diff --git a/py/builtin.c b/py/builtin.c
index d29a2bf..b565ed4 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -8,6 +8,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -91,7 +92,7 @@
     switch (n_args) {
         case 0: return mp_const_false;
         case 1: if (rt_is_true(args[0])) { return mp_const_true; } else { return mp_const_false; }
-        default: nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "bool() takes at most 1 argument (%d given)", (void*)(machine_int_t)n_args));
+        default: nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "bool() takes at most 1 argument (%d given)", (void*)(machine_int_t)n_args));
     }
 }
 
@@ -147,7 +148,7 @@
         str[1] = '\0';
         return mp_obj_new_str(qstr_from_str_take(str, 2));
     } else {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_ValueError, "chr() arg not in range(0x110000)"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_ValueError, "chr() arg not in range(0x110000)"));
     }
 }
 
@@ -165,7 +166,7 @@
         revs_args[0] = MP_OBJ_NEW_SMALL_INT(i1 % i2);
         return rt_build_tuple(2, revs_args);
     } else {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "unsupported operand type(s) for divmod(): '%s' and '%s'", mp_obj_get_type_str(o1_in), mp_obj_get_type_str(o2_in)));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "unsupported operand type(s) for divmod(): '%s' and '%s'", mp_obj_get_type_str(o1_in), mp_obj_get_type_str(o2_in)));
     }
 }
 
@@ -235,7 +236,7 @@
     } else if (MP_OBJ_IS_TYPE(o_in, &dict_type)) {
         len = mp_obj_dict_len(o_in);
     } else {
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "object of type '%s' has no len()", mp_obj_get_type_str(o_in)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "object of type '%s' has no len()", mp_obj_get_type_str(o_in)));
     }
     return MP_OBJ_NEW_SMALL_INT(len);
 }
@@ -254,7 +255,7 @@
             }
             return list;
         }
-        default: nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "list() takes at most 1 argument (%d given)", (void*)(machine_int_t)n_args));
+        default: nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "list() takes at most 1 argument (%d given)", (void*)(machine_int_t)n_args));
     }
 }
 
@@ -270,7 +271,7 @@
             }
         }
         if (max_obj == NULL) {
-            nlr_jump(mp_obj_new_exception_msg(rt_q_ValueError, "max() arg is an empty sequence"));
+            nlr_jump(mp_obj_new_exception_msg(MP_QSTR_ValueError, "max() arg is an empty sequence"));
         }
         return max_obj;
     } else {
@@ -297,7 +298,7 @@
             }
         }
         if (min_obj == NULL) {
-            nlr_jump(mp_obj_new_exception_msg(rt_q_ValueError, "min() arg is an empty sequence"));
+            nlr_jump(mp_obj_new_exception_msg(MP_QSTR_ValueError, "min() arg is an empty sequence"));
         }
         return min_obj;
     } else {
@@ -315,7 +316,7 @@
 static mp_obj_t mp_builtin_next(mp_obj_t o) {
     mp_obj_t ret = rt_iternext(o);
     if (ret == mp_const_stop_iteration) {
-        nlr_jump(mp_obj_new_exception(qstr_from_str_static("StopIteration")));
+        nlr_jump(mp_obj_new_exception(MP_QSTR_StopIteration));
     } else {
         return ret;
     }
@@ -328,7 +329,7 @@
     if (strlen(str) == 1) {
         return mp_obj_new_int(str[0]);
     } else {
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "ord() expected a character, but string of length %d found", (void*)(machine_int_t)strlen(str)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "ord() expected a character, but string of length %d found", (void*)(machine_int_t)strlen(str)));
     }
 }
 
@@ -336,7 +337,7 @@
     switch (n_args) {
         case 2: return rt_binary_op(RT_BINARY_OP_POWER, args[0], args[1]);
         case 3: return rt_binary_op(RT_BINARY_OP_MODULO, rt_binary_op(RT_BINARY_OP_POWER, args[0], args[1]), args[2]); // TODO optimise...
-        default: nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "pow expected at most 3 arguments, got %d", (void*)(machine_int_t)n_args));
+        default: nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "pow expected at most 3 arguments, got %d", (void*)(machine_int_t)n_args));
     }
 }
 
@@ -362,7 +363,7 @@
         case 1: return mp_obj_new_range(0, mp_obj_get_int(args[0]), 1);
         case 2: return mp_obj_new_range(mp_obj_get_int(args[0]), mp_obj_get_int(args[1]), 1);
         case 3: return mp_obj_new_range(mp_obj_get_int(args[0]), mp_obj_get_int(args[1]), mp_obj_get_int(args[2]));
-        default: nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "range expected at most 3 arguments, got %d", (void*)(machine_int_t)n_args));
+        default: nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "range expected at most 3 arguments, got %d", (void*)(machine_int_t)n_args));
     }
 }
 
@@ -391,7 +392,7 @@
     switch (n_args) {
         case 1: value = mp_obj_new_int(0); break;
         case 2: value = args[1]; break;
-        default: nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "sum expected at most 2 arguments, got %d", (void*)(machine_int_t)n_args));
+        default: nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "sum expected at most 2 arguments, got %d", (void*)(machine_int_t)n_args));
     }
     mp_obj_t iterable = rt_getiter(args[0]);
     mp_obj_t item;
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 47dbf21..90a0fc3 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -58,7 +58,9 @@
         return mp_const_none;
     }
 
-    if (!mp_compile(pn, false)) {
+    mp_obj_t module_fun = mp_compile(pn, false);
+
+    if (module_fun == mp_const_none) {
         // TODO handle compile error correctly
         rt_locals_set(old_locals);
         rt_globals_set(old_globals);
@@ -66,7 +68,6 @@
     }
 
     // complied successfully, execute it
-    mp_obj_t module_fun = rt_make_function_from_id(1); // TODO we should return from mp_compile the unique_code_id for the module
     nlr_buf_t nlr;
     if (nlr_push(&nlr) == 0) {
         rt_call_function_0(module_fun);
diff --git a/py/compile.c b/py/compile.c
index 8db7c6d..f8fa2cb 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -7,6 +7,7 @@
 
