].num = code.size();
+    emit(op_callh, const_string_map.at(node->get_field()), node->get_location());
+    code[jmp_direct_point].num = code.size();
+}
+
 void codegen::call_vector_gen(call_vector* node) {
     // maybe this place can use callv-const if ast's first child is ast_num
     if (node->get_slices().size()==1 &&
@@ -455,6 +475,8 @@ void codegen::mcall(expr* node) {
                 call_vector_gen(reinterpret_cast<call_vector*>(tmp)); break;
             case expr_type::ast_callf:
                 call_func_gen(reinterpret_cast<call_function*>(tmp)); break;
+            case expr_type::ast_null_access:
+                null_access_gen(reinterpret_cast<null_access*>(tmp)); break;
             default: break;
         }
     }
@@ -467,6 +489,8 @@ void codegen::mcall(expr* node) {
             mcall_vec(reinterpret_cast<call_vector*>(tmp)); break;
         case expr_type::ast_callf:
             die("bad left-value: function call", tmp->get_location()); break;
+        case expr_type::ast_null_access:
+            die("bad left-value: null access test", tmp->get_location()); break;
         default:
             die("bad left-value: unknown call", tmp->get_location()); break;
     }
@@ -1059,6 +1083,9 @@ void codegen::binary_gen(binary_operator* node) {
             calc_gen(node->get_right());
             emit(op_btand, 0, node->get_location());
             return;
+        case binary_operator::binary_type::null_chain:
+            null_chain_gen(node);
+            return;
         default: break;
     }
     switch(node->get_operator_type()) {
@@ -1174,6 +1201,23 @@ void codegen::binary_gen(binary_operator* node) {
     }
 }
 
+void codegen::null_chain_gen(binary_operator* node) {
+    calc_gen(node->get_left());
+    emit(op_pnil, 0, node->get_location());
+    emit(op_eq, 0, node->get_location());
+
+    const auto jmp_false_point = code.size();
+    emit(op_jf, 0, node->get_location());
+
+    calc_gen(node->get_right());
+    const auto jmp_direct_point = code.size();
+    emit(op_jmp, 0, node->get_location());
+
+    code[jmp_false_point].num = code.size();
+    emit(op_pop, 0, node->get_location());
+    code[jmp_direct_point].num = code.size();
+}
+
 void codegen::trino_gen(ternary_operator* node) {
     calc_gen(node->get_condition());
     usize label_jump_false = code.size();
diff --git a/src/nasal_codegen.h b/src/nasal_codegen.h
index 8501426..ffa2ab7 100644
--- a/src/nasal_codegen.h
+++ b/src/nasal_codegen.h
@@ -123,6 +123,7 @@ private:
     void call_gen(call_expr*);
     void call_identifier(identifier*);
     void call_hash_gen(call_hash*);
+    void null_access_gen(null_access*);
     void call_vector_gen(call_vector*);
     void call_func_gen(call_function*);
     void mcall(expr*);
@@ -148,6 +149,7 @@ private:
     void and_gen(binary_operator*);
     void unary_gen(unary_operator*);
     void binary_gen(binary_operator*);
+    void null_chain_gen(binary_operator*);
     void trino_gen(ternary_operator*);
     void calc_gen(expr*);
     void repl_mode_info_output_gen(expr*);
diff --git a/src/nasal_dbg.cpp b/src/nasal_dbg.cpp
index cc1bc9c..4f6c4cb 100644
--- a/src/nasal_dbg.cpp
+++ b/src/nasal_dbg.cpp
@@ -46,8 +46,9 @@ void operand_line_counter::dump_operand_count() const {
         if (!rate) {
             break;
         }
-        std::clog << " " << oprand_name_table[i.first] << " : ";
-        std::clog << i.second << " (" << rate << "%)\n";
+        std::clog << " ";
+        std::clog << operand_name_table.at(static_cast<op_code_type>(i.first));
+        std::clog << " : " << i.second << " (" << rate << "%)\n";
     }
     std::clog << " total  : " << total << '\n';
 }
diff --git a/src/nasal_dbg.h b/src/nasal_dbg.h
index 61b03fc..c554941 100644
--- a/src/nasal_dbg.h
+++ b/src/nasal_dbg.h
@@ -40,55 +40,6 @@ public:
 };
 
 class dbg: public vm {
-private:
-    typedef void (dbg::*nasal_vm_func)();
-    const nasal_vm_func operand_function[op_ret + 1] = {
-        nullptr,        &dbg::o_repl,
-        &dbg::o_intl,   &dbg::o_loadg,
-        &dbg::o_loadl,  &dbg::o_loadu,
-        &dbg::o_pnum,   &dbg::o_pnil,
-        &dbg::o_pstr,   &dbg::o_newv,
-        &dbg::o_newh,   &dbg::o_newf,
-        &dbg::o_happ,   &dbg::o_para,
-        &dbg::o_deft,   &dbg::o_dyn,
-        &dbg::o_lnot,   &dbg::o_usub,
-        &dbg::o_bnot,   &dbg::o_btor,
-        &dbg::o_btxor,  &dbg::o_btand,
-        &dbg::o_add,    &dbg::o_sub,
-        &dbg::o_mul,    &dbg::o_div,
-        &dbg::o_lnk,    &dbg::o_addc,
-        &dbg::o_subc,   &dbg::o_mulc,
-        &dbg::o_divc,   &dbg::o_lnkc,
-        &dbg::o_addeq,  &dbg::o_subeq,
-        &dbg::o_muleq,  &dbg::o_diveq,
-        &dbg::o_lnkeq,  &dbg::o_bandeq,
-        &dbg::o_boreq,  &dbg::o_bxoreq,
-        &dbg::o_addeqc, &dbg::o_subeqc,
-        &dbg::o_muleqc, &dbg::o_diveqc,
-        &dbg::o_lnkeqc, &dbg::o_addecp,
-        &dbg::o_subecp, &dbg::o_mulecp,
-        &dbg::o_divecp, &dbg::o_lnkecp,
-        &dbg::o_meq,    &dbg::o_eq,
-        &dbg::o_neq,    &dbg::o_less,
-        &dbg::o_leq,    &dbg::o_grt,
-        &dbg::o_geq,    &dbg::o_lessc,
-        &dbg::o_leqc,   &dbg::o_grtc,
-        &dbg::o_geqc,   &dbg::o_pop,
-        &dbg::o_jmp,    &dbg::o_jt,
-        &dbg::o_jf,     &dbg::o_cnt,
-        &dbg::o_findex, &dbg::o_feach,
-        &dbg::o_callg,  &dbg::o_calll,
-        &dbg::o_upval,  &dbg::o_callv,
-        &dbg::o_callvi, &dbg::o_callh,
-        &dbg::o_callfv, &dbg::o_callfh,
-        &dbg::o_callb,  &dbg::o_slcbeg,
-        &dbg::o_slcend, &dbg::o_slc,
-        &dbg::o_slc2,   &dbg::o_mcallg,
-        &dbg::o_mcalll, &dbg::o_mupval,
-        &dbg::o_mcallv, &dbg::o_mcallh,
-        &dbg::o_ret
-    };
-
 private:
     enum class cmd_kind {
         cmd_error,
diff --git a/src/nasal_gc.cpp b/src/nasal_gc.cpp
index fb1f21e..6f2a999 100644
--- a/src/nasal_gc.cpp
+++ b/src/nasal_gc.cpp
@@ -1,4 +1,5 @@
-#include "nasal_gc.h"
+#include "nasal_gc.h"
+#include "util/util.h"
 
