diff --git a/py/builtin.h b/py/builtin.h
index a63752c5fa..115d059911 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -73,6 +73,7 @@ MP_DECLARE_CONST_FUN_OBJ(mp_op_setitem_obj);
 MP_DECLARE_CONST_FUN_OBJ(mp_op_delitem_obj);
 
 extern const mp_obj_module_t mp_module___main__;
+extern const mp_obj_module_t mp_module_builtins;
 extern const mp_obj_module_t mp_module_array;
 extern const mp_obj_module_t mp_module_collections;
 extern const mp_obj_module_t mp_module_io;
@@ -83,6 +84,9 @@ extern const mp_obj_module_t mp_module_struct;
 extern const mp_obj_module_t mp_module_sys;
 extern const mp_obj_module_t mp_module_gc;
 
+extern const mp_obj_dict_t mp_module_builtins_globals;
+extern mp_obj_dict_t *mp_module_builtins_override_dict;
+
 struct _dummy_t;
 extern struct _dummy_t mp_sys_stdin_obj;
 extern struct _dummy_t mp_sys_stdout_obj;
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 2910f8d974..a8f6a81741 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -44,7 +44,6 @@
 #include "runtime0.h"
 #include "runtime.h"
 #include "builtin.h"
-#include "builtintables.h"
 
 #if 0 // print debugging info
 #define DEBUG_PRINT (1)
@@ -56,6 +55,14 @@
 
 #define PATH_SEP_CHAR '/'
 