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "lexer.h"
 #include "parse.h"
 #include "scope.h"
@@ -38,20 +39,6 @@
 #define EMIT_OPT_ASM_THUMB      (4)
 
 typedef struct _compiler_t {
-    qstr qstr___class__;
-    qstr qstr___locals__;
-    qstr qstr___name__;
-    qstr qstr___module__;
-    qstr qstr___qualname__;
-    qstr qstr___doc__;
-    qstr qstr_assertion_error;
-    qstr qstr_micropython;
-    qstr qstr_byte_code;
-    qstr qstr_native;
-    qstr qstr_viper;
-    qstr qstr_asm_thumb;
-    qstr qstr_range;
-
     bool is_repl;
     pass_kind_t pass;
     bool had_error; // try to keep compiler clean from nlr
@@ -202,7 +189,7 @@
 }
 
 static scope_t *scope_new_and_link(compiler_t *comp, scope_kind_t kind, mp_parse_node_t pn, uint emit_options) {
-    scope_t *scope = scope_new(kind, pn, rt_get_unique_code_id(kind == SCOPE_MODULE), emit_options);
+    scope_t *scope = scope_new(kind, pn, rt_get_unique_code_id(), emit_options);
     scope->parent = comp->scope_cur;
     scope->next = NULL;
     if (comp->scope_head == NULL) {
@@ -903,7 +890,7 @@
 
 // returns true if it was a built-in decorator (even if the built-in had an error)
 static bool compile_built_in_decorator(compiler_t *comp, int name_len, mp_parse_node_t *name_nodes, uint *emit_options) {
-    if (MP_PARSE_NODE_LEAF_ARG(name_nodes[0]) != comp->qstr_micropython) {
+    if (MP_PARSE_NODE_LEAF_ARG(name_nodes[0]) != MP_QSTR_micropython) {
         return false;
     }
 
@@ -913,16 +900,16 @@
     }
 
     qstr attr = MP_PARSE_NODE_LEAF_ARG(name_nodes[1]);
-    if (attr == comp->qstr_byte_code) {
+    if (attr == MP_QSTR_byte_code) {
         *emit_options = EMIT_OPT_BYTE_CODE;
 #if MICROPY_EMIT_NATIVE
-    } else if (attr == comp->qstr_native) {
+    } else if (attr == MP_QSTR_native) {
         *emit_options = EMIT_OPT_NATIVE_PYTHON;
-    } else if (attr == comp->qstr_viper) {
+    } else if (attr == MP_QSTR_viper) {
         *emit_options = EMIT_OPT_VIPER;
 #endif
 #if MICROPY_EMIT_INLINE_THUMB
-    } else if (attr == comp->qstr_asm_thumb) {
+    } else if (attr == MP_QSTR_asm_thumb) {
         *emit_options = EMIT_OPT_ASM_THUMB;
 #endif
     } else {
@@ -1329,7 +1316,7 @@
 void compile_assert_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
     int l_end = comp_next_label(comp);
     c_if_cond(comp, pns->nodes[0], true, l_end);
-    EMIT(load_id, comp->qstr_assertion_error);
+    EMIT(load_id, MP_QSTR_assertion_error);
     if (!MP_PARSE_NODE_IS_NULL(pns->nodes[1])) {
         // assertion message
         compile_node(comp, pns->nodes[1]);
@@ -1495,7 +1482,7 @@
     // for viper it will be much, much faster
     if (/*comp->scope_cur->emit_options == EMIT_OPT_VIPER &&*/ MP_PARSE_NODE_IS_ID(pns->nodes[0]) && MP_PARSE_NODE_IS_STRUCT_KIND(pns->nodes[1], PN_power)) {
         mp_parse_node_struct_t *pns_it = (mp_parse_node_struct_t*)pns->nodes[1];
-        if (MP_PARSE_NODE_IS_ID(pns_it->nodes[0]) && MP_PARSE_NODE_LEAF_ARG(pns_it->nodes[0]) == comp->qstr_range && MP_PARSE_NODE_IS_STRUCT_KIND(pns_it->nodes[1], PN_trailer_paren) && MP_PARSE_NODE_IS_NULL(pns_it->nodes[2])) {
+        if (MP_PARSE_NODE_IS_ID(pns_it->nodes[0]) && MP_PARSE_NODE_LEAF_ARG(pns_it->nodes[0]) == MP_QSTR_range && MP_PARSE_NODE_IS_STRUCT_KIND(pns_it->nodes[1], PN_trailer_paren) && MP_PARSE_NODE_IS_NULL(pns_it->nodes[2])) {
             mp_parse_node_t pn_range_args = ((mp_parse_node_struct_t*)pns_it->nodes[1])->nodes[0];
             mp_parse_node_t *args;
             int n_args = list_get(&pn_range_args, PN_arglist, &args);
@@ -1743,7 +1730,7 @@
     if (MP_PARSE_NODE_IS_NULL(pns->nodes[1])) {
         if (comp->is_repl && comp->scope_cur->kind == SCOPE_MODULE) {
             // for REPL, evaluate then print the expression
-            EMIT(load_id, qstr_from_str_static("__repl_print__"));
+            EMIT(load_id, MP_QSTR___repl_print__);
             compile_node(comp, pns->nodes[0]);
             EMIT(call_function, 1, 0, false, false);
             EMIT(pop_top);
@@ -2683,7 +2670,7 @@
             if (kind == MP_PARSE_NODE_STRING) {
                 compile_node(comp, pns->nodes[0]); // a doc string
                 // store doc string
-                EMIT(store_id, comp->qstr___doc__);
+                EMIT(store_id, MP_QSTR___doc__);
             }
         }
     }
@@ -2796,35 +2783,35 @@
 
         if (comp->pass == PASS_1) {
             bool added;
-            id_info_t *id_info = scope_find_or_add_id(scope, comp->qstr___class__, &added);
+            id_info_t *id_info = scope_find_or_add_id(scope, MP_QSTR___class__, &added);
             assert(added);
             id_info->kind = ID_INFO_KIND_LOCAL;
-            id_info = scope_find_or_add_id(scope, comp->qstr___locals__, &added);
+            id_info = scope_find_or_add_id(scope, MP_QSTR___locals__, &added);
             assert(added);
             id_info->kind = ID_INFO_KIND_LOCAL;
             id_info->param = true;
             scope->num_params = 1; // __locals__ is the parameter
         }
 