 namespace nasal {
 
@@ -258,15 +259,27 @@ void gc::clear() {
 }
 
 void gc::info() const {
+    util::windows_code_page_manager wm;
+    wm.set_utf8_output();
 
     using std::left;
     using std::setw;
     using std::setfill;
+    using std::setprecision;
 
     const char* used_table_name[] = {
-        "object type", "gc count", "alloc count", "memory size",
-        "detail", "time spend", "gc time", "avg time", "max gc",
-        "max mark", "max sweep", nullptr
+        "object type",
+        "gc count",
+        "alloc count",
+        "memory size",
+        "detail",
+        "time spend",
+        "gc time",
+        "avg time",
+        "max gc",
+        "max mark",
+        "max sweep",
+        nullptr
     };
     const char* name[] = {
         "string",
@@ -297,21 +310,33 @@ void gc::info() const {
         len = std::to_string(size[i]).length();
         indent = indent<len? len:indent;
     }
-    auto indent_string = std::string("--");
+    auto indent_string = std::string("──");
     for(usize i = 0; i<indent; ++i) {
-        indent_string += "-";
+        indent_string += "─";
     }
-    const auto last_line = "+" + indent_string + "+" +
-        indent_string + "-" + indent_string + "-" + indent_string + "+";
-    indent_string = indent_string + "+" +
-        indent_string + "+" + indent_string + "+" + indent_string;
+    const auto first_line = "╭" + indent_string + "┬" +
+                            indent_string + "─" +
+                            indent_string + "─" +
+                            indent_string + "╮";
+    const auto mid_line = "├" + indent_string + "┼" +
+                          indent_string + "┼" +
+                          indent_string + "┼" +
+                          indent_string + "┤";
+    const auto another_mid_line = "├" + indent_string + "┼" +
+                                  indent_string + "┴" +
+                                  indent_string + "┴" +
+                                  indent_string + "┤";
+    const auto last_line = "╰" + indent_string + "┴" +
+                           indent_string + "─" +
+                           indent_string + "─" +
+                           indent_string + "╯";
 
-    std::clog << "\n+" << indent_string << "+\n";
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "object type";
-    std::clog << " | " << left << setw(indent) << setfill(' ') << "gc count";
-    std::clog << " | " << left << setw(indent) << setfill(' ') << "alloc count";
-    std::clog << " | " << left << setw(indent) << setfill(' ') << "memory size";
-    std::clog << " |\n+" << indent_string << "+\n";
+    std::clog << "\n" << first_line << "\n";
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "object type";
+    std::clog << " │ " << left << setw(indent) << setfill(' ') << "gc count";
+    std::clog << " │ " << left << setw(indent) << setfill(' ') << "alloc count";
+    std::clog << " │ " << left << setw(indent) << setfill(' ') << "memory size";
+    std::clog << " │\n" << mid_line << "\n";
 
     double total = 0;
     for(u8 i = 0; i<gc_type_size; ++i) {
@@ -319,46 +344,54 @@ void gc::info() const {
             continue;
         }
         total += static_cast<f64>(gcnt[i]);
-        std::clog << "| " << left << setw(indent) << setfill(' ') << name[i];
-        std::clog << " | " << left << setw(indent) << setfill(' ') << gcnt[i];
-        std::clog << " | " << left << setw(indent) << setfill(' ') << acnt[i];
-        std::clog << " | " << left << setw(indent) << setfill(' ') << size[i];
-        std::clog << " |\n";
+        std::clog << "│ " << left << setw(indent) << setfill(' ') << name[i];
+        std::clog << " │ " << left << setw(indent) << setfill(' ') << gcnt[i];
+        std::clog << " │ " << left << setw(indent) << setfill(' ') << acnt[i];
+        std::clog << " │ " << left << setw(indent) << setfill(' ') << size[i];
+        std::clog << " │\n";
     }
-    std::clog << "+" << indent_string << "+\n";
+    std::clog << mid_line << "\n";
 
-    auto den = std::chrono::high_resolution_clock::duration::period::den;
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "detail";
-    std::clog << " | " << left << setw(indent) << setfill(' ') << "time spend";
-    std::clog << " | " << left << setw(indent) << setfill('x') << "x";
-    std::clog << " | " << left << setw(indent) << setfill('x') << "x";
-    std::clog << " |\n+" << indent_string << "+\n";
+    const auto den = std::chrono::high_resolution_clock::duration::period::den;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "detail";
+    std::clog << " │ " << left << setw(indent) << setfill(' ') << "time spend";
+    std::clog << " │ " << left << setw(indent) << setfill('x') << "x";
+    std::clog << " │ " << left << setw(indent) << setfill('x') << "x";
+    std::clog << " │\n" << another_mid_line << "\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "gc time";
-    std::clog << " | " << setw(indent-3) << std::setprecision(4) << worktime*1.0/den*1000 << " ms";
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    const auto gc_time = worktime*1.0/den*1000;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "gc time";
+    std::clog << " │ " << setw(indent-3) << setprecision(4) << gc_time << " ms";
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "avg time";
-    std::clog << " | " << setw(indent-3) << std::setprecision(4) << worktime*1.0/den*1000/total << " ms";
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    const auto avg_time = worktime*1.0/den*1000/total;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "avg time";
+    std::clog << " │ " << setw(indent-3) << setprecision(4) << avg_time << " ms";
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "max gc";
-    std::clog << " | " << setw(indent-3) << std::setprecision(4) << max_time*1.0/den*1000 << " ms";
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    const auto max_gc = max_time*1.0/den*1000;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "max gc";
+    std::clog << " │ " << setw(indent-3) << setprecision(4) << max_gc << " ms";
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "max mark";
-    std::clog << " | " << setw(indent-3) << std::setprecision(4) << max_mark_time*1.0/den*1000 << " ms";
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    const auto max_mark = max_mark_time*1.0/den*1000;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "max mark";
+    std::clog << " │ " << setw(indent-3) << setprecision(4) << max_mark << " ms";
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "max sweep";
-    std::clog << " | " << setw(indent-3) << std::setprecision(4) << max_sweep_time*1.0/den*1000 << " ms";
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    const auto max_sweep = max_sweep_time*1.0/den*1000;
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "max sweep";
+    std::clog << " │ " << setw(indent-3) << setprecision(4) << max_sweep << " ms";
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
-    std::clog << "| " << left << setw(indent) << setfill(' ') << "concurrent";
-    std::clog << " | " << setw(indent) << (flag_concurrent_mark_triggered? "true":"false");
-    std::clog << setw(indent*2+7) << " " << "|\n";
+    std::clog << "│ " << left << setw(indent) << setfill(' ') << "concurrent";
+    std::clog << " │ " << setw(indent)
+              << (flag_concurrent_mark_triggered? "true":"false");
+    std::clog << setw(indent*2+7) << " " << "│\n";
 