+#if MICROPY_MODULE_WEAK_LINKS
+STATIC const mp_map_elem_t mp_builtin_module_weak_links_table[] = {
+    MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
+};
+
+STATIC MP_DEFINE_CONST_MAP(mp_builtin_module_weak_links_map, mp_builtin_module_weak_links_table);
+#endif
+
 bool mp_obj_is_package(mp_obj_t module) {
     mp_obj_t dest[2];
     mp_load_method_maybe(module, MP_QSTR___path__, dest);
@@ -270,7 +277,7 @@ mp_obj_t mp_builtin___import__(mp_uint_t n_args, const mp_obj_t *args) {
                 #if MICROPY_MODULE_WEAK_LINKS
                 // check if there is a weak link to this module
                 if (i == mod_len) {
-                    mp_map_elem_t *el = mp_map_lookup((mp_map_t*)&mp_builtin_module_weak_links_dict_obj.map, MP_OBJ_NEW_QSTR(mod_name), MP_MAP_LOOKUP);
+                    mp_map_elem_t *el = mp_map_lookup((mp_map_t*)&mp_builtin_module_weak_links_map, MP_OBJ_NEW_QSTR(mod_name), MP_MAP_LOOKUP);
                     if (el == NULL) {
                         goto no_exist;
                     }
diff --git a/py/builtintables.c b/py/builtintables.c
deleted file mode 100644
index ebf8ddf4a7..0000000000
--- a/py/builtintables.c
+++ /dev/null
@@ -1,261 +0,0 @@
-/*
- * This file is part of the Micro Python project, http://micropython.org/
- *
- * The MIT License (MIT)
- *
- * Copyright (c) 2013, 2014 Damien P. George
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-
-#include <stdlib.h>
-
-#include "mpconfig.h"
-#include "misc.h"
-#include "qstr.h"
-#include "obj.h"
-#include "builtin.h"
-#include "builtintables.h"
-#include "objarray.h"
-
-STATIC const mp_map_elem_t mp_builtin_object_table[] = {
-    // built-in core functions
-    { MP_OBJ_NEW_QSTR(MP_QSTR___build_class__), (mp_obj_t)&mp_builtin___build_class___obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR___import__), (mp_obj_t)&mp_builtin___import___obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR___repl_print__), (mp_obj_t)&mp_builtin___repl_print___obj },
-
-    // built-in types
-    { MP_OBJ_NEW_QSTR(MP_QSTR_bool), (mp_obj_t)&mp_type_bool },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_bytes), (mp_obj_t)&mp_type_bytes },
-#if MICROPY_PY_BUILTINS_BYTEARRAY
-    { MP_OBJ_NEW_QSTR(MP_QSTR_bytearray), (mp_obj_t)&mp_type_bytearray },
-#endif
-#if MICROPY_PY_BUILTINS_COMPLEX
-    { MP_OBJ_NEW_QSTR(MP_QSTR_complex), (mp_obj_t)&mp_type_complex },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_dict), (mp_obj_t)&mp_type_dict },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_enumerate), (mp_obj_t)&mp_type_enumerate },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_filter), (mp_obj_t)&mp_type_filter },
-#if MICROPY_PY_BUILTINS_FLOAT
-    { MP_OBJ_NEW_QSTR(MP_QSTR_float), (mp_obj_t)&mp_type_float },
-#endif
-#if MICROPY_PY_BUILTINS_FROZENSET
-    { MP_OBJ_NEW_QSTR(MP_QSTR_frozenset), (mp_obj_t)&mp_type_frozenset },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_int), (mp_obj_t)&mp_type_int },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_list), (mp_obj_t)&mp_type_list },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_map), (mp_obj_t)&mp_type_map },
-#if MICROPY_PY_BUILTINS_MEMORYVIEW
-    { MP_OBJ_NEW_QSTR(MP_QSTR_memoryview), (mp_obj_t)&mp_type_memoryview },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_object), (mp_obj_t)&mp_type_object },
-#if MICROPY_PY_BUILTINS_PROPERTY
-    { MP_OBJ_NEW_QSTR(MP_QSTR_property), (mp_obj_t)&mp_type_property },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_range), (mp_obj_t)&mp_type_range },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_reversed), (mp_obj_t)&mp_type_reversed },
-#if MICROPY_PY_BUILTINS_SET
-    { MP_OBJ_NEW_QSTR(MP_QSTR_set), (mp_obj_t)&mp_type_set },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_str), (mp_obj_t)&mp_type_str },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_super), (mp_obj_t)&mp_type_super },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_tuple), (mp_obj_t)&mp_type_tuple },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_type), (mp_obj_t)&mp_type_type },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_zip), (mp_obj_t)&mp_type_zip },
-
-    { MP_OBJ_NEW_QSTR(MP_QSTR_classmethod), (mp_obj_t)&mp_type_classmethod },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_staticmethod), (mp_obj_t)&mp_type_staticmethod },
-
-    // built-in objects
-    { MP_OBJ_NEW_QSTR(MP_QSTR_Ellipsis), (mp_obj_t)&mp_const_ellipsis_obj },
-
-    // built-in user functions
-    { MP_OBJ_NEW_QSTR(MP_QSTR_abs), (mp_obj_t)&mp_builtin_abs_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_all), (mp_obj_t)&mp_builtin_all_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_any), (mp_obj_t)&mp_builtin_any_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_bin), (mp_obj_t)&mp_builtin_bin_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_callable), (mp_obj_t)&mp_builtin_callable_obj },
-#if MICROPY_PY_BUILTINS_COMPILE
-    { MP_OBJ_NEW_QSTR(MP_QSTR_compile), (mp_obj_t)&mp_builtin_compile_obj },
-#endif
-    { MP_OBJ_NEW_QSTR(MP_QSTR_chr), (mp_obj_t)&mp_builtin_chr_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_dir), (mp_obj_t)&mp_builtin_dir_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_divmod), (mp_obj_t)&mp_builtin_divmod_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_eval), (mp_obj_t)&mp_builtin_eval_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_exec), (mp_obj_t)&mp_builtin_exec_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_getattr), (mp_obj_t)&mp_builtin_getattr_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_globals), (mp_obj_t)&mp_builtin_globals_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_hasattr), (mp_obj_t)&mp_builtin_hasattr_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_hash), (mp_obj_t)&mp_builtin_hash_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_hex), (mp_obj_t)&mp_builtin_hex_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