diff --git a/long_xadd.c b/long_xadd.c
index c5579c5..07a4fbd 100644
--- a/long_xadd.c
+++ b/long_xadd.c
@@ -1,5 +1,13 @@
 
-
+/**
+ * Add two longs in xmem zero page
+ *
+ * r0 = ptr to firts arg
+ * r1 = ptr to second arg
+ * r7 = length
+ *
+ * result is in r0*
+ */
 
 void long_xadd() __naked
 {