     std::clog << last_line << "\n";
+
+    wm.restore_code_page();
 }
 
 var gc::alloc(const vm_type type) {
diff --git a/src/nasal_lexer.cpp b/src/nasal_lexer.cpp
index 7ff1f21..425b700 100644
--- a/src/nasal_lexer.cpp
+++ b/src/nasal_lexer.cpp
@@ -35,12 +35,16 @@ bool lexer::is_str(char c) {
     return c=='\'' || c=='\"' || c=='`';
 }
 
+bool lexer::is_quesmark(char c) {
+    return c=='?';
+}
+
 bool lexer::is_single_opr(char c) {
     return (
         c=='(' || c==')' || c=='[' || c==']' ||
         c=='{' || c=='}' || c==',' || c==';' ||
-        c==':' || c=='?' || c=='`' || c=='@' ||
-        c=='%' || c=='$' || c=='\\'
+        c==':' || c=='`' || c=='@' || c=='%' ||
+        c=='$' || c=='\\'
     );
 }
 
@@ -103,6 +107,8 @@ void lexer::open(const std::string& file) {
 }
 
 tok lexer::get_type(const std::string& str) {
+    // search token type from mapper
+    // if cannot find, just return null
     return token_mapper.count(str)? token_mapper.at(str):tok::tk_null;
 }
 
@@ -333,6 +339,24 @@ token lexer::str_gen() {
     };
 }
 
+token lexer::quesmark_gen() {
+    u64 begin_line = line;
+    u64 begin_column = column;
+    std::string str(1, res[ptr]);
+    ++column;
+    ++ptr;
+    if (ptr < res.size() && (res[ptr]=='?' || res[ptr]=='.')) {
+        str += res[ptr];
+        ++column;
+        ++ptr;
+    }
+    return {
+        {begin_line, begin_column, line, column, filename},
+        get_type(str),
+        str
+    };
+}
+
 token lexer::single_opr() {
     u64 begin_line = line;
     u64 begin_column = column;
@@ -398,6 +422,8 @@ const error& lexer::scan(const std::string& file) {
             toks.push_back(num_gen());
         } else if (is_str(res[ptr])) {
             toks.push_back(str_gen());
+        } else if (is_quesmark(res[ptr])) {
+            toks.push_back(quesmark_gen());
         } else if (is_single_opr(res[ptr])) {
             toks.push_back(single_opr());
         } else if (res[ptr]=='.') {
diff --git a/src/nasal_lexer.h b/src/nasal_lexer.h
index 3a00f2b..2286504 100644
--- a/src/nasal_lexer.h
+++ b/src/nasal_lexer.h
@@ -50,6 +50,8 @@ enum class tok {
     tk_dot,      // .
     tk_ellipsis, // ...
     tk_quesmark, // ?
+    tk_quesques, // ??
+    tk_quesdot,  // ?.
     tk_colon,    // :
     tk_add,      // operator +
     tk_sub,      // operator -
@@ -79,9 +81,10 @@ enum class tok {
 };
 
 struct token {
-    span loc; // location
-    tok type; // token type
+    span loc;        // location
+    tok type;        // token type
     std::string str; // content
+
     token() = default;
     token(const token&) = default;
 };
@@ -94,10 +97,12 @@ private:
     std::string filename;
     std::string res;
 
+private:
     error err;
     u64 invalid_char;
     std::vector<token> toks;
 
+private:
     const std::unordered_map<std::string, tok> token_mapper = {
         {"use"     , tok::tk_use     },
         {"true"    , tok::tk_true    },
@@ -128,6 +133,8 @@ private:
         {"."       , tok::tk_dot     },
         {"..."     , tok::tk_ellipsis},
         {"?"       , tok::tk_quesmark},
+        {"??"      , tok::tk_quesques},
+        {"?."      , tok::tk_quesdot },
         {":"       , tok::tk_colon   },
         {"+"       , tok::tk_add     },
         {"-"       , tok::tk_sub     },
@@ -155,6 +162,7 @@ private:
         {">="      , tok::tk_geq     }
     };
 
+private:
     tok get_type(const std::string&);
     bool skip(char);
     bool is_id(char);
@@ -162,6 +170,7 @@ private:
     bool is_oct(char);
     bool is_dec(char);
     bool is_str(char);
+    bool is_quesmark(char);
     bool is_single_opr(char);
     bool is_calc_opr(char);
 
@@ -173,14 +182,17 @@ private:
     token id_gen();
     token num_gen();
     token str_gen();
+    token quesmark_gen();
     token single_opr();
     token dots();
     token calc_opr();
+
 public:
     lexer(): line(1), column(0), ptr(0),
-             filename(""), res(""), invalid_char(0) {}
+             filename(""), res(""),
+             invalid_char(0) {}
     const error& scan(const std::string&);
-    const std::vector<token>& result() const {return toks;}
+    const auto& result() const {return toks;}
 };
 
