diff --git a/src/pkg/sync/asm_arm.s b/src/pkg/sync/asm_arm.s
new file mode 100644
index 0000000000..a6ad9d949c
--- /dev/null
+++ b/src/pkg/sync/asm_arm.s
@@ -0,0 +1,14 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// func cas(val *int32, old, new int32) bool
+// Atomically:
+//	if *val == old {
+//		*val = new;
+//		return true;
+//	}else
+//		return false;
+
+TEXT	sync·cas+0(SB),0,$12
+	TODO