-        EMIT(load_id, comp->qstr___locals__);
+        EMIT(load_id, MP_QSTR___locals__);
         EMIT(store_locals);
-        EMIT(load_id, comp->qstr___name__);
-        EMIT(store_id, comp->qstr___module__);
+        EMIT(load_id, MP_QSTR___name__);
+        EMIT(store_id, MP_QSTR___module__);
         EMIT(load_const_id, MP_PARSE_NODE_LEAF_ARG(pns->nodes[0])); // 0 is class name
-        EMIT(store_id, comp->qstr___qualname__);
+        EMIT(store_id, MP_QSTR___qualname__);
 
         check_for_doc_string(comp, pns->nodes[2]);
         compile_node(comp, pns->nodes[2]); // 2 is class body
 
-        id_info_t *id = scope_find(scope, comp->qstr___class__);
+        id_info_t *id = scope_find(scope, MP_QSTR___class__);
         assert(id != NULL);
         if (id->kind == ID_INFO_KIND_LOCAL) {
             EMIT(load_const_tok, MP_TOKEN_KW_NONE);
         } else {
 #if MICROPY_EMIT_CPYTHON
-            EMIT(load_closure, comp->qstr___class__, 0); // XXX check this is the correct local num
+            EMIT(load_closure, MP_QSTR___class__, 0); // XXX check this is the correct local num
 #else
-            EMIT(load_fast, comp->qstr___class__, 0); // XXX check this is the correct local num
+            EMIT(load_fast, MP_QSTR___class__, 0); // XXX check this is the correct local num
 #endif
         }
         EMIT(return_value);
@@ -2917,7 +2904,7 @@
     scope->num_locals = 0;
     for (int i = 0; i < scope->id_info_len; i++) {
         id_info_t *id = &scope->id_info[i];
-        if (scope->kind == SCOPE_CLASS && id->qstr == comp->qstr___class__) {
+        if (scope->kind == SCOPE_CLASS && id->qstr == MP_QSTR___class__) {
             // __class__ is not counted as a local; if it's used then it becomes a ID_INFO_KIND_CELL
             continue;
         }
@@ -3021,20 +3008,6 @@
 mp_obj_t mp_compile(mp_parse_node_t pn, bool is_repl) {
     compiler_t *comp = m_new(compiler_t, 1);
 
-    comp->qstr___class__ = qstr_from_str_static("__class__");
-    comp->qstr___locals__ = qstr_from_str_static("__locals__");
-    comp->qstr___name__ = qstr_from_str_static("__name__");
-    comp->qstr___module__ = qstr_from_str_static("__module__");
-    comp->qstr___qualname__ = qstr_from_str_static("__qualname__");
-    comp->qstr___doc__ = qstr_from_str_static("__doc__");
-    comp->qstr_assertion_error = qstr_from_str_static("AssertionError");
-    comp->qstr_micropython = qstr_from_str_static("micropython");
-    comp->qstr_byte_code = qstr_from_str_static("byte_code");
-    comp->qstr_native = qstr_from_str_static("native");
-    comp->qstr_viper = qstr_from_str_static("viper");
-    comp->qstr_asm_thumb = qstr_from_str_static("asm_thumb");
-    comp->qstr_range = qstr_from_str_static("range");
-
     comp->is_repl = is_repl;
     comp->had_error = false;
 
@@ -3048,10 +3021,10 @@
     pn = fold_constants(pn);
 
     // set the outer scope
-    scope_new_and_link(comp, SCOPE_MODULE, pn, EMIT_OPT_NONE);
+    scope_t *module_scope = scope_new_and_link(comp, SCOPE_MODULE, pn, EMIT_OPT_NONE);
 
     // compile pass 1
-    comp->emit = emit_pass1_new(comp->qstr___class__);
+    comp->emit = emit_pass1_new(MP_QSTR___class__);
     comp->emit_method_table = &emit_pass1_method_table;
     comp->emit_inline_asm = NULL;
     comp->emit_inline_asm_method_table = NULL;
@@ -3162,10 +3135,11 @@
     } else {
 #if MICROPY_EMIT_CPYTHON
         // can't create code, so just return true
+        (void)module_scope; // to suppress warning that module_scope is unused
         return mp_const_true;
 #else
         // return function that executes the outer module
-        return rt_make_function_from_id(1);
+        return rt_make_function_from_id(module_scope->unique_code_id);
 #endif
     }
 }
diff --git a/py/mpqstr.h b/py/mpqstr.h
new file mode 100644
index 0000000..1440fb3
--- /dev/null
+++ b/py/mpqstr.h
@@ -0,0 +1,13 @@
+// See mpqstrraw.h for a list of qstr's that are available as constants.
+// Reference them as MP_QSTR_xxxx.
+//
+// Note: it would be possible to define MP_QSTR_xxx as qstr_from_str_static("xxx")
+// for qstrs that are referenced this way, but you don't want to have them in ROM.
+
+enum {
+    MP_QSTR_nil = 0,
+#define Q(id) MP_QSTR_##id,
+#include "mpqstrraw.h"
+#undef Q
+    MP_QSTR_number_of,
+} category_t;
diff --git a/py/mpqstrraw.h b/py/mpqstrraw.h
new file mode 100644
index 0000000..85cf1ff
--- /dev/null
+++ b/py/mpqstrraw.h
@@ -0,0 +1,63 @@
+// All the qstr definitions in this file are available as constants.
+// That is, they are in ROM and you can reference them simple as MP_QSTR_xxxx.
+
+Q(__build_class__)
+Q(__class__)
+Q(__doc__)
+Q(__init__)
+Q(__locals__)
+Q(__main__)
+Q(__module__)
+Q(__name__)
+Q(__next__)
+Q(__qualname__)
+Q(__repl_print__)
+
+Q(assertion_error)
+Q(micropython)
+Q(byte_code)
+Q(native)
+Q(viper)
+Q(asm_thumb)
+
+Q(StopIteration)
+
+Q(AttributeError)
+Q(IndexError)
+Q(KeyError)
+Q(NameError)
+Q(TypeError)
+Q(SyntaxError)
+Q(ValueError)
+
+Q(abs)
+Q(all)
+Q(any)
+Q(bool)
+Q(callable)
+Q(chr)
+Q(complex)
+Q(dict)
+Q(divmod)
+Q(float)
+Q(hash)
+Q(int)
+Q(iter)
+Q(len)
+Q(list)
+Q(max)
+Q(min)
+Q(next)
+Q(ord)
+Q(pow)
+Q(print)
+Q(range)
+Q(set)
+Q(sum)
+Q(type)
+
+Q(append)
+Q(pop)
+Q(sort)
+Q(join)
+Q(format)
diff --git a/py/obj.c b/py/obj.c
index d88d0ac..2b834ff 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -7,6 +7,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -144,7 +145,7 @@
     } else if (MP_OBJ_IS_TYPE(arg, &float_type)) {
         return mp_obj_float_get(arg);
     } else {
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "can't convert %s to float", mp_obj_get_type_str(arg)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "can't convert %s to float", mp_obj_get_type_str(arg)));
     }
 }
 