_id), (mp_obj_t)&mp_builtin_id_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_isinstance), (mp_obj_t)&mp_builtin_isinstance_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_issubclass), (mp_obj_t)&mp_builtin_issubclass_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_iter), (mp_obj_t)&mp_builtin_iter_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_len), (mp_obj_t)&mp_builtin_len_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_locals), (mp_obj_t)&mp_builtin_locals_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_max), (mp_obj_t)&mp_builtin_max_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_min), (mp_obj_t)&mp_builtin_min_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_next), (mp_obj_t)&mp_builtin_next_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_oct), (mp_obj_t)&mp_builtin_oct_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ord), (mp_obj_t)&mp_builtin_ord_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_pow), (mp_obj_t)&mp_builtin_pow_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_print), (mp_obj_t)&mp_builtin_print_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_repr), (mp_obj_t)&mp_builtin_repr_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_round), (mp_obj_t)&mp_builtin_round_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_sorted), (mp_obj_t)&mp_builtin_sorted_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_sum), (mp_obj_t)&mp_builtin_sum_obj },
-
-    // built-in exceptions
-    { MP_OBJ_NEW_QSTR(MP_QSTR_BaseException), (mp_obj_t)&mp_type_BaseException },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ArithmeticError), (mp_obj_t)&mp_type_ArithmeticError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_AssertionError), (mp_obj_t)&mp_type_AssertionError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_AttributeError), (mp_obj_t)&mp_type_AttributeError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_EOFError), (mp_obj_t)&mp_type_EOFError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_Exception), (mp_obj_t)&mp_type_Exception },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_GeneratorExit), (mp_obj_t)&mp_type_GeneratorExit },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ImportError), (mp_obj_t)&mp_type_ImportError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_IndentationError), (mp_obj_t)&mp_type_IndentationError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_IndexError), (mp_obj_t)&mp_type_IndexError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_KeyError), (mp_obj_t)&mp_type_KeyError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_LookupError), (mp_obj_t)&mp_type_LookupError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_MemoryError), (mp_obj_t)&mp_type_MemoryError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_NameError), (mp_obj_t)&mp_type_NameError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_NotImplementedError), (mp_obj_t)&mp_type_NotImplementedError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_OSError), (mp_obj_t)&mp_type_OSError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_OverflowError), (mp_obj_t)&mp_type_OverflowError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_RuntimeError), (mp_obj_t)&mp_type_RuntimeError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_StopIteration), (mp_obj_t)&mp_type_StopIteration },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_SyntaxError), (mp_obj_t)&mp_type_SyntaxError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_SystemExit), (mp_obj_t)&mp_type_SystemExit },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_TypeError), (mp_obj_t)&mp_type_TypeError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ValueError), (mp_obj_t)&mp_type_ValueError },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ZeroDivisionError), (mp_obj_t)&mp_type_ZeroDivisionError },
-    // Somehow CPython managed to have OverflowError not inherit from ValueError ;-/
-    // TODO: For MICROPY_CPYTHON_COMPAT==0 use ValueError to avoid exc proliferation
-
-    // Extra builtins as defined by a port
-    MICROPY_PORT_BUILTINS
-};
-
-const mp_obj_dict_t mp_builtin_object_dict_obj = {
-    .base = {&mp_type_dict},
-    .map = {
-        .all_keys_are_qstrs = 1,
-        .table_is_fixed_array = 1,
-        .used = MP_ARRAY_SIZE(mp_builtin_object_table),
-        .alloc = MP_ARRAY_SIZE(mp_builtin_object_table),
-        .table = (mp_map_elem_t*)mp_builtin_object_table,
-    },
-};
-
-STATIC const mp_obj_module_t mp_module_builtins = {
-    .base = { &mp_type_module },
-    .name = MP_QSTR_builtins,
-    .globals = (mp_obj_dict_t*)&mp_builtin_object_dict_obj,
-};
-
-STATIC const mp_map_elem_t mp_builtin_module_table[] = {
-    { MP_OBJ_NEW_QSTR(MP_QSTR___main__), (mp_obj_t)&mp_module___main__ },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_builtins), (mp_obj_t)&mp_module_builtins },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_micropython), (mp_obj_t)&mp_module_micropython },
-
-#if MICROPY_PY_ARRAY
-    { MP_OBJ_NEW_QSTR(MP_QSTR_array), (mp_obj_t)&mp_module_array },
-#endif
-#if MICROPY_PY_IO
-    { MP_OBJ_NEW_QSTR(MP_QSTR__io), (mp_obj_t)&mp_module_io },
-#endif
-#if MICROPY_PY_COLLECTIONS
-    { MP_OBJ_NEW_QSTR(MP_QSTR__collections), (mp_obj_t)&mp_module_collections },
-#endif
-#if MICROPY_PY_STRUCT
-    { MP_OBJ_NEW_QSTR(MP_QSTR_struct), (mp_obj_t)&mp_module_struct },
-#endif
-
-#if MICROPY_PY_BUILTINS_FLOAT
-#if MICROPY_PY_MATH
-    { MP_OBJ_NEW_QSTR(MP_QSTR_math), (mp_obj_t)&mp_module_math },
-#endif
-#if MICROPY_PY_CMATH
-    { MP_OBJ_NEW_QSTR(MP_QSTR_cmath), (mp_obj_t)&mp_module_cmath },
-#endif
-#endif
-#if MICROPY_PY_SYS
-    { MP_OBJ_NEW_QSTR(MP_QSTR_sys), (mp_obj_t)&mp_module_sys },
-#endif
-#if MICROPY_PY_GC && MICROPY_ENABLE_GC
-    { MP_OBJ_NEW_QSTR(MP_QSTR_gc), (mp_obj_t)&mp_module_gc },
-#endif
-
-    // extmod modules
-
-#if MICROPY_PY_UCTYPES
