diff --git a/sys/sys/rman.h b/sys/sys/rman.h
index 6535f200a850..14275cb7ed1c 100644
--- a/sys/sys/rman.h
+++ b/sys/sys/rman.h
@@ -84,7 +84,10 @@ struct u_rman {
  * We use a linked list rather than a bitmap because we need to be able to
  * represent potentially huge objects (like all of a processor's physical
  * address space).  That is also why the indices are defined to have type
- * `unsigned long' -- that being the largest integral type in Standard C.
+ * `unsigned long' -- that being the largest integral type in ISO C (1990).
+ * The 1999 version of C allows `long long'; we may need to switch to that
+ * at some point in the future, particularly if we want to support 36-bit
+ * addresses on IA32 hardware.
  */
 CIRCLEQ_HEAD(resource_head, resource);
 struct	resource {