@@ -164,7 +165,7 @@
     } else if (MP_OBJ_IS_TYPE(arg, &complex_type)) {
         mp_obj_complex_get(arg, real, imag);
     } else {
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "can't convert %s to complex", mp_obj_get_type_str(arg)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "can't convert %s to complex", mp_obj_get_type_str(arg)));
     }
 }
 #endif
@@ -188,11 +189,11 @@
             mp_obj_list_get(o_in, &seq_len, &seq_items);
         }
         if (seq_len != n) {
-            nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_IndexError, "requested length %d but object has length %d", (void*)n, (void*)(machine_uint_t)seq_len));
+            nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_IndexError, "requested length %d but object has length %d", (void*)n, (void*)(machine_uint_t)seq_len));
         }
         return seq_items;
     } else {
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "object '%s' is not a tuple or list", mp_obj_get_type_str(o_in)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "object '%s' is not a tuple or list", mp_obj_get_type_str(o_in)));
     }
 }
 
@@ -204,10 +205,10 @@
             i += len;
         }
         if (i < 0 || i >= len) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_IndexError, "%s index out of range", type->name));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_IndexError, "%s index out of range", type->name));
         }
         return i;
     } else {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "%s indices must be integers, not %s", type->name, mp_obj_get_type_str(index)));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "%s indices must be integers, not %s", type->name, mp_obj_get_type_str(index)));
     }
 }
diff --git a/py/objclass.c b/py/objclass.c
index 203923a..f223c5f 100644
--- a/py/objclass.c
+++ b/py/objclass.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime.h"
 #include "map.h"
@@ -25,7 +26,7 @@
     mp_obj_t o = mp_obj_new_instance(self_in);
 
     // look for __init__ function
-    mp_map_elem_t *init_fn = mp_qstr_map_lookup(self->locals, qstr_from_str_static("__init__"), false);
+    mp_map_elem_t *init_fn = mp_qstr_map_lookup(self->locals, MP_QSTR___init__, false);
 
     if (init_fn != NULL) {
         // call __init__ function
@@ -40,13 +41,13 @@
             m_del(mp_obj_t, args2, n_args + 1);
         }
         if (init_ret != mp_const_none) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "__init__() should return None, not '%s'", mp_obj_get_type_str(init_ret)));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "__init__() should return None, not '%s'", mp_obj_get_type_str(init_ret)));
         }
 
     } else {
         // TODO
         if (n_args != 0) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "function takes 0 positional arguments but %d were given", (void*)(machine_int_t)n_args));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "function takes 0 positional arguments but %d were given", (void*)(machine_int_t)n_args));
         }
     }
 
diff --git a/py/objdict.c b/py/objdict.c
index 50ce279..acf1a9f 100644
--- a/py/objdict.c
+++ b/py/objdict.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -42,7 +43,7 @@
             // dict load
             mp_map_elem_t *elem = mp_map_lookup_helper(&o->map, rhs_in, false);
             if (elem == NULL) {
-                nlr_jump(mp_obj_new_exception_msg(rt_q_KeyError, "<value>"));
+                nlr_jump(mp_obj_new_exception_msg(MP_QSTR_KeyError, "<value>"));
             } else {
                 return elem->value;
             }
diff --git a/py/objfun.c b/py/objfun.c
index e998bd2..c478386 100644
--- a/py/objfun.c
+++ b/py/objfun.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "map.h"
 #include "runtime.h"
@@ -24,7 +25,7 @@
 
         // check number of arguments
         if (n_args != self->n_args_min) {
-            nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args_min, (const char*)(machine_int_t)n_args));
+            nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args_min, (const char*)(machine_int_t)n_args));
         }
 
         // dispatch function call
@@ -47,9 +48,9 @@
         // function takes a variable number of arguments
 
         if (n_args < self->n_args_min) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "<fun name>() missing %d required positional arguments: <list of names of params>", (const char*)(machine_int_t)(self->n_args_min - n_args)));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "<fun name>() missing %d required positional arguments: <list of names of params>", (const char*)(machine_int_t)(self->n_args_min - n_args)));
         } else if (n_args > self->n_args_max) {
-            nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "<fun name> expected at most %d arguments, got %d", (void*)(machine_int_t)self->n_args_max, (void*)(machine_int_t)n_args));
+            nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "<fun name> expected at most %d arguments, got %d", (void*)(machine_int_t)self->n_args_max, (void*)(machine_int_t)n_args));
         }
 
         // TODO really the args need to be passed in as a Python tuple, as the form f(*[1,2]) can be used to pass var args
@@ -141,7 +142,7 @@
     mp_obj_fun_bc_t *self = self_in;
 
     if (n_args != self->n_args) {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args, (const char*)(machine_int_t)n_args));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args, (const char*)(machine_int_t)n_args));
     }
 
     // optimisation: allow the compiler to optimise this tail call for
@@ -250,7 +251,7 @@
     mp_obj_fun_asm_t *self = self_in;
 
     if (n_args != self->n_args) {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args, (const char*)(machine_int_t)n_args));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)self->n_args, (const char*)(machine_int_t)n_args));
     }
 
     machine_uint_t ret;
diff --git a/py/objgenerator.c b/py/objgenerator.c
index ecdd72d..8492122 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime.h"
 #include "bc.h"
@@ -29,7 +30,7 @@
     const byte *bc_code;
     mp_obj_fun_bc_get(self_fun, &bc_n_args, &bc_n_state, &bc_code);
     if (n_args != bc_n_args) {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)bc_n_args, (const char*)(machine_int_t)n_args));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_TypeError, "function takes %d positional arguments but %d were given", (const char*)(machine_int_t)bc_n_args, (const char*)(machine_int_t)n_args));
     }
 
     return mp_obj_new_gen_instance(bc_code, self->n_state, n_args, args);
diff --git a/py/objinstance.c b/py/objinstance.c
index e5d23af..6cfcdf6 100644
--- a/py/objinstance.c
+++ b/py/objinstance.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime.h"
 #include "map.h"
@@ -44,7 +45,7 @@
             return elem->value;
         }
     }
-    nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(self_in), qstr_str(attr)));
+    nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(self_in), qstr_str(attr)));
 }
 