-    { MP_OBJ_NEW_QSTR(MP_QSTR_uctypes), (mp_obj_t)&mp_module_uctypes },
-#endif
-#if MICROPY_PY_UZLIB
-    { MP_OBJ_NEW_QSTR(MP_QSTR_uzlib), (mp_obj_t)&mp_module_uzlib },
-#endif
-#if MICROPY_PY_UJSON
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ujson), (mp_obj_t)&mp_module_ujson },
-#endif
-#if MICROPY_PY_URE
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ure), (mp_obj_t)&mp_module_ure },
-#endif
-#if MICROPY_PY_UHEAPQ
-    { MP_OBJ_NEW_QSTR(MP_QSTR_uheapq), (mp_obj_t)&mp_module_uheapq },
-#endif
-#if MICROPY_PY_UHASHLIB
-    { MP_OBJ_NEW_QSTR(MP_QSTR_uhashlib), (mp_obj_t)&mp_module_uhashlib },
-#endif
-#if MICROPY_PY_UBINASCII
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ubinascii), (mp_obj_t)&mp_module_ubinascii },
-#endif
-
-    // extra builtin modules as defined by a port
-    MICROPY_PORT_BUILTIN_MODULES
-};
-
-const mp_obj_dict_t mp_builtin_module_dict_obj = {
-    .base = {&mp_type_dict},
-    .map = {
-        .all_keys_are_qstrs = 1,
-        .table_is_fixed_array = 1,
-        .used = MP_ARRAY_SIZE(mp_builtin_module_table),
-        .alloc = MP_ARRAY_SIZE(mp_builtin_module_table),
-        .table = (mp_map_elem_t*)mp_builtin_module_table,
-    },
-};
-
-#if MICROPY_MODULE_WEAK_LINKS
-STATIC const mp_map_elem_t mp_builtin_module_weak_links_table[] = {
-    MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
-};
-
-const mp_obj_dict_t mp_builtin_module_weak_links_dict_obj = {
-    .base = {&mp_type_dict},
-    .map = {
-        .all_keys_are_qstrs = 1,
-        .table_is_fixed_array = 1,
-        .used = MP_ARRAY_SIZE(mp_builtin_module_weak_links_table),
-        .alloc = MP_ARRAY_SIZE(mp_builtin_module_weak_links_table),
-        .table = (mp_map_elem_t*)mp_builtin_module_weak_links_table,
-    },
-};
-#endif
diff --git a/py/builtintables.h b/py/builtintables.h
deleted file mode 100644
index 5a865a78b4..0000000000
--- a/py/builtintables.h
+++ /dev/null
@@ -1,32 +0,0 @@
-/*
- * This file is part of the Micro Python project, http://micropython.org/
- *
- * The MIT License (MIT)
- *
- * Copyright (c) 2013, 2014 Damien P. George
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-
-extern const mp_obj_dict_t mp_builtin_object_dict_obj;
-extern const mp_obj_dict_t mp_builtin_module_dict_obj;
-
-#if MICROPY_MODULE_WEAK_LINKS
-extern const mp_obj_dict_t mp_builtin_module_weak_links_dict_obj;
-#endif
diff --git a/py/builtin.c b/py/modbuiltins.c
similarity index 73%
rename from py/builtin.c
rename to py/modbuiltins.c
index 6bcde43645..181b08eb13 100644
--- a/py/builtin.c
+++ b/py/modbuiltins.c
@@ -561,3 +561,131 @@ MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_id_obj, mp_obj_id);
 MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_len_obj, mp_obj_len);
 MP_DEFINE_CONST_FUN_OBJ_0(mp_builtin_globals_obj, mp_globals_get);
 MP_DEFINE_CONST_FUN_OBJ_0(mp_builtin_locals_obj, mp_locals_get);
+
+STATIC const mp_map_elem_t mp_module_builtins_globals_table[] = {
+    // built-in core functions
+    { MP_OBJ_NEW_QSTR(MP_QSTR___build_class__), (mp_obj_t)&mp_builtin___build_class___obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR___import__), (mp_obj_t)&mp_builtin___import___obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR___repl_print__), (mp_obj_t)&mp_builtin___repl_print___obj },
+
+    // built-in types
+    { MP_OBJ_NEW_QSTR(MP_QSTR_bool), (mp_obj_t)&mp_type_bool },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_bytes), (mp_obj_t)&mp_type_bytes },
+#if MICROPY_PY_BUILTINS_BYTEARRAY
+    { MP_OBJ_NEW_QSTR(MP_QSTR_bytearray), (mp_obj_t)&mp_type_bytearray },
+#endif
+#if MICROPY_PY_BUILTINS_COMPLEX
+    { MP_OBJ_NEW_QSTR(MP_QSTR_complex), (mp_obj_t)&mp_type_complex },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_dict), (mp_obj_t)&mp_type_dict },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_enumerate), (mp_obj_t)&mp_type_enumerate },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_filter), (mp_obj_t)&mp_type_filter },
+#if MICROPY_PY_BUILTINS_FLOAT
+    { MP_OBJ_NEW_QSTR(MP_QSTR_float), (mp_obj_t)&mp_type_float },
+#endif
+#if MICROPY_PY_BUILTINS_FROZENSET
+    { MP_OBJ_NEW_QSTR(MP_QSTR_frozenset), (mp_obj_t)&mp_type_frozenset },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_int), (mp_obj_t)&mp_type_int },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_list), (mp_obj_t)&mp_type_list },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_map), (mp_obj_t)&mp_type_map },
+#if MICROPY_PY_BUILTINS_MEMORYVIEW
+    { MP_OBJ_NEW_QSTR(MP_QSTR_memoryview), (mp_obj_t)&mp_type_memoryview },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_object), (mp_obj_t)&mp_type_object },
+#if MICROPY_PY_BUILTINS_PROPERTY
+    { MP_OBJ_NEW_QSTR(MP_QSTR_property), (mp_obj_t)&mp_type_property },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_range), (mp_obj_t)&mp_type_range },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_reversed), (mp_obj_t)&mp_type_reversed },
+#if MICROPY_PY_BUILTINS_SET
+    { MP_OBJ_NEW_QSTR(MP_QSTR_set), (mp_obj_t)&mp_type_set },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_str), (mp_obj_t)&mp_type_str },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_super), (mp_obj_t)&mp_type_super },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_tuple), (mp_obj_t)&mp_type_tuple },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_type), (mp_obj_t)&mp_type_type },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_zip), (mp_obj_t)&mp_type_zip },
+
+    { MP_OBJ_NEW_QSTR(MP_QSTR_classmethod), (mp_obj_t)&mp_type_classmethod },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_staticmethod), (mp_obj_t)&mp_type_staticmethod },
+
+    // built-in objects
+    { MP_OBJ_NEW_QSTR(MP_QSTR_Ellipsis), (mp_obj_t)&mp_const_ellipsis_obj },
+
+    // built-in user functions
