diff --git a/ia64.s b/ia64.s
index 6320922cca..92415b4e97 100644
--- a/ia64.s
+++ b/ia64.s
@@ -10,6 +10,10 @@
 // It's because BSP is updated by br.call/brl.call (not alloc instruction).
 // So rb_ia64_flushrs flushes stack frames including caller's one.
 // rb_ia64_bsp returns the address next to caller's register stack frame. 
+//
+// See also
+// Intel Itanium Architecture Software Developer's Manual
+// Volume 2: System Architecture.
 //
 	.file	"ia64.c"
 	.text