 void mp_obj_instance_load_method(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
diff --git a/py/objlist.c b/py/objlist.c
index e371057..30d3a41 100644
--- a/py/objlist.c
+++ b/py/objlist.c
@@ -6,6 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -81,7 +82,7 @@
     assert(MP_OBJ_IS_TYPE(args[0], &list_type));
     mp_obj_list_t *self = args[0];
     if (self->len == 0) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_IndexError, "pop from empty list"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_IndexError, "pop from empty list"));
     }
     uint index = mp_get_index(self->base.type, self->len, n_args == 1 ? mp_obj_new_int(-1) : args[1]);
     mp_obj_t ret = self->items[index];
diff --git a/py/objstr.c b/py/objstr.c
index 54e6f37..03a7618 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -7,6 +7,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -61,7 +62,7 @@
             } else {
                 // Message doesn't match CPython, but we don't have so much bytes as they
                 // to spend them on verbose wording
-                nlr_jump(mp_obj_new_exception_msg(rt_q_TypeError, "index must be int"));
+                nlr_jump(mp_obj_new_exception_msg(MP_QSTR_TypeError, "index must be int"));
             }
 
         case RT_BINARY_OP_ADD:
@@ -134,7 +135,7 @@
     return mp_obj_new_str(qstr_from_str_take(joined_str, required_len + 1));
 
 bad_arg:
-    nlr_jump(mp_obj_new_exception_msg(rt_q_TypeError, "?str.join expecting a list of str's"));
+    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_TypeError, "?str.join expecting a list of str's"));
 }
 
 void vstr_printf_wrapper(void *env, const char *fmt, ...) {
@@ -158,7 +159,7 @@
                 vstr_add_char(vstr, '{');
             } else if (*str == '}') {
                 if (arg_i >= n_args) {
-                    nlr_jump(mp_obj_new_exception_msg(rt_q_IndexError, "tuple index out of range"));
+                    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_IndexError, "tuple index out of range"));
                 }
                 mp_obj_print_helper(vstr_printf_wrapper, vstr, args[arg_i]);
                 arg_i++;
diff --git a/py/qstr.c b/py/qstr.c
index 0dd8a04..0ed7aa9 100644
--- a/py/qstr.c
+++ b/py/qstr.c
@@ -2,55 +2,110 @@
 #include <string.h>
 
 #include "misc.h"
+#include "mpqstr.h"
 
-static int qstrs_alloc;
-static int qstrs_len;
-static const char **qstrs;
+// NOTE: we are using linear arrays to store and search for qstr's (unique strings, interned strings)
+// ultimately we will replace this with a static hash table of some kind
+// also probably need to include the length in the string data, to allow null bytes in the string
+
+#if 0 // print debugging info
+#include <stdio.h>
+#define DEBUG_printf(args...) printf(args)
+#else // don't print debugging info
+#define DEBUG_printf(args...) (void)0
+#endif
+
+typedef struct _qstr_pool_t {
+    struct _qstr_pool_t *prev;
+    uint total_prev_len;
+    uint alloc;
+    uint len;
+    const char *qstrs[];
+} qstr_pool_t;
+
+const static qstr_pool_t const_pool = {
+    NULL,               // no previous pool
+    0,                  // no previous pool
+    10,                 // set so that the first dynamically allocated pool is twice this size; must be <= the len (just below)
+    MP_QSTR_number_of,  // corresponds to number of strings in array just below
+    {
+        "nil", // must be first, since 0 qstr is nil
+#define Q(id) #id,
+#include "mpqstrraw.h"
+#undef Q
+    },
+};
+
+static qstr_pool_t *last_pool = (qstr_pool_t*)&const_pool; // we won't modify the const_pool since it has no allocated room left
 
 void qstr_init(void) {
-    qstrs_alloc = 400;
-    qstrs_len = 1;
-    qstrs = m_new(const char*, qstrs_alloc);
-    qstrs[0] = "nil";
+    // nothing to do!
 }
 
 static qstr qstr_add(const char *str) {
-    if (qstrs_len >= qstrs_alloc) {
-        qstrs = m_renew(const char*, qstrs, qstrs_alloc, qstrs_alloc * 2);
-        qstrs_alloc *= 2;
+    DEBUG_printf("QSTR: add %s\n", str);
+
+    // make sure we have room in the pool for a new qstr
+    if (last_pool->len >= last_pool->alloc) {
+        qstr_pool_t *pool = m_new_obj_var(qstr_pool_t, const char*, last_pool->alloc * 2);
+        pool->prev = last_pool;
+        pool->total_prev_len = last_pool->total_prev_len + last_pool->len;
+        pool->alloc = last_pool->alloc * 2;
+        pool->len = 0;
+        last_pool = pool;
+        DEBUG_printf("QSTR: allocate new pool of size %d\n", last_pool->alloc);
     }
-    qstrs[qstrs_len++] = str;
-    return qstrs_len - 1;
+
+    // add the new qstr
+    last_pool->qstrs[last_pool->len++] = str;
+
+    // return id for the newly-added qstr
+    return last_pool->total_prev_len + last_pool->len - 1;
 }
 
 qstr qstr_from_str_static(const char *str) {
-    for (int i = 0; i < qstrs_len; i++) {
-        if (strcmp(qstrs[i], str) == 0) {
-            return i;
+    for (qstr_pool_t *pool = last_pool; pool != NULL; pool = pool->prev) {
+        for (const char **qstr = pool->qstrs, **qstr_top = pool->qstrs + pool->len; qstr < qstr_top; qstr++) {
+            if (strcmp(*qstr, str) == 0) {
+                return pool->total_prev_len + (qstr - pool->qstrs);
+            }
         }
     }
     return qstr_add(str);
 }
 
 qstr qstr_from_str_take(char *str, int alloc_len) {
-    for (int i = 0; i < qstrs_len; i++) {
-        if (strcmp(qstrs[i], str) == 0) {
-            m_del(char, str, alloc_len);
-            return i;
+    for (qstr_pool_t *pool = last_pool; pool != NULL; pool = pool->prev) {
+        for (const char **qstr = pool->qstrs, **qstr_top = pool->qstrs + pool->len; qstr < qstr_top; qstr++) {
+            if (strcmp(*qstr, str) == 0) {
+                m_del(char, str, alloc_len);
+                return pool->total_prev_len + (qstr - pool->qstrs);
+            }
         }
     }
     return qstr_add(str);
 }
 
 qstr qstr_from_strn_copy(const char *str, int len) {
-    for (int i = 0; i < qstrs_len; i++) {
-        if (strncmp(qstrs[i], str, len) == 0 && qstrs[i][len] == '\0') {
-            return i;
+    for (qstr_pool_t *pool = last_pool; pool != NULL; pool = pool->prev) {
+        for (const char **qstr = pool->qstrs, **qstr_top = pool->qstrs + pool->len; qstr < qstr_top; qstr++) {
+            if (strncmp(*qstr, str, len) == 0 && (*qstr)[len] == '\0') {
+                return pool->total_prev_len + (qstr - pool->qstrs);
+            }
         }
     }
     return qstr_add(strndup(str, len));
 }
 