+    { MP_OBJ_NEW_QSTR(MP_QSTR_abs), (mp_obj_t)&mp_builtin_abs_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_all), (mp_obj_t)&mp_builtin_all_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_any), (mp_obj_t)&mp_builtin_any_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_bin), (mp_obj_t)&mp_builtin_bin_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_callable), (mp_obj_t)&mp_builtin_callable_obj },
+#if MICROPY_PY_BUILTINS_COMPILE
+    { MP_OBJ_NEW_QSTR(MP_QSTR_compile), (mp_obj_t)&mp_builtin_compile_obj },
+#endif
+    { MP_OBJ_NEW_QSTR(MP_QSTR_chr), (mp_obj_t)&mp_builtin_chr_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_dir), (mp_obj_t)&mp_builtin_dir_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_divmod), (mp_obj_t)&mp_builtin_divmod_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_eval), (mp_obj_t)&mp_builtin_eval_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_exec), (mp_obj_t)&mp_builtin_exec_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_getattr), (mp_obj_t)&mp_builtin_getattr_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_globals), (mp_obj_t)&mp_builtin_globals_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_hasattr), (mp_obj_t)&mp_builtin_hasattr_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_hash), (mp_obj_t)&mp_builtin_hash_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_hex), (mp_obj_t)&mp_builtin_hex_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_id), (mp_obj_t)&mp_builtin_id_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_isinstance), (mp_obj_t)&mp_builtin_isinstance_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_issubclass), (mp_obj_t)&mp_builtin_issubclass_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_iter), (mp_obj_t)&mp_builtin_iter_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_len), (mp_obj_t)&mp_builtin_len_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_locals), (mp_obj_t)&mp_builtin_locals_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_max), (mp_obj_t)&mp_builtin_max_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_min), (mp_obj_t)&mp_builtin_min_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_next), (mp_obj_t)&mp_builtin_next_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_oct), (mp_obj_t)&mp_builtin_oct_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ord), (mp_obj_t)&mp_builtin_ord_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_pow), (mp_obj_t)&mp_builtin_pow_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_print), (mp_obj_t)&mp_builtin_print_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_repr), (mp_obj_t)&mp_builtin_repr_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_round), (mp_obj_t)&mp_builtin_round_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_sorted), (mp_obj_t)&mp_builtin_sorted_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_sum), (mp_obj_t)&mp_builtin_sum_obj },
+
+    // built-in exceptions
+    { MP_OBJ_NEW_QSTR(MP_QSTR_BaseException), (mp_obj_t)&mp_type_BaseException },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ArithmeticError), (mp_obj_t)&mp_type_ArithmeticError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_AssertionError), (mp_obj_t)&mp_type_AssertionError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_AttributeError), (mp_obj_t)&mp_type_AttributeError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_EOFError), (mp_obj_t)&mp_type_EOFError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_Exception), (mp_obj_t)&mp_type_Exception },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_GeneratorExit), (mp_obj_t)&mp_type_GeneratorExit },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ImportError), (mp_obj_t)&mp_type_ImportError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_IndentationError), (mp_obj_t)&mp_type_IndentationError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_IndexError), (mp_obj_t)&mp_type_IndexError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_KeyError), (mp_obj_t)&mp_type_KeyError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_LookupError), (mp_obj_t)&mp_type_LookupError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_MemoryError), (mp_obj_t)&mp_type_MemoryError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_NameError), (mp_obj_t)&mp_type_NameError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_NotImplementedError), (mp_obj_t)&mp_type_NotImplementedError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_OSError), (mp_obj_t)&mp_type_OSError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_OverflowError), (mp_obj_t)&mp_type_OverflowError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_RuntimeError), (mp_obj_t)&mp_type_RuntimeError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_StopIteration), (mp_obj_t)&mp_type_StopIteration },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_SyntaxError), (mp_obj_t)&mp_type_SyntaxError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_SystemExit), (mp_obj_t)&mp_type_SystemExit },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_TypeError), (mp_obj_t)&mp_type_TypeError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ValueError), (mp_obj_t)&mp_type_ValueError },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ZeroDivisionError), (mp_obj_t)&mp_type_ZeroDivisionError },
+    // Somehow CPython managed to have OverflowError not inherit from ValueError ;-/
+    // TODO: For MICROPY_CPYTHON_COMPAT==0 use ValueError to avoid exc proliferation
+
+    // Extra builtins as defined by a port
+    MICROPY_PORT_BUILTINS
+};
+
+MP_DEFINE_CONST_DICT(mp_module_builtins_globals, mp_module_builtins_globals_table);
+
+const mp_obj_module_t mp_module_builtins = {
+    .base = { &mp_type_module },
+    .name = MP_QSTR_builtins,
+    .globals = (mp_obj_dict_t*)&mp_module_builtins_globals,
+};
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 4bd7bfeab7..880e469873 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -292,6 +292,11 @@ typedef double mp_float_t;
 #define MICROPY_MODULE_WEAK_LINKS (0)
 #endif
 
