diff --git a/prjxray/util.py b/prjxray/util.py
index 16f28455..7baa127c 100644
--- a/prjxray/util.py
+++ b/prjxray/util.py
@@ -103,6 +103,23 @@ def parse_db_line(line):
     return tag, bits, None
 
 
+def parse_db_lines(fn):
+    with open(fn, "r") as f:
+        for line in f:
+            yield line, parse_db_line(line)
+
+
+def write_db_lines(fn, entries):
+    new_lines = []
+    for tag, bits in entries.items():
+        new_line = " ".join([tag] + sorted(bits))
+        new_lines.append(new_line)
+
+    with open(fn, "w") as f:
+        for line in sorted(new_lines):
+            print(line, file=f)
+
+
 def parse_tagbit(x):
     # !30_07
     if x[0] == '!':
@@ -186,14 +203,12 @@ def gen_tile_bits(db_root, tilej, strict=False, verbose=False):
         elif not os.path.exists(fn):
             continue
 
-        with open(fn, "r") as f:
-            for line in f:
-                tag, bits, mode = parse_db_line(line)
-                assert mode is None
-                for bitstr in bits:
-                    # 31_06
-                    _bit_inv, (bit_addroff, bit_bitoff) = parse_tagbit(bitstr)
-                    yield (baseaddr + bit_addroff, bitbase + bit_bitoff, tag)
+        for line, (tag, bits, mode) in parse_db_lines(fn):
+            assert mode is None
+            for bitstr in bits:
+                # 31_06
+                _bit_inv, (bit_addroff, bit_bitoff) = parse_tagbit(bitstr)
+                yield (baseaddr + bit_addroff, bitbase + bit_bitoff, tag)
 
 
 def specn():
diff --git a/utils/groupmask.py b/utils/groupmask.py
new file mode 100644
index 00000000..db658ff1
--- /dev/null
+++ b/utils/groupmask.py
@@ -0,0 +1,81 @@
+#/usr/bin/env python3
+
+import sys, os, re
+from prjxray import util
+
+
+def index_masks(fn_in, groups_in):
+    """Return a dictionary with the bits active in each group for the specified list of groups"""
+    # Only analyze the given groups
+    groups = {}
+    for group in groups_in:
+        groups[group] = set()
+
+    # Index bits
+    for line, (tag, bits, mode) in util.parse_db_lines(fn_in):
+        assert not mode, "Unresolved tag: %s" % (line, )
+        prefix = tag[0:tag.rfind(".")]
+        group = groups.get(prefix, None)
+        # Drop groups we aren't interested in
+        if group is None:
+            continue
+        for bit in bits:
+            bit = bit.replace("!", "")
+            group.add(bit)
+
+    # Verify we were able to find all groups
+    for groupk, groupv in groups.items():
+        assert len(groupv), "Bad group %s" % groupk
+
+    return groups
+
+
+def apply_masks(fn_in, groups):
+    """Add 0 entries ("!") to .db entries based on groups definition"""
+    new_db = {}
+    for line, (tag, bits, mode) in util.parse_db_lines(fn_in):
+        assert not mode, "Unresolved tag: %s" % (line, )
+        prefix = tag[0:tag.rfind(".")]
+        group = groups.get(prefix, None)
+        if group:
+            bits = set(bits)
+            for bit in group:
+                if bit not in bits:
+                    bits.add("!" + bit)
+            bits = frozenset(bits)
+        new_db[tag] = bits
+    return new_db
+
+
+def load_groups(fn):
+    ret = []
+    for l in open(fn, "r"):
+        ret.append(l.strip())
+    return ret
+
+
+def run(fn_in, fn_out, groups_fn, verbose=False):
+    groups_in = load_groups(groups_fn)
+    groups = index_masks(fn_in, groups_in)
+    new_db = apply_masks(fn_in, groups)
+    util.write_db_lines(fn_out, new_db)
+
+
+def main():
+    import argparse
+
+    parser = argparse.ArgumentParser(description='Create multi-bit entries')
+    parser.add_argument('--verbose', action='store_true', help='')
+    parser.add_argument(
+        '--groups-fn',
+        default="groups.grp",
+        help='File containing one group per line to parse')
+    parser.add_argument('fn_in', help='')
+    parser.add_argument('fn_out', help='')
+    args = parser.parse_args()
+
+    run(args.fn_in, args.fn_out, args.groups_fn, verbose=args.verbose)
+
+
+if __name__ == '__main__':
+    main()