diff --git a/ia64.s b/ia64.s
index ba5241daf2..3865712266 100644
--- a/ia64.s
+++ b/ia64.s
@@ -6,6 +6,11 @@
 //   void rb_ia64_flushrs(void) { __builtin_ia64_flushrs(); }
 //   void *rb_ia64_bsp(void) { return __builtin_ia64_bsp(); }
 // 
+// Note that rb_ia64_flushrs and rb_ia64_bsp works in its own stack frame.
+// 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. 
+//
 	.file	"ia64.c"
 	.text
 	.align 16