diff --git a/py/dynruntime.h b/py/dynruntime.h
index 435b85622a..3e660f19fd 100644
--- a/py/dynruntime.h
+++ b/py/dynruntime.h
@@ -209,7 +209,7 @@ static inline void *mp_obj_malloc_helper_dyn(size_t num_bytes, const mp_obj_type
     mp_obj_t old_globals = mp_fun_table.swap_globals(self->context->module.globals); \
     mp_raw_code_truncated_t rc; \
     rc.kind = MP_CODE_NATIVE_VIPER; \
-    rc.scope_flags = 0; \
+    rc.is_generator = 0; \
     (void)rc;
 
 #define MP_DYNRUNTIME_INIT_EXIT \
diff --git a/py/emitglue.c b/py/emitglue.c
index 0ec126fe94..c0003c5cd5 100644
--- a/py/emitglue.c
+++ b/py/emitglue.c
@@ -71,7 +71,7 @@ void mp_emit_glue_assign_bytecode(mp_raw_code_t *rc, const byte *code,
     uint16_t scope_flags) {
 
     rc->kind = MP_CODE_BYTECODE;
-    rc->scope_flags = scope_flags;
+    rc->is_generator = (scope_flags & MP_SCOPE_FLAG_GENERATOR) != 0;
     rc->fun_data = code;
     #if MICROPY_PERSISTENT_CODE_SAVE || MICROPY_DEBUG_PRINTERS
     rc->fun_data_len = len;
@@ -133,7 +133,7 @@ void mp_emit_glue_assign_native(mp_raw_code_t *rc, mp_raw_code_kind_t kind, void
     #endif
 
     rc->kind = kind;
-    rc->scope_flags = scope_flags;
+    rc->is_generator = (scope_flags & MP_SCOPE_FLAG_GENERATOR) != 0;
     rc->fun_data = fun_data;
 
     #if MICROPY_PERSISTENT_CODE_SAVE || MICROPY_DEBUG_PRINTERS
@@ -191,7 +191,7 @@ mp_obj_t mp_make_function_from_raw_code(const mp_raw_code_t *rc, const mp_module
         case MP_CODE_NATIVE_VIPER:
             fun = mp_obj_new_fun_native(def_args, rc->fun_data, context, rc->children);
             // Check for a generator function, and if so change the type of the object
-            if ((rc->scope_flags & MP_SCOPE_FLAG_GENERATOR) != 0) {
+            if (rc->is_generator) {
                 ((mp_obj_base_t *)MP_OBJ_TO_PTR(fun))->type = &mp_type_native_gen_wrap;
             }
             break;
@@ -206,7 +206,7 @@ mp_obj_t mp_make_function_from_raw_code(const mp_raw_code_t *rc, const mp_module
             assert(rc->kind == MP_CODE_BYTECODE);
             fun = mp_obj_new_fun_bc(def_args, rc->fun_data, context, rc->children);
             // check for generator functions and if so change the type of the object
-            if ((rc->scope_flags & MP_SCOPE_FLAG_GENERATOR) != 0) {
+            if (rc->is_generator) {
                 ((mp_obj_base_t *)MP_OBJ_TO_PTR(fun))->type = &mp_type_gen_wrap;
             }
 