+// convert qstr id to pointer to its string
 const char *qstr_str(qstr qstr) {
-    return qstrs[qstr];
+    // search
+    for (qstr_pool_t *pool = last_pool; pool != NULL; pool = pool->prev) {
+        if (qstr >= pool->total_prev_len) {
+            return pool->qstrs[qstr - pool->total_prev_len];
+        }
+    }
+
+    // not found, return nil
+    return const_pool.qstrs[0];
 }
diff --git a/py/runtime.c b/py/runtime.c
index 3144321..a1f9ee3 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -11,6 +11,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
+#include "mpqstr.h"
 #include "obj.h"
 #include "runtime0.h"
 #include "runtime.h"
@@ -27,22 +28,6 @@
 #define DEBUG_OP_printf(args...) (void)0
 #endif
 
-// TODO make these predefined so they don't take up RAM
-qstr rt_q_append;
-qstr rt_q_pop;
-qstr rt_q_sort;
-qstr rt_q_join;
-qstr rt_q_format;
-qstr rt_q___build_class__;
-qstr rt_q___next__;
-qstr rt_q_AttributeError;
-qstr rt_q_IndexError;
-qstr rt_q_KeyError;
-qstr rt_q_NameError;
-qstr rt_q_TypeError;
-qstr rt_q_SyntaxError;
-qstr rt_q_ValueError;
-
 // locals and globals need to be pointers because they can be the same in outer module scope
 static mp_map_t *map_locals;
 static mp_map_t *map_globals;
@@ -83,74 +68,58 @@
 #endif
 
 void rt_init(void) {
-    rt_q_append = qstr_from_str_static("append");
-    rt_q_pop = qstr_from_str_static("pop");
-    rt_q_sort = qstr_from_str_static("sort");
-    rt_q_join = qstr_from_str_static("join");
-    rt_q_format = qstr_from_str_static("format");
-    rt_q___build_class__ = qstr_from_str_static("__build_class__");
-    rt_q___next__ = qstr_from_str_static("__next__");
-    rt_q_AttributeError = qstr_from_str_static("AttributeError");
-    rt_q_IndexError = qstr_from_str_static("IndexError");
-    rt_q_KeyError = qstr_from_str_static("KeyError");
-    rt_q_NameError = qstr_from_str_static("NameError");
-    rt_q_TypeError = qstr_from_str_static("TypeError");
-    rt_q_SyntaxError = qstr_from_str_static("SyntaxError");
-    rt_q_ValueError = qstr_from_str_static("ValueError");
-
     // locals = globals for outer module (see Objects/frameobject.c/PyFrame_New())
     map_locals = map_globals = mp_map_new(MP_MAP_QSTR, 1);
-    mp_qstr_map_lookup(map_globals, qstr_from_str_static("__name__"), true)->value = mp_obj_new_str(qstr_from_str_static("__main__"));
+    mp_qstr_map_lookup(map_globals, MP_QSTR___name__, true)->value = mp_obj_new_str(MP_QSTR___main__);
 
     // init built-in hash table
     mp_map_init(&map_builtins, MP_MAP_QSTR, 3);
 
     // built-in exceptions (TODO, make these proper classes)
-    mp_qstr_map_lookup(&map_builtins, rt_q_AttributeError, true)->value = mp_obj_new_exception(rt_q_AttributeError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_IndexError, true)->value = mp_obj_new_exception(rt_q_IndexError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_KeyError, true)->value = mp_obj_new_exception(rt_q_KeyError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_NameError, true)->value = mp_obj_new_exception(rt_q_NameError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_TypeError, true)->value = mp_obj_new_exception(rt_q_TypeError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_SyntaxError, true)->value = mp_obj_new_exception(rt_q_SyntaxError);
-    mp_qstr_map_lookup(&map_builtins, rt_q_ValueError, true)->value = mp_obj_new_exception(rt_q_ValueError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_AttributeError, true)->value = mp_obj_new_exception(MP_QSTR_AttributeError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_IndexError, true)->value = mp_obj_new_exception(MP_QSTR_IndexError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_KeyError, true)->value = mp_obj_new_exception(MP_QSTR_KeyError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_NameError, true)->value = mp_obj_new_exception(MP_QSTR_NameError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_TypeError, true)->value = mp_obj_new_exception(MP_QSTR_TypeError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_SyntaxError, true)->value = mp_obj_new_exception(MP_QSTR_SyntaxError);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_ValueError, true)->value = mp_obj_new_exception(MP_QSTR_ValueError);
 
     // built-in core functions
-    mp_qstr_map_lookup(&map_builtins, rt_q___build_class__, true)->value = rt_make_function_2(mp_builtin___build_class__);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("__repl_print__"), true)->value = rt_make_function_1(mp_builtin___repl_print__);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR___build_class__, true)->value = rt_make_function_2(mp_builtin___build_class__);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR___repl_print__, true)->value = rt_make_function_1(mp_builtin___repl_print__);
 