 }
diff --git a/src/nasal_opcode.cpp b/src/nasal_opcode.cpp
index b8ed77d..23b4901 100644
--- a/src/nasal_opcode.cpp
+++ b/src/nasal_opcode.cpp
@@ -3,31 +3,6 @@
 
 namespace nasal {
 
-const char* oprand_name_table[] = {
-    "exit  ", "repl  ", "intl  ", "loadg ",
-    "loadl ", "loadu ", "pnum  ", "pnil  ",
-    "pstr  ", "newv  ", "newh  ", "newf  ",
-    "happ  ", "para  ", "def   ", "dyn   ",
-    "lnot  ", "usub  ", "bitnot", "bitor ",
-    "bitxor", "bitand", "add   ", "sub   ",
-    "mult  ", "div   ", "lnk   ", "addc  ",
-    "subc  ", "multc ", "divc  ", "lnkc  ",
-    "addeq ", "subeq ", "muleq ", "diveq ",
-    "lnkeq ", "bandeq", "boreq ", "bxoreq",
-    "addeqc", "subeqc", "muleqc", "diveqc",
-    "lnkeqc", "addecp", "subecp", "mulecp",
-    "divecp", "lnkecp", "meq   ", "eq    ",
-    "neq   ", "less  ", "leq   ", "grt   ",
-    "geq   ", "lessc ", "leqc  ", "grtc  ",
-    "geqc  ", "pop   ", "jmp   ", "jt    ",
-    "jf    ", "cnt   ", "findx ", "feach ",
-    "callg ", "calll ", "upval ", "callv ",
-    "callvi", "callh ", "callfv", "callfh",
-    "callb ", "slcbeg", "slcend", "slice ",
-    "slice2", "mcallg", "mcalll", "mupval",
-    "mcallv", "mcallh", "ret   "
-};
-
 void codestream::set(const f64* number_list,
                      const std::string* string_list,
                      const nasal_builtin_table* native_table,
@@ -58,7 +33,7 @@ void codestream::dump(std::ostream& out) const {
     }
 
     // dump operand name
-    out << "    " << oprand_name_table[op] << "  ";
+    out << "    " << operand_name_table.at(static_cast<op_code_type>(op)) << "  ";
 
     switch(op) {
         case op_addeq:
diff --git a/src/nasal_opcode.h b/src/nasal_opcode.h
index 55e81c8..e64ade5 100644
--- a/src/nasal_opcode.h
+++ b/src/nasal_opcode.h
@@ -14,6 +14,7 @@ enum op_code_type: u8 {
     op_loadg,   // load global value
     op_loadl,   // load local value
     op_loadu,   // load upvalue
+    op_dup,     // copy value on stack top
     op_pnum,    // push constant number to the stack
     op_pnil,    // push constant nil to the stack
     op_pstr,    // push constant std::string to the stack
@@ -97,6 +98,97 @@ enum op_code_type: u8 {
     op_ret      // return
 };
 
+static std::unordered_map<op_code_type, std::string> operand_name_table = {
+    {op_code_type::op_exit, "exit  "},
+    {op_code_type::op_repl, "repl  "},
+    {op_code_type::op_intl, "intl  "},
+    {op_code_type::op_loadg, "loadg "},
+    {op_code_type::op_loadl, "loadl "},
+    {op_code_type::op_loadu, "loadu "},
+    {op_code_type::op_dup, "dup   "},
+    {op_code_type::op_pnum, "pnum  "},
+    {op_code_type::op_pnil, "pnil  "},
+    {op_code_type::op_pstr, "pstr  "},
+    {op_code_type::op_newv, "newv  "},
+    {op_code_type::op_newh, "newh  "},
+    {op_code_type::op_newf, "newf  "},
+    {op_code_type::op_happ, "happ  "},
+    {op_code_type::op_para, "para  "},
+    {op_code_type::op_deft, "def   "},
+    {op_code_type::op_dyn, "dyn   "},
+    {op_code_type::op_lnot, "lnot  "},
+    {op_code_type::op_usub, "usub  "},
+    {op_code_type::op_bnot, "bitnot"},
+    {op_code_type::op_btor, "bitor "},
+    {op_code_type::op_btxor, "bitxor"},
+    {op_code_type::op_btand, "bitand"},
+    {op_code_type::op_add, "add   "},
+    {op_code_type::op_sub, "sub   "},
+    {op_code_type::op_mul, "mult  "},
+    {op_code_type::op_div, "div   "},
+    {op_code_type::op_lnk, "lnk   "},
+    {op_code_type::op_addc, "addc  "},
+    {op_code_type::op_subc, "subc  "},
+    {op_code_type::op_mulc, "multc "},
+    {op_code_type::op_divc, "divc  "},
+    {op_code_type::op_lnkc, "lnkc  "},
+    {op_code_type::op_addeq, "addeq "},
+    {op_code_type::op_subeq, "subeq "},
+    {op_code_type::op_muleq, "muleq "},
+    {op_code_type::op_diveq, "diveq "},
+    {op_code_type::op_lnkeq, "lnkeq "},
+    {op_code_type::op_btandeq, "bandeq"},
+    {op_code_type::op_btoreq, "boreq "},
+    {op_code_type::op_btxoreq, "bxoreq"},
+    {op_code_type::op_addeqc, "addeqc"},
+    {op_code_type::op_subeqc, "subeqc"},
+    {op_code_type::op_muleqc, "muleqc"},
+    {op_code_type::op_diveqc, "diveqc"},
+    {op_code_type::op_lnkeqc, "lnkeqc"},
+    {op_code_type::op_addecp, "addecp"},
+    {op_code_type::op_subecp, "subecp"},
+    {op_code_type::op_mulecp, "mulecp"},
+    {op_code_type::op_divecp, "divecp"},
+    {op_code_type::op_lnkecp, "lnkecp"},
+    {op_code_type::op_meq, "meq   "},
+    {op_code_type::op_eq, "eq    "},
+    {op_code_type::op_neq, "neq   "},
+    {op_code_type::op_less, "less  "},
+    {op_code_type::op_leq, "leq   "},
+    {op_code_type::op_grt, "grt   "},
+    {op_code_type::op_geq, "geq   "},
+    {op_code_type::op_lessc, "lessc "},
+    {op_code_type::op_leqc, "leqc  "},
+    {op_code_type::op_grtc, "grtc  "},
+    {op_code_type::op_geqc, "geqc  "},
+    {op_code_type::op_pop, "pop   "},
+    {op_code_type::op_jmp, "jmp   "},
+    {op_code_type::op_jt, "jt    "},
+    {op_code_type::op_jf, "jf    "},
+    {op_code_type::op_cnt, "cnt   "},
+    {op_code_type::op_findex, "findx "},
+    {op_code_type::op_feach, "feach "},
+    {op_code_type::op_callg, "callg "},
+    {op_code_type::op_calll, "calll "},
+    {op_code_type::op_upval, "upval "},
+    {op_code_type::op_callv, "callv "},
+    {op_code_type::op_callvi, "callvi"},
+    {op_code_type::op_callh, "callh "},
+    {op_code_type::op_callfv, "callfv"},
+    {op_code_type::op_callfh, "callfh"},
+    {op_code_type::op_callb, "callb "},
+    {op_code_type::op_slcbeg, "slcbeg"},
+    {op_code_type::op_slcend, "slcend"},
+    {op_code_type::op_slc, "slice "},
+    {op_code_type::op_slc2, "slice2"},
+    {op_code_type::op_mcallg, "mcallg"},
+    {op_code_type::op_mcalll, "mcalll"},
+    {op_code_type::op_mupval, "mupval"},
+    {op_code_type::op_mcallv, "mcallv"},
+    {op_code_type::op_mcallh, "mcallh"},
+    {op_code_type::op_ret, "ret   "}
+};
+
 struct opcode {
     u8  op;   // opcode
     u16 fidx; // source code file index
@@ -127,6 +219,4 @@ public:
 
 std::ostream& operator<<(std::ostream&, const codestream&);
 