+// Whether you can override builtins in the builtins module
+#ifndef MICROPY_CAN_OVERRIDE_BUILTINS
+#define MICROPY_CAN_OVERRIDE_BUILTINS (0)
+#endif
+
 /*****************************************************************************/
 /* Fine control over Python builtins, classes, modules, etc                  */
 
diff --git a/py/obj.h b/py/obj.h
index 6db76405e9..ea85542bd6 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -97,9 +97,18 @@ typedef struct _mp_obj_base_t mp_obj_base_t;
 #define MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(obj_name, n_args_min, n_args_max, fun_name) MP_DEFINE_CONST_FUN_OBJ_VOID_PTR(obj_name, false, n_args_min, n_args_max, (mp_fun_var_t)fun_name)
 #define MP_DEFINE_CONST_FUN_OBJ_KW(obj_name, n_args_min, fun_name) MP_DEFINE_CONST_FUN_OBJ_VOID_PTR(obj_name, true, n_args_min, MP_OBJ_FUN_ARGS_MAX, (mp_fun_kw_t)fun_name)
 
-// This macro is used to define constant dict objects
+// These macros are used to define constant map/dict objects
 // You can put "static" in front of the definition to make it local
 
+#define MP_DEFINE_CONST_MAP(map_name, table_name) \
+    const mp_map_t map_name = { \
+        .all_keys_are_qstrs = 1, \
+        .table_is_fixed_array = 1, \
+        .used = MP_ARRAY_SIZE(table_name), \
+        .alloc = MP_ARRAY_SIZE(table_name), \
+        .table = (mp_map_elem_t*)table_name, \
+    }
+
 #define MP_DEFINE_CONST_DICT(dict_name, table_name) \
     const mp_obj_dict_t dict_name = { \
         .base = {&mp_type_dict}, \
diff --git a/py/objmodule.c b/py/objmodule.c
index 7f765ff95c..5ffb368bf5 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -34,7 +34,7 @@
 #include "obj.h"
 #include "objmodule.h"
 #include "runtime.h"
-#include "builtintables.h"
+#include "builtin.h"
 
 STATIC mp_map_t mp_loaded_modules_map; // TODO: expose as sys.modules
 
@@ -65,13 +65,28 @@ STATIC void module_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
 
