Clarify.

diff --git a/doc/vm.texi b/doc/vm.texi
index 72bd6b9470f1c367ca40f302f20ef2a4e28ec20d..ef2eb80050c77efc5b08cdc5da6f6696e35aed2d 100644 (file)
--- a/doc/vm.texi
+++ b/doc/vm.texi
@@ -278,6 +278,12 @@ Some way of translating from physical page frames back to virtual page
 frames, so that when you evict a physical page from its frame, you can
 invalidate its translation(s).
 
+It is possible to do this translation without adding a new data
+structure, by modifying the code in @file{userprog/pagedir.c}.  However,
+if you do that you'll need to carefully study and understand section 3.7
+in @bibref{IA32-v3}, and in practice it is probably easier to add a new
+data structure.
+
 @item
 Some way of finding a page on disk if it is not in memory.  You won't
 need this data structure until problem 3-2, but planning ahead is a