-extern const char* oprand_name_table[];
-
 }
\ No newline at end of file
diff --git a/src/nasal_parse.cpp b/src/nasal_parse.cpp
index 811c6ce..e6ea255 100644
--- a/src/nasal_parse.cpp
+++ b/src/nasal_parse.cpp
@@ -82,7 +82,11 @@ void parse::match(tok type, const char* info) {
             case tok::tk_num: die(thisspan, "expected number"); break;
             case tok::tk_str: die(thisspan, "expected string"); break;
             case tok::tk_id: die(thisspan, "expected identifier"); break;
-            default: die(thisspan, "expected \""+tokname.at(type)+"\""); break;
+            default:
+                die(thisspan,
+                    "expected \"" + token_name_mapper.at(type)+"\""
+                );
+                break;
         }
         return;
     }
@@ -94,7 +98,8 @@ bool parse::lookahead(tok type) {
 }
 
 bool parse::is_call(tok type) {
-    return type==tok::tk_lcurve || type==tok::tk_lbracket || type==tok::tk_dot;
+    return type==tok::tk_lcurve || type==tok::tk_lbracket ||
+           type==tok::tk_dot || type==tok::tk_quesdot;
 }
 
 bool parse::check_comma(const tok* panic_set) {
@@ -564,7 +569,7 @@ expr* parse::and_expr() {
 }
 
 expr* parse::cmp_expr() {
-    auto node = additive_expr();
+    auto node = null_chain_expr();
     while(tok::tk_cmpeq<=toks[ptr].type && toks[ptr].type<=tok::tk_geq) {
         auto tmp = new binary_operator(toks[ptr].loc);
         switch(toks[ptr].type) {
@@ -578,6 +583,21 @@ expr* parse::cmp_expr() {
         }
         tmp->set_left(node);
         match(toks[ptr].type);
+        tmp->set_right(null_chain_expr());
+        update_location(tmp);
+        node = tmp;
+    }
+    update_location(node);
+    return node;
+}
+
+expr* parse::null_chain_expr() {
+    auto node = additive_expr();
+    while(lookahead(tok::tk_quesques)) {
+        auto tmp = new binary_operator(toks[ptr].loc);
+        tmp->set_operator_type(binary_operator::binary_type::null_chain);
+        tmp->set_left(node);
+        match(tok::tk_quesques);
         tmp->set_right(additive_expr());
         update_location(tmp);
         node = tmp;
@@ -718,6 +738,7 @@ call* parse::call_scalar() {
         case tok::tk_lcurve:   return callf(); break;
         case tok::tk_lbracket: return callv(); break;
         case tok::tk_dot:      return callh(); break;
+        case tok::tk_quesdot:  return null_access_call(); break;
         default: break;
     }
     // unreachable
@@ -733,6 +754,15 @@ call_hash* parse::callh() {
     return node;
 }
 
+null_access* parse::null_access_call() {
+    const auto& begin_loc = toks[ptr].loc;
+    match(tok::tk_quesdot);
+    auto node = new null_access(begin_loc, toks[ptr].str);
+    update_location(node);
+    match(tok::tk_id, "expected hashmap key"); // get key
+    return node;
+}
+
 call_vector* parse::callv() {
     // panic set for this token is not ','
     // this is the FIRST set of subvec
diff --git a/src/nasal_parse.h b/src/nasal_parse.h
index 3cb334f..f589d63 100644
--- a/src/nasal_parse.h
+++ b/src/nasal_parse.h
@@ -23,7 +23,7 @@ private:
     error err;
 
 private:
-    const std::unordered_map<tok, std::string> tokname = {
+    const std::unordered_map<tok, std::string> token_name_mapper = {
         {tok::tk_true    , "true"    },
         {tok::tk_false   , "false"   },
         {tok::tk_use     , "use"     },
@@ -53,6 +53,8 @@ private:
         {tok::tk_dot     , "."       },
         {tok::tk_ellipsis, "..."     },
         {tok::tk_quesmark, "?"       },
+        {tok::tk_quesques, "??"      },
+        {tok::tk_quesdot , "?."      },
         {tok::tk_colon   , ":"       },
         {tok::tk_add     , "+"       },
         {tok::tk_sub     , "-"       },
@@ -117,12 +119,14 @@ private:
     expr* or_expr();
     expr* and_expr();
     expr* cmp_expr();
+    expr* null_chain_expr();
     expr* additive_expr();
     expr* multive_expr();
     unary_operator* unary();
     expr* scalar();
     call* call_scalar();
     call_hash* callh();
+    null_access* null_access_call();
     call_vector* callv();
     call_function* callf();
     slice_vector* subvec();
@@ -153,8 +157,9 @@ public:
     }
 