 STATIC bool module_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
     mp_obj_module_t *self = self_in;
+    mp_obj_dict_t *dict = self->globals;
+    if (dict->map.table_is_fixed_array) {
+        #if MICROPY_CAN_OVERRIDE_BUILTINS
+        if (dict == &mp_module_builtins_globals) {
+            if (mp_module_builtins_override_dict == NULL) {
+                mp_module_builtins_override_dict = mp_obj_new_dict(1);
+            }
+            dict = mp_module_builtins_override_dict;
+        } else
+        #endif
+        {
+            // can't delete or store to fixed map
+            return false;
+        }
+    }
     if (value == MP_OBJ_NULL) {
         // delete attribute
-        mp_obj_dict_delete(self->globals, MP_OBJ_NEW_QSTR(attr));
+        mp_obj_dict_delete(dict, MP_OBJ_NEW_QSTR(attr));
     } else {
         // store attribute
         // TODO CPython allows STORE_ATTR to a module, but is this the correct implementation?
-        mp_obj_dict_store(self->globals, MP_OBJ_NEW_QSTR(attr), value);
+        mp_obj_dict_store(dict, MP_OBJ_NEW_QSTR(attr), value);
     }
     return true;
 }
@@ -117,6 +132,69 @@ mp_obj_dict_t *mp_obj_module_get_globals(mp_obj_t self_in) {
 /******************************************************************************/
 // Global module table and related functions
 
+STATIC const mp_map_elem_t mp_builtin_module_table[] = {
+    { MP_OBJ_NEW_QSTR(MP_QSTR___main__), (mp_obj_t)&mp_module___main__ },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_builtins), (mp_obj_t)&mp_module_builtins },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_micropython), (mp_obj_t)&mp_module_micropython },
+
+#if MICROPY_PY_ARRAY
+    { MP_OBJ_NEW_QSTR(MP_QSTR_array), (mp_obj_t)&mp_module_array },
+#endif
+#if MICROPY_PY_IO
+    { MP_OBJ_NEW_QSTR(MP_QSTR__io), (mp_obj_t)&mp_module_io },
+#endif
+#if MICROPY_PY_COLLECTIONS
+    { MP_OBJ_NEW_QSTR(MP_QSTR__collections), (mp_obj_t)&mp_module_collections },
+#endif
+#if MICROPY_PY_STRUCT
+    { MP_OBJ_NEW_QSTR(MP_QSTR_struct), (mp_obj_t)&mp_module_struct },
+#endif
+
+#if MICROPY_PY_BUILTINS_FLOAT
+#if MICROPY_PY_MATH
+    { MP_OBJ_NEW_QSTR(MP_QSTR_math), (mp_obj_t)&mp_module_math },
+#endif
+#if MICROPY_PY_CMATH
+    { MP_OBJ_NEW_QSTR(MP_QSTR_cmath), (mp_obj_t)&mp_module_cmath },
+#endif
+#endif
+#if MICROPY_PY_SYS
+    { MP_OBJ_NEW_QSTR(MP_QSTR_sys), (mp_obj_t)&mp_module_sys },
+#endif
+#if MICROPY_PY_GC && MICROPY_ENABLE_GC
+    { MP_OBJ_NEW_QSTR(MP_QSTR_gc), (mp_obj_t)&mp_module_gc },
+#endif
+
+    // extmod modules
+
+#if MICROPY_PY_UCTYPES
+    { MP_OBJ_NEW_QSTR(MP_QSTR_uctypes), (mp_obj_t)&mp_module_uctypes },
+#endif
+#if MICROPY_PY_UZLIB
+    { MP_OBJ_NEW_QSTR(MP_QSTR_uzlib), (mp_obj_t)&mp_module_uzlib },
+#endif
+#if MICROPY_PY_UJSON
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ujson), (mp_obj_t)&mp_module_ujson },
+#endif
+#if MICROPY_PY_URE
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ure), (mp_obj_t)&mp_module_ure },
+#endif
+#if MICROPY_PY_UHEAPQ
+    { MP_OBJ_NEW_QSTR(MP_QSTR_uheapq), (mp_obj_t)&mp_module_uheapq },
+#endif
+#if MICROPY_PY_UHASHLIB
+    { MP_OBJ_NEW_QSTR(MP_QSTR_uhashlib), (mp_obj_t)&mp_module_uhashlib },
+#endif
+#if MICROPY_PY_UBINASCII
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ubinascii), (mp_obj_t)&mp_module_ubinascii },
+#endif
+
+    // extra builtin modules as defined by a port
+    MICROPY_PORT_BUILTIN_MODULES
+};
+
+STATIC MP_DEFINE_CONST_MAP(mp_builtin_module_map, mp_builtin_module_table);
+
 void mp_module_init(void) {
     mp_map_init(&mp_loaded_modules_map, 3);
 }
@@ -132,7 +210,7 @@ mp_obj_t mp_module_get(qstr module_name) {
 