diff --git a/py/emitglue.h b/py/emitglue.h
index b79283541b..125ab2ccd5 100644
--- a/py/emitglue.h
+++ b/py/emitglue.h
@@ -57,8 +57,8 @@ typedef enum {
 // mpy file: instance in RAM, created when .mpy file is loaded (same comments as above)
 // frozen: instance in ROM
 typedef struct _mp_raw_code_t {
-    uint32_t kind : 3; // of type mp_raw_code_kind_t
-    uint32_t scope_flags : 7;
+    uint8_t kind; // of type mp_raw_code_kind_t; only 3 bits used
+    bool is_generator;
     const void *fun_data;
     struct _mp_raw_code_t **children;
     #if MICROPY_PERSISTENT_CODE_SAVE || MICROPY_DEBUG_PRINTERS
@@ -88,8 +88,8 @@ typedef struct _mp_raw_code_t {
 // only needed when the kind is MP_CODE_NATIVE_ASM.  So this struct can be used when the
 // kind is MP_CODE_BYTECODE, MP_CODE_NATIVE_PY or MP_CODE_NATIVE_VIPER, to reduce its size.
 typedef struct _mp_raw_code_truncated_t {
-    uint32_t kind : 3;
-    uint32_t scope_flags : 7;
+    uint8_t kind;
+    bool is_generator;
     const void *fun_data;
     struct _mp_raw_code_t **children;
     #if MICROPY_PERSISTENT_CODE_SAVE || MICROPY_DEBUG_PRINTERS
diff --git a/py/persistentcode.c b/py/persistentcode.c
index 83806febb3..7b002e7fcf 100644
--- a/py/persistentcode.c
+++ b/py/persistentcode.c
@@ -577,7 +577,9 @@ STATIC void save_raw_code(mp_print_t *print, const mp_raw_code_t *rc) {
         mp_print_uint(print, rc->prelude_offset);
     } else if (rc->kind == MP_CODE_NATIVE_VIPER || rc->kind == MP_CODE_NATIVE_ASM) {
         // Save basic scope info for viper and asm
-        mp_print_uint(print, rc->scope_flags & MP_SCOPE_FLAG_ALL_SIG);
+        // Viper/asm functions don't support generator, variable args, or default keyword args
+        // so (scope_flags & MP_SCOPE_FLAG_ALL_SIG) for these functions is always 0.
+        mp_print_uint(print, 0);
         #if MICROPY_EMIT_INLINE_ASM
         if (rc->kind == MP_CODE_NATIVE_ASM) {
             mp_print_uint(print, rc->asm_n_pos_args);
diff --git a/py/runtime0.h b/py/runtime0.h
index 69af38ddcb..9c6f0e079f 100644
--- a/py/runtime0.h
+++ b/py/runtime0.h
@@ -26,8 +26,11 @@
 #ifndef MICROPY_INCLUDED_PY_RUNTIME0_H
 #define MICROPY_INCLUDED_PY_RUNTIME0_H
 
-// The first four must fit in 8 bits, see emitbc.c
-// The remaining must fit in 16 bits, see scope.h
+// These constants are used by:
+// - mp_raw_code_t::is_generator (only MP_SCOPE_FLAG_GENERATOR)
+// - scope_t::scope_flags (16 bits)
+// - MP_BC_PRELUDE_SIG_ENCODE macro, masked by MP_SCOPE_FLAG_ALL_SIG (4 bits)
+// - tools/mpy_ld.py, when generating mpy files (maximum 7 bits)
 #define MP_SCOPE_FLAG_ALL_SIG      (0x0f)
 #define MP_SCOPE_FLAG_GENERATOR    (0x01)
 #define MP_SCOPE_FLAG_VARKEYWORDS  (0x02)
diff --git a/tools/mpy-tool.py b/tools/mpy-tool.py
index 9b824099ea..744800c670 100755
--- a/tools/mpy-tool.py
+++ b/tools/mpy-tool.py
@@ -126,6 +126,7 @@ MP_PERSISTENT_OBJ_FLOAT = 8
 MP_PERSISTENT_OBJ_COMPLEX = 9
 MP_PERSISTENT_OBJ_TUPLE = 10
 
+MP_SCOPE_FLAG_GENERATOR = 0x01
 MP_SCOPE_FLAG_VIPERRELOC = 0x10
 MP_SCOPE_FLAG_VIPERRODATA = 0x20
 MP_SCOPE_FLAG_VIPERBSS = 0x40
@@ -912,7 +913,7 @@ class RawCode(object):
             raw_code_type = "mp_raw_code_truncated_t"
         print("static const %s raw_code_%s = {" % (raw_code_type, self.escaped_name))
         print("    .kind = %s," % RawCode.code_kind_str[self.code_kind])
-        print("    .scope_flags = 0x%02x," % self.scope_flags)
+        print("    .is_generator = %d," % bool(self.scope_flags & MP_SCOPE_FLAG_GENERATOR))
         print("    .fun_data = fun_data_%s," % self.escaped_name)
         if len(self.children):
             print("    .children = (void *)&children_%s," % self.escaped_name)