 public:
-    parse(): ptr(0), in_func_depth(0), in_loop_depth(0),
-             toks(nullptr), root(nullptr) {}
+    parse(): ptr(0), in_func_depth(0),
+             in_loop_depth(0), toks(nullptr),
+             root(nullptr) {}
     ~parse() {delete root;}
     const error& compile(const lexer&);
     static void easter_egg();
diff --git a/src/nasal_vm.cpp b/src/nasal_vm.cpp
index 9515c1a..006a392 100644
--- a/src/nasal_vm.cpp
+++ b/src/nasal_vm.cpp
@@ -429,28 +429,94 @@ void vm::run(const codegen& gen,
 #ifndef _MSC_VER
     // using labels as values/computed goto
     const void* oprs[] = {
-        &&vmexit, &&repl,   &&intl,   &&loadg,
-        &&loadl,  &&loadu,  &&pnum,   &&pnil,
-        &&pstr,   &&newv,   &&newh,   &&newf,
-        &&happ,   &&para,   &&deft,   &&dyn,
-        &&lnot,   &&usub,   &&bnot,   &&btor,
-        &&btxor,  &&btand,  &&add,    &&sub,
-        &&mul,    &&div,    &&lnk,    &&addc,
-        &&subc,   &&mulc,   &&divc,   &&lnkc,
-        &&addeq,  &&subeq,  &&muleq,  &&diveq,
-        &&lnkeq,  &&bandeq, &&boreq,  &&bxoreq,
-        &&addeqc, &&subeqc, &&muleqc, &&diveqc,
-        &&lnkeqc, &&addecp, &&subecp, &&mulecp,
-        &&divecp, &&lnkecp, &&meq,    &&eq,
-        &&neq,    &&less,   &&leq,    &&grt,
-        &&geq,    &&lessc,  &&leqc,   &&grtc,
-        &&geqc,   &&pop,    &&jmp,    &&jt,
-        &&jf,     &&cnt,    &&findex, &&feach,
-        &&callg,  &&calll,  &&upval,  &&callv,
-        &&callvi, &&callh,  &&callfv, &&callfh,
-        &&callb,  &&slcbeg, &&slcend, &&slc,
-        &&slc2,   &&mcallg, &&mcalll, &&mupval,
-        &&mcallv, &&mcallh, &&ret
+        &&vmexit,
+        &&repl,
+        &&intl,
+        &&loadg,
+        &&loadl,
+        &&loadu,
+        &&dup,
+        &&pnum,
+        &&pnil,
+        &&pstr,
+        &&newv,
+        &&newh,
+        &&newf,
+        &&happ,
+        &&para,
+        &&deft,
+        &&dyn,
+        &&lnot,
+        &&usub,
+        &&bnot,
+        &&btor,
+        &&btxor,
+        &&btand,
+        &&add,
+        &&sub,
+        &&mul,
+        &&div,
+        &&lnk,
+        &&addc,
+        &&subc,
+        &&mulc,
+        &&divc,
+        &&lnkc,
+        &&addeq,
+        &&subeq,
+        &&muleq,
+        &&diveq,
+        &&lnkeq,
+        &&bandeq,
+        &&boreq,
+        &&bxoreq,
+        &&addeqc,
+        &&subeqc,
+        &&muleqc,
+        &&diveqc,
+        &&lnkeqc,
+        &&addecp,
+        &&subecp,
+        &&mulecp,
+        &&divecp,
+        &&lnkecp,
+        &&meq,
+        &&eq,
+        &&neq,
+        &&less,
+        &&leq,
+        &&grt,
+        &&geq,
+        &&lessc,
+        &&leqc,
+        &&grtc,
+        &&geqc,
+        &&pop,
+        &&jmp,
+        &&jt,
+        &&jf,
+        &&cnt,
+        &&findex,
+        &&feach,
+        &&callg,
+        &&calll,
+        &&upval,
+        &&callv,
+        &&callvi,
+        &&callh,
+        &&callfv,
+        &&callfh,
+        &&callb,
+        &&slcbeg,
+        &&slcend,
+        &&slc,
+        &&slc2,
+        &&mcallg,
+        &&mcalll,
+        &&mupval,
+        &&mcallv,
+        &&mcallh,
+        &&ret
     };
     std::vector<const void*> code;
     for(const auto& i : gen.codes()) {
@@ -460,56 +526,9 @@ void vm::run(const codegen& gen,
     // goto the first operand
     goto *code[ctx.pc];
 #else
-    typedef void (vm::*nafunc)();
-    const nafunc oprs[] = {
-        nullptr,       &vm::o_repl,
-        &vm::o_intl,   &vm::o_loadg,
-        &vm::o_loadl,  &vm::o_loadu,
-        &vm::o_pnum,   &vm::o_pnil,
-        &vm::o_pstr,   &vm::o_newv,
-        &vm::o_newh,   &vm::o_newf,
-        &vm::o_happ,   &vm::o_para,
-        &vm::o_deft,   &vm::o_dyn,
-        &vm::o_lnot,   &vm::o_usub,
-        &vm::o_bnot,   &vm::o_btor,
-        &vm::o_btxor,  &vm::o_btand,
-        &vm::o_add,    &vm::o_sub,
-        &vm::o_mul,    &vm::o_div,
-        &vm::o_lnk,    &vm::o_addc,
-        &vm::o_subc,   &vm::o_mulc,
-        &vm::o_divc,   &vm::o_lnkc,
-        &vm::o_addeq,  &vm::o_subeq,
-        &vm::o_muleq,  &vm::o_diveq,
-        &vm::o_lnkeq,  &vm::o_bandeq,
-        &vm::o_boreq,  &vm::o_bxoreq,
-        &vm::o_addeqc, &vm::o_subeqc,
-        &vm::o_muleqc, &vm::o_diveqc,
-        &vm::o_lnkeqc, &vm::o_addecp,
-        &vm::o_subecp, &vm::o_mulecp,
-        &vm::o_divecp, &vm::o_lnkecp,
-        &vm::o_meq,    &vm::o_eq,
-        &vm::o_neq,    &vm::o_less,
-        &vm::o_leq,    &vm::o_grt,
-        &vm::o_geq,    &vm::o_lessc,
-        &vm::o_leqc,   &vm::o_grtc,
-        &vm::o_geqc,   &vm::o_pop,
-        &vm::o_jmp,    &vm::o_jt,
-        &vm::o_jf,     &vm::o_cnt,
-        &vm::o_findex, &vm::o_feach,
-        &vm::o_callg,  &vm::o_calll,
-        &vm::o_upval,  &vm::o_callv,
-        &vm::o_callvi, &vm::o_callh,
-        &vm::o_callfv, &vm::o_callfh,
-        &vm::o_callb,  &vm::o_slcbeg,
-        &vm::o_slcend, &vm::o_slc,
-        &vm::o_slc2,   &vm::o_mcallg,
-        &vm::o_mcalll, &vm::o_mupval,
-        &vm::o_mcallv, &vm::o_mcallh,
-        &vm::o_ret
-    };
-    std::vector<nafunc> code;
+    std::vector<nasal_vm_func> code;
     for(const auto& i : gen.codes()) {
-        code.push_back(oprs[i.op]);
+        code.push_back(operand_function[i.op]);
         imm.push_back(i.num);
     }
     while(code[ctx.pc]) {
@@ -520,7 +539,7 @@ void vm::run(const codegen& gen,
         ++ctx.pc;
     }
 #endif
-
+// all nasal programs should end here
 vmexit:
     if (verbose) {
         ngc.info();
@@ -551,6 +570,7 @@ intl:   exec_nodie(o_intl  ); // -0
 loadg:  exec_nodie(o_loadg ); // -1
 loadl:  exec_nodie(o_loadl ); // -1
 loadu:  exec_nodie(o_loadu ); // -1
+dup:    exec_check(o_dup   ); // +1
 pnum:   exec_check(o_pnum  ); // +1
 pnil:   exec_check(o_pnil  ); // +1
 pstr:   exec_check(o_pstr  ); // +1
diff --git a/src/nasal_vm.h b/src/nasal_vm.h
index 8ed1109..392dfaa 100644
--- a/src/nasal_vm.h
+++ b/src/nasal_vm.h
@@ -93,6 +93,7 @@ protected:
     inline void o_loadg();
     inline void o_loadl();
     inline void o_loadu();
+    inline void o_dup();
     inline void o_pnum();
     inline void o_pnil();
     inline void o_pstr();
@@ -175,6 +176,100 @@ protected:
     inline void o_mcallh();
     inline void o_ret();
 