     if (el == NULL) {
         // module not found, look for builtin module names
-        el = mp_map_lookup((mp_map_t*)&mp_builtin_module_dict_obj.map, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP);
+        el = mp_map_lookup((mp_map_t*)&mp_builtin_module_map, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP);
         if (el == NULL) {
             return MP_OBJ_NULL;
         }
diff --git a/py/py.mk b/py/py.mk
index 52c284e519..99fc0223ce 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -91,11 +91,10 @@ PY_O_BASENAME = \
 	sequence.o \
 	stream.o \
 	binary.o \
-	builtin.o \
 	builtinimport.o \
 	builtinevex.o \
-	builtintables.o \
 	modarray.o \
+	modbuiltins.o \
 	modcollections.o \
 	modgc.o \
 	modio.o \
diff --git a/py/runtime.c b/py/runtime.c
index 463e325d22..0690bd8b05 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -42,7 +42,6 @@
 #include "runtime.h"
 #include "emitglue.h"
 #include "builtin.h"
-#include "builtintables.h"
 #include "bc.h"
 #include "smallint.h"
 #include "objgenerator.h"
@@ -78,6 +77,10 @@ const mp_obj_module_t mp_module___main__ = {
     .globals = (mp_obj_dict_t*)&dict_main,
 };
 
+#if MICROPY_CAN_OVERRIDE_BUILTINS
+mp_obj_dict_t *mp_module_builtins_override_dict;
+#endif
+
 void mp_init(void) {
     qstr_init();
     mp_stack_ctrl_init();
@@ -106,6 +109,11 @@ void mp_init(void) {
 
     // locals = globals for outer module (see Objects/frameobject.c/PyFrame_New())
     dict_locals = dict_globals = &dict_main;
+
+    #if MICROPY_CAN_OVERRIDE_BUILTINS
+    // start with no extensions to builtins
+    mp_module_builtins_override_dict = NULL;
+    #endif
 }
 
 void mp_deinit(void) {
@@ -162,8 +170,16 @@ mp_obj_t mp_load_global(qstr qstr) {
     DEBUG_OP_printf("load global %s\n", qstr_str(qstr));
     mp_map_elem_t *elem = mp_map_lookup(&dict_globals->map, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP);
     if (elem == NULL) {
-        // TODO lookup in dynamic table of builtins first
-        elem = mp_map_lookup((mp_map_t*)&mp_builtin_object_dict_obj.map, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP);
+        #if MICROPY_CAN_OVERRIDE_BUILTINS
+        if (mp_module_builtins_override_dict != NULL) {
+            // lookup in additional dynamic table of builtins first
+            elem = mp_map_lookup(&mp_module_builtins_override_dict->map, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP);
+            if (elem != NULL) {
+                return elem->value;
+            }
+        }
+        #endif
+        elem = mp_map_lookup((mp_map_t*)&mp_module_builtins_globals.map, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP);
         if (elem == NULL) {
             if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
                 nlr_raise(mp_obj_new_exception_msg(&mp_type_NameError,
@@ -179,8 +195,15 @@ mp_obj_t mp_load_global(qstr qstr) {
 
 mp_obj_t mp_load_build_class(void) {
     DEBUG_OP_printf("load_build_class\n");
-    // TODO lookup __build_class__ in dynamic table of builtins first
-    // ... else no user-defined __build_class__, return builtin one
+    #if MICROPY_CAN_OVERRIDE_BUILTINS
+    if (mp_module_builtins_override_dict != NULL) {
+        // lookup in additional dynamic table of builtins first
+        mp_map_elem_t *elem = mp_map_lookup(&mp_module_builtins_override_dict->map, MP_OBJ_NEW_QSTR(MP_QSTR___build_class__), MP_MAP_LOOKUP);
+        if (elem != NULL) {
+            return elem->value;
+        }
+    }
+    #endif
     return (mp_obj_t)&mp_builtin___build_class___obj;
 }
 
diff --git a/stmhal/mpconfigport.h b/stmhal/mpconfigport.h
index d730173b0a..1b2cf4abfa 100644
--- a/stmhal/mpconfigport.h
+++ b/stmhal/mpconfigport.h
@@ -50,6 +50,7 @@
 #define MICROPY_LFN_CODE_PAGE       (437) /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_MODULE_WEAK_LINKS   (1)
+#define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
diff --git a/tests/basics/builtin_override.py b/tests/basics/builtin_override.py
new file mode 100644
index 0000000000..e245985ad9
--- /dev/null
+++ b/tests/basics/builtin_override.py
@@ -0,0 +1,13 @@
+# test overriding builtins
+
+import builtins
+
+# override generic builtin
+builtins.abs = lambda x: x + 1
+print(abs(1))
+
+# __build_class__ is handled in a special way
+builtins.__build_class__ = lambda x, y: ('class', y)
+class A:
+    pass
+print(A)
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index e08da770ad..51d100b722 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -44,6 +44,7 @@
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (1)
+#define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
diff --git a/windows/mpconfigport.h b/windows/mpconfigport.h
index 5da912dec6..1ec0c41aff 100644
--- a/windows/mpconfigport.h
+++ b/windows/mpconfigport.h
@@ -46,6 +46,7 @@
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (0)
+#define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)