     // built-in user functions
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("abs"), true)->value = rt_make_function_1(mp_builtin_abs);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("all"), true)->value = rt_make_function_1(mp_builtin_all);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("any"), true)->value = rt_make_function_1(mp_builtin_any);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("bool"), true)->value = rt_make_function_var(0, mp_builtin_bool);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("callable"), true)->value = rt_make_function_1(mp_builtin_callable);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("chr"), true)->value = rt_make_function_1(mp_builtin_chr);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_abs, true)->value = rt_make_function_1(mp_builtin_abs);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_all, true)->value = rt_make_function_1(mp_builtin_all);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_any, true)->value = rt_make_function_1(mp_builtin_any);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_bool, true)->value = rt_make_function_var(0, mp_builtin_bool);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_callable, true)->value = rt_make_function_1(mp_builtin_callable);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_chr, true)->value = rt_make_function_1(mp_builtin_chr);
 #if MICROPY_ENABLE_FLOAT
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("complex"), true)->value = (mp_obj_t)&mp_builtin_complex_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_complex, true)->value = (mp_obj_t)&mp_builtin_complex_obj;
 #endif
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("dict"), true)->value = rt_make_function_0(mp_builtin_dict);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("divmod"), true)->value = rt_make_function_2(mp_builtin_divmod);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_dict, true)->value = rt_make_function_0(mp_builtin_dict);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_divmod, true)->value = rt_make_function_2(mp_builtin_divmod);
 #if MICROPY_ENABLE_FLOAT
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("float"), true)->value = (mp_obj_t)&mp_builtin_float_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_float, true)->value = (mp_obj_t)&mp_builtin_float_obj;
 #endif
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("hash"), true)->value = (mp_obj_t)&mp_builtin_hash_obj;
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("int"), true)->value = (mp_obj_t)&mp_builtin_int_obj;
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("iter"), true)->value = (mp_obj_t)&mp_builtin_iter_obj;
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("len"), true)->value = rt_make_function_1(mp_builtin_len);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("list"), true)->value = rt_make_function_var(0, mp_builtin_list);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("max"), true)->value = rt_make_function_var(1, mp_builtin_max);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("min"), true)->value = rt_make_function_var(1, mp_builtin_min);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("next"), true)->value = (mp_obj_t)&mp_builtin_next_obj;
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("ord"), true)->value = rt_make_function_1(mp_builtin_ord);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("pow"), true)->value = rt_make_function_var(2, mp_builtin_pow);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("print"), true)->value = rt_make_function_var(0, mp_builtin_print);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("range"), true)->value = rt_make_function_var(1, mp_builtin_range);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("set"), true)->value = (mp_obj_t)&mp_builtin_set_obj;
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("sum"), true)->value = rt_make_function_var(1, mp_builtin_sum);
-    mp_qstr_map_lookup(&map_builtins, qstr_from_str_static("type"), true)->value = (mp_obj_t)&mp_builtin_type_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_hash, true)->value = (mp_obj_t)&mp_builtin_hash_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_int, true)->value = (mp_obj_t)&mp_builtin_int_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_iter, true)->value = (mp_obj_t)&mp_builtin_iter_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_len, true)->value = rt_make_function_1(mp_builtin_len);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_list, true)->value = rt_make_function_var(0, mp_builtin_list);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_max, true)->value = rt_make_function_var(1, mp_builtin_max);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_min, true)->value = rt_make_function_var(1, mp_builtin_min);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_next, true)->value = (mp_obj_t)&mp_builtin_next_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_ord, true)->value = rt_make_function_1(mp_builtin_ord);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_pow, true)->value = rt_make_function_var(2, mp_builtin_pow);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_print, true)->value = rt_make_function_var(0, mp_builtin_print);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_range, true)->value = rt_make_function_var(1, mp_builtin_range);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_set, true)->value = (mp_obj_t)&mp_builtin_set_obj;
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_sum, true)->value = rt_make_function_var(1, mp_builtin_sum);
+    mp_qstr_map_lookup(&map_builtins, MP_QSTR_type, true)->value = (mp_obj_t)&mp_builtin_type_obj;
 
-
-    next_unique_code_id = 2; // 1 is reserved for the __main__ module scope
+    next_unique_code_id = 1; // 0 indicates "no code"
     unique_codes = NULL;
 
 #ifdef WRITE_CODE
@@ -166,12 +135,8 @@
 #endif
 }
 
-int rt_get_unique_code_id(bool is_main_module) {
-    if (is_main_module) {
-        return 1;
-    } else {
-        return next_unique_code_id++;
-    }
+int rt_get_unique_code_id(void) {
+    return next_unique_code_id++;
 }
 
 static void alloc_unique_codes(void) {
@@ -186,7 +151,7 @@
 void rt_assign_byte_code(int unique_code_id, byte *code, uint len, int n_args, int n_locals, int n_stack, bool is_generator) {
     alloc_unique_codes();
 
-    assert(unique_code_id < next_unique_code_id);
+    assert(1 <= unique_code_id && unique_code_id < next_unique_code_id);
     unique_codes[unique_code_id].kind = MP_CODE_BYTE;
     unique_codes[unique_code_id].n_args = n_args;
     unique_codes[unique_code_id].n_locals = n_locals;
@@ -355,7 +320,7 @@
         }
     }
     if (*s != 0) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_SyntaxError, "invalid syntax for number"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_SyntaxError, "invalid syntax for number"));
     }
     if (exp_neg) {
         exp_val = -exp_val;
@@ -373,7 +338,7 @@
         return mp_obj_new_float(dec_val);
     }
 #else
-    nlr_jump(mp_obj_new_exception_msg(rt_q_SyntaxError, "decimal numbers not supported"));
+    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_SyntaxError, "decimal numbers not supported"));
 #endif
 }
 
@@ -391,7 +356,7 @@
         if (elem == NULL) {
             elem = mp_qstr_map_lookup(&map_builtins, qstr, false);
             if (elem == NULL) {
-                nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_NameError, "name '%s' is not defined", qstr_str(qstr)));
+                nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_NameError, "name '%s' is not defined", qstr_str(qstr)));
             }
         }
     }
@@ -405,7 +370,7 @@
     if (elem == NULL) {
         elem = mp_qstr_map_lookup(&map_builtins, qstr, false);
         if (elem == NULL) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_NameError, "name '%s' is not defined", qstr_str(qstr)));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_NameError, "name '%s' is not defined", qstr_str(qstr)));
         }
     }
     return elem->value;
@@ -413,9 +378,9 @@
 
 mp_obj_t rt_load_build_class(void) {
     DEBUG_OP_printf("load_build_class\n");
-    mp_map_elem_t *elem = mp_qstr_map_lookup(&map_builtins, rt_q___build_class__, false);
+    mp_map_elem_t *elem = mp_qstr_map_lookup(&map_builtins, MP_QSTR___build_class__, false);
     if (elem == NULL) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_NameError, "name '__build_class__' is not defined"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_NameError, "name '__build_class__' is not defined"));
     }
     return elem->value;
 }
@@ -465,7 +430,7 @@
             }
         }
         // TODO specify in error message what the operator is
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "bad operand type for unary operator: '%s'", o->type->name));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "bad operand type for unary operator: '%s'", o->type->name));
     }
 }
 