+protected:
+    // for debugger and MSVC(does not support labels as values)
+    typedef void (vm::*nasal_vm_func)();
+    const nasal_vm_func operand_function[op_ret + 1] = {
+        nullptr,
+        &vm::o_repl,
+        &vm::o_intl,
+        &vm::o_loadg,
+        &vm::o_loadl,
+        &vm::o_loadu,
+        &vm::o_dup,
+        &vm::o_pnum,
+        &vm::o_pnil,
+        &vm::o_pstr,
+        &vm::o_newv,
+        &vm::o_newh,
+        &vm::o_newf,
+        &vm::o_happ,
+        &vm::o_para,
+        &vm::o_deft,
+        &vm::o_dyn,
+        &vm::o_lnot,
+        &vm::o_usub,
+        &vm::o_bnot,
+        &vm::o_btor,
+        &vm::o_btxor,
+        &vm::o_btand,
+        &vm::o_add,
+        &vm::o_sub,
+        &vm::o_mul,
+        &vm::o_div,
+        &vm::o_lnk,
+        &vm::o_addc,
+        &vm::o_subc,
+        &vm::o_mulc,
+        &vm::o_divc,
+        &vm::o_lnkc,
+        &vm::o_addeq,
+        &vm::o_subeq,
+        &vm::o_muleq,
+        &vm::o_diveq,
+        &vm::o_lnkeq,
+        &vm::o_bandeq,
+        &vm::o_boreq,
+        &vm::o_bxoreq,
+        &vm::o_addeqc,
+        &vm::o_subeqc,
+        &vm::o_muleqc,
+        &vm::o_diveqc,
+        &vm::o_lnkeqc,
+        &vm::o_addecp,
+        &vm::o_subecp,
+        &vm::o_mulecp,
+        &vm::o_divecp,
+        &vm::o_lnkecp,
+        &vm::o_meq,
+        &vm::o_eq,
+        &vm::o_neq,
+        &vm::o_less,
+        &vm::o_leq,
+        &vm::o_grt,
+        &vm::o_geq,
+        &vm::o_lessc,
+        &vm::o_leqc,
+        &vm::o_grtc,
+        &vm::o_geqc,
+        &vm::o_pop,
+        &vm::o_jmp,
+        &vm::o_jt,
+        &vm::o_jf,
+        &vm::o_cnt,
+        &vm::o_findex,
+        &vm::o_feach,
+        &vm::o_callg,
+        &vm::o_calll,
+        &vm::o_upval,
+        &vm::o_callv,
+        &vm::o_callvi,
+        &vm::o_callh,
+        &vm::o_callfv,
+        &vm::o_callfh,
+        &vm::o_callb,
+        &vm::o_slcbeg,
+        &vm::o_slcend,
+        &vm::o_slc,
+        &vm::o_slc2,
+        &vm::o_mcallg,
+        &vm::o_mcalll,
+        &vm::o_mupval,
+        &vm::o_mcallv,
+        &vm::o_mcallh,
+        &vm::o_ret
+    };
+
 public:
 
     /* constructor of vm instance */
@@ -248,6 +343,11 @@ inline void vm::o_loadu() {
                     .upval()[imm[ctx.pc]&0xffff] = (ctx.top--)[0];
 }
 
+inline void vm::o_dup() {
+    ctx.top[1] = ctx.top[0];
+    ++ctx.top;
+}
+
 inline void vm::o_pnum() {
     (++ctx.top)[0] = var::num(const_number[imm[ctx.pc]]);
 }
diff --git a/test/ascii-art.nas b/test/ascii-art.nas
index 574c912..72505b1 100644
--- a/test/ascii-art.nas
+++ b/test/ascii-art.nas
@@ -18,7 +18,7 @@ var table_character_set = [
     "┷", "┳", "⊥", "﹃", "﹄", "╮", "╭", "╯", "╰", "╳"
 ];
 
-var selection_box = ["○", "◉", "☐", "▣"];
+var selection_box = ["○", "◉", "☐", "▣", "☑", "☒"];
 
 var char_ttf=[
     ["    ","    ","    ","    ","    ","    "],
diff --git a/test/occupation.nas b/test/occupation.nas
index ffae78b..348bbc3 100644
--- a/test/occupation.nas
+++ b/test/occupation.nas
@@ -7,8 +7,8 @@ use std.io;
 use std.unix;
 use std.math;
 
-var is_windows_platform=os.platform()=="windows";
-var is_macos_platform=os.platform()=="macOS";
+var is_windows_platform = os.platform()=="windows";
+var is_macos_platform = os.platform()=="macOS";
 
 if (is_windows_platform) {
     runtime.windows.set_utf8_output();
@@ -17,72 +17,72 @@ if (is_windows_platform) {
 var cpu_stat = func() {
     if (is_windows_platform or is_macos_platform)
         return nil;
-    var cpu=split("\n",io.readfile("/proc/stat"))[0];
-    cpu=split(" ",cpu);
-    cpu={
-        name:cpu[0],
-        user:cpu[1],
-        nice:cpu[2],
-        system:cpu[3],
-        idle:cpu[4],
-        iowait:cpu[5],
-        irq:cpu[6],
-        softirq:cpu[7],
+    var cpu = split("\n", io.readfile("/proc/stat"))[0];
+    cpu = split(" ", cpu);
+    cpu = {
+        name: cpu[0],
+        user: cpu[1],
+        nice: cpu[2],
+        system: cpu[3],
+        idle: cpu[4],
+        iowait: cpu[5],
+        irq: cpu[6],
+        softirq: cpu[7],
     };
     return cpu;
 }
 
