diff --git a/prjxray/db.py b/prjxray/db.py
index 56ea14f0..daecc2ea 100644
--- a/prjxray/db.py
+++ b/prjxray/db.py
@@ -42,6 +42,8 @@ class Database(object):
         self.tile_segbits = {}
         self.site_types = {}
 
+        per_part_features = {}
+
         for f in os.listdir(self.db_root):
             if f.endswith('.json') and f.startswith('tile_type_'):
                 tile_type = f[len('tile_type_'):-len('.json')].lower()
@@ -85,15 +87,32 @@ class Database(object):
 
                 self.site_types[site_type_name] = os.path.join(self.db_root, f)
 
+            if f.endswith('_required_features.fasm'):
+                part = f[:-len('_required_features.fasm')]
+
+                with open(os.path.join(self.db_root, f), "r") as fp:
+                    features = []
+                    for line in fp:
+                        line = line.strip()
+                        if len(line) > 0:
+                            features.append(line)
+
+                    per_part_features[part] = set(features)
+
         self.tile_types_obj = {}
 
-        # Load set of required fasm features for given part.
-        self.required_features = {}
-        fname = os.path.join(self.db_root, "required_features.json")
+        # Find common required features for all supported parts
+        if len(per_part_features) > 0:
+            common_features = set.intersection(*per_part_features.values())
+            for features in per_part_features.values():
+                features -= common_features
+        else:
+            common_features = set()
 
-        if os.path.isfile(fname):
-            with open(fname, "r") as fp:
-                self.required_features = json.load(fp)
+        self.required_features = {
+            "always_required": common_features,
+            "per_part_required": per_part_features
+        }
 
     def get_tile_types(self):
         """ Return list of tile types """
@@ -171,16 +190,8 @@ class Database(object):
         if "always_required" in self.required_features:
             features |= set(self.required_features["always_required"])
 
-        # Append / remove features for the specific part. If a feature string
-        # starts with "-" then the feature needs not to be present for that
-        # part.
+        # Append list of part specific features
         if part is not None:
-            for f in self.required_features["per_part_required"][part]:
-
-                if f.startswith("-"):
-                    f = f[1:]
-                    features -= set([f])
-                else:
-                    features |= set([f])
+            features |= set(self.required_features["per_part_required"][part])
 
         return features
diff --git a/utils/fasm2frames.py b/utils/fasm2frames.py
index 8776103d..f540fc7e 100755
--- a/utils/fasm2frames.py
+++ b/utils/fasm2frames.py
@@ -81,6 +81,7 @@ def find_pudc_b(db):
 
 def run(
         db_root,
+        part,
         filename_in,
         f_out,
         sparse=False,
@@ -121,8 +122,7 @@ def run(
                 '\n'.join(roi_j['required_features']))
 
     # Get required extra features for the part
-    # TODO: Specify the part explicitly ?
-    required_features = db.get_required_fasm_features(None)
+    required_features = db.get_required_fasm_features(part)
     extra_features += fasm.parse_fasm_string('\n'.join(required_features))
 
     assembler.parse_fasm_filename(filename_in, extra_features=extra_features)
@@ -170,7 +170,13 @@ def main():
         db_root_kwargs['required'] = False
         db_root_kwargs['default'] = os.path.join(database_dir, database)
 
+    def_part = os.getenv("XRAY_PART")
+
     parser.add_argument('--db-root', help="Database root.", **db_root_kwargs)
+    parser.add_argument(
+        '--part',
+        default=def_part,
+        help="Part name. When not given defaults to XRAY_PART env. var.")
     parser.add_argument(
         '--sparse', action='store_true', help="Don't zero fill all frames")
     parser.add_argument(
@@ -192,6 +198,7 @@ def main():
     args = parser.parse_args()
     run(
         db_root=args.db_root,
+        part=args.part,
         filename_in=args.fn_in,
         f_out=open(args.fn_out, 'w'),
         sparse=args.sparse,