@@ -544,7 +509,7 @@
     }
 
     // TODO specify in error message what the operator is
-    nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "unsupported operand type for binary operator: '%s'", mp_obj_get_type_str(lhs)));
+    nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "unsupported operand type for binary operator: '%s'", mp_obj_get_type_str(lhs)));
 }
 
 mp_obj_t rt_compare_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
@@ -693,13 +658,13 @@
     DEBUG_OP_printf("calling function %p(n_args=%d, args=%p)\n", fun_in, n_args, args);
 
     if (MP_OBJ_IS_SMALL_INT(fun_in)) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_TypeError, "'int' object is not callable"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_TypeError, "'int' object is not callable"));
     } else {
         mp_obj_base_t *fun = fun_in;
         if (fun->type->call_n != NULL) {
             return fun->type->call_n(fun_in, n_args, args);
         } else {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "'%s' object is not callable", fun->type->name));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "'%s' object is not callable", fun->type->name));
         }
     }
 }
@@ -756,14 +721,14 @@
             mp_obj_list_get(seq_in, &seq_len, &seq_items);
         }
         if (seq_len < num) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_ValueError, "need more than %d values to unpack", (void*)(machine_uint_t)seq_len));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_ValueError, "need more than %d values to unpack", (void*)(machine_uint_t)seq_len));
         } else if (seq_len > num) {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_ValueError, "too many values to unpack (expected %d)", (void*)(machine_uint_t)num));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_ValueError, "too many values to unpack (expected %d)", (void*)(machine_uint_t)num));
         }
         memcpy(items, seq_items, num * sizeof(mp_obj_t));
     } else {
         // TODO call rt_getiter and extract via rt_iternext
-        nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "'%s' object is not iterable", mp_obj_get_type_str(seq_in)));
+        nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "'%s' object is not iterable", mp_obj_get_type_str(seq_in)));
     }
 }
 
@@ -807,12 +772,12 @@
     }
 
 no_attr:
-    nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(base), qstr_str(attr)));
+    nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(base), qstr_str(attr)));
 }
 
 void rt_load_method(mp_obj_t base, qstr attr, mp_obj_t *dest) {
     DEBUG_OP_printf("load method %s\n", qstr_str(attr));
-    if (MP_OBJ_IS_TYPE(base, &gen_instance_type) && attr == rt_q___next__) {
+    if (MP_OBJ_IS_TYPE(base, &gen_instance_type) && attr == MP_QSTR___next__) {
         dest[1] = (mp_obj_t)&mp_builtin_next_obj;
         dest[0] = base;
         return;
@@ -850,7 +815,7 @@
         mp_map_t *globals = mp_obj_module_get_globals(base);
         mp_qstr_map_lookup(globals, attr, true)->value = value;
     } else {
-        nlr_jump(mp_obj_new_exception_msg_2_args(rt_q_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(base), qstr_str(attr)));
+        nlr_jump(mp_obj_new_exception_msg_2_args(MP_QSTR_AttributeError, "'%s' object has no attribute '%s'", mp_obj_get_type_str(base), qstr_str(attr)));
     }
 }
 
@@ -869,26 +834,26 @@
 
 mp_obj_t rt_getiter(mp_obj_t o_in) {
     if (MP_OBJ_IS_SMALL_INT(o_in)) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_TypeError, "'int' object is not iterable"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_TypeError, "'int' object is not iterable"));
     } else {
         mp_obj_base_t *o = o_in;
         if (o->type->getiter != NULL) {
             return o->type->getiter(o_in);
         } else {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "'%s' object is not iterable", o->type->name));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "'%s' object is not iterable", o->type->name));
         }
     }
 }
 
 mp_obj_t rt_iternext(mp_obj_t o_in) {
     if (MP_OBJ_IS_SMALL_INT(o_in)) {
-        nlr_jump(mp_obj_new_exception_msg(rt_q_TypeError, "? 'int' object is not iterable"));
+        nlr_jump(mp_obj_new_exception_msg(MP_QSTR_TypeError, "? 'int' object is not iterable"));
     } else {
         mp_obj_base_t *o = o_in;
         if (o->type->iternext != NULL) {
             return o->type->iternext(o_in);
         } else {
-            nlr_jump(mp_obj_new_exception_msg_1_arg(rt_q_TypeError, "? '%s' object is not iterable", o->type->name));
+            nlr_jump(mp_obj_new_exception_msg_1_arg(MP_QSTR_TypeError, "? '%s' object is not iterable", o->type->name));
         }
     }
 }
diff --git a/py/runtime.h b/py/runtime.h
index cf91802..96f1671 100644
--- a/py/runtime.h
+++ b/py/runtime.h
@@ -1,18 +1,3 @@
-extern qstr rt_q_append;
-extern qstr rt_q_pop;
-extern qstr rt_q_sort;
-extern qstr rt_q_join;
-extern qstr rt_q_format;
-extern qstr rt_q___build_class__;
-extern qstr rt_q___next__;
-extern qstr rt_q_AttributeError;
-extern qstr rt_q_IndexError;
-extern qstr rt_q_KeyError;
-extern qstr rt_q_NameError;
-extern qstr rt_q_TypeError;
-extern qstr rt_q_SyntaxError;
-extern qstr rt_q_ValueError;
-
 int rt_is_true(mp_obj_t arg);
 
 mp_obj_t rt_load_const_dec(qstr qstr);
diff --git a/py/runtime0.h b/py/runtime0.h
index 8ec2c05..97dbe5d 100644
--- a/py/runtime0.h
+++ b/py/runtime0.h
@@ -81,7 +81,7 @@
 
 void rt_init(void);
 void rt_deinit(void);
-int rt_get_unique_code_id(bool is_main_module);
+int rt_get_unique_code_id(void);
 void rt_assign_byte_code(int unique_code_id, byte *code, uint len, int n_args, int n_locals, int n_stack, bool is_generator);
 void rt_assign_native_code(int unique_code_id, void *f, uint len, int n_args);
 void rt_assign_inline_asm_code(int unique_code_id, void *f, uint len, int n_args);