 var cpu_occupation = func() {
-    var first_in=1;
+    var first_in = 1;
     while(1) {
-        var cpu0=cpu_stat();
+        var cpu0 = cpu_stat();
         if (first_in) {
             unix.sleep(0.1);
-            first_in=0;
+            first_in = 0;
         } else {
-            for(var i=0;i<10;i+=1) {
+            for(var i = 0; i < 10; i += 1) {
                 unix.sleep(0.1);
                 coroutine.yield(nil);
             }
         }
-        var cpu1=cpu_stat();
+        var cpu1 = cpu_stat();
         if (is_windows_platform or is_macos_platform) {
             coroutine.yield(0);
             continue;
         }
-        var t0=cpu0.user+cpu0.nice+cpu0.system+cpu0.idle+cpu0.iowait+cpu0.irq+cpu0.softirq;
-        var t1=cpu1.user+cpu1.nice+cpu1.system+cpu1.idle+cpu1.iowait+cpu1.irq+cpu1.softirq;
-        var interval=cpu1.idle-cpu0.idle;
-        coroutine.yield(t0==t1?0:(1-interval/(t1-t0))*100);
+        var t0 = cpu0.user+cpu0.nice+cpu0.system+cpu0.idle+cpu0.iowait+cpu0.irq+cpu0.softirq;
+        var t1 = cpu1.user+cpu1.nice+cpu1.system+cpu1.idle+cpu1.iowait+cpu1.irq+cpu1.softirq;
+        var interval = cpu1.idle-cpu0.idle;
+        coroutine.yield(t0==t1? 0:(1-interval/(t1-t0))*100);
     }
 }
 
 var mem_occupation = func() {
     if (is_windows_platform or is_macos_platform)
         return {MemTotal:math.inf,MemFree:math.inf};
-    var meminfo=split("\n",io.readfile("/proc/meminfo"));
-    var mem_res={};
-    forindex(var i;meminfo) {
-        var tmp=split(" ",meminfo[i])[0:1];
-        tmp[0]=substr(tmp[0],0,size(tmp[0])-1);
-        mem_res[tmp[0]]=num(tmp[1]);
+    var meminfo = split("\n",io.readfile("/proc/meminfo"));
+    var mem_res = {};
+    forindex(var i; meminfo) {
+        var tmp = split(" ", meminfo[i])[0:1];
+        tmp[0] = substr(tmp[0], 0, size(tmp[0])-1);
+        mem_res[tmp[0]] = num(tmp[1]);
     }
     return mem_res;
 }
 
 var random_generator = func() {
-    var rise=[" ","▁","▂","▃","▄","▅","▆","▇","█"];
-    var total=0;
-    var statistics=[];
-    setsize(statistics,70);
+    var rise = [" ", "▁", "▂", "▃", "▄", "▅", "▆", "▇", "█"];
+    var total = 0;
+    var statistics = [];
+    setsize(statistics, 70);
     while(1) {
         for(var i=0;i<10;i+=1) {
             total+=1;
             var u=rand()*rand()*(rand()>0.5?-1:1);
             statistics[int(size(statistics)/2+u*size(statistics)/2)]+=1;
         }
-        var s=["","",""];
-        foreach(var st;statistics) {
+        var s = ["", "", ""];
+        foreach(var st; statistics) {
             var max_rate=100/size(statistics);
             var rate=st/total*100;
             for(var i=size(s)-1;i>=0;i-=1) {
@@ -97,12 +97,12 @@ var random_generator = func() {
         }
         var tmp="";
         for(var i=0;i<size(statistics);i+=1) {
-            tmp~="-";
+            tmp~="─";
         }
-        println("\e[16;1H \e[32m|",s[0],"|\e[0m");
-        println("\e[17;1H \e[32m|",s[1],"|\e[0m");
-        println("\e[18;1H \e[32m|",s[2],"|\e[0m");
-        println("\e[19;1H \e[32m+"~tmp~"+\e[0m");
+        println("\e[16;1H \e[32m│", s[0], "│\e[0m");
+        println("\e[17;1H \e[32m│", s[1], "│\e[0m");
+        println("\e[18;1H \e[32m│", s[2], "│\e[0m");
+        println("\e[19;1H \e[32m╰", tmp, "╯\e[0m");
         coroutine.yield();
     }
 }
@@ -164,7 +164,7 @@ func() {
 
         var tmp="";
         for(var i=0;i<70;i+=1) {
-            tmp~="-";
+            tmp~="─";
         }
 
         var s=["","",""];
@@ -181,11 +181,11 @@ func() {
                 }
             }
         }
-        println("\e[7;1H \e[32m+"~tmp~"+\e[0m");
-        println("\e[8;1H \e[32m|",s[0],"|\e[0m");
-        println("\e[9;1H \e[32m|",s[1],"|\e[0m");
-        println("\e[10;1H \e[32m|",s[2],"|\e[0m");
-        println("\e[11;1H \e[32m+"~tmp~"+\e[0m");
+        println("\e[7;1H \e[32m╭"~tmp~"╮\e[0m");
+        println("\e[8;1H \e[32m│",s[0],"│\e[0m");
+        println("\e[9;1H \e[32m│",s[1],"│\e[0m");
+        println("\e[10;1H \e[32m│",s[2],"│\e[0m");
+        println("\e[11;1H \e[32m├"~tmp~"┤\e[0m");
 
         var s=["","",""];
         foreach(var occ;mem_occupation_log) {
@@ -201,10 +201,10 @@ func() {
                 }
             }
         }
-        println("\e[12;1H \e[32m|",s[0],"|\e[0m");
-        println("\e[13;1H \e[32m|",s[1],"|\e[0m");
-        println("\e[14;1H \e[32m|",s[2],"|\e[0m");
-        println("\e[15;1H \e[32m+"~tmp~"+\e[0m");
+        println("\e[12;1H \e[32m│",s[0],"│\e[0m");
+        println("\e[13;1H \e[32m│",s[1],"│\e[0m");
+        println("\e[14;1H \e[32m│",s[2],"│\e[0m");
+        println("\e[15;1H \e[32m├"~tmp~"┤\e[0m");
 
         println("\e[20;1H Press 'q' to quit.");
     }
diff --git a/test/scalar.nas b/test/scalar.nas
index 3773749..5a085de 100644
--- a/test/scalar.nas
+++ b/test/scalar.nas
@@ -267,3 +267,16 @@ for(var i = 1; i<=10; i += 1) {
         die("test failed: expect " ~ i ~ ", but get " ~ closure_tester[1]());
     }
 }
+
+func() {
+    var a = nil;
+    var b = nil;
+    var c = nil;
+    println(a??b??c??"a??b??c?? -> should print this text");
+
+    var a = {b: 2};
+    println(a?.b); # should be 2
+
+    var a = nil;
+    println(a?.b); # should be nil
+}();