Comments.

[pintos-anon] / src / userprog / pagedir.c

diff --git a/src/userprog/pagedir.c b/src/userprog/pagedir.c
index fa8f244d88220a954e43ebc880e1140d242643f4..000aefea700be64ded51049debd81ffae0199328 100644 (file)
--- a/src/userprog/pagedir.c
+++ b/src/userprog/pagedir.c
@@ -121,7 +121,9 @@ void *
 pagedir_get_page (uint32_t *pd, const void *uaddr) 
 {
   uint32_t *pte = lookup_page (pd, (void *) uaddr, false);
-  return pte != NULL && *pte != 0 ? pte_get_page (*pte) : NULL;
+  return (pte != NULL && *pte != 0
+          ? (uint8_t *) pte_get_page (*pte) + pg_ofs (uaddr)
+          : NULL);
 }
 
 /* Clears any mapping for user virtual address UPAGE in page
@@ -185,6 +187,11 @@ pagedir_activate (uint32_t *pd)
 {
   if (pd == NULL)
     pd = base_page_dir;
+
+  /* Store the physical address of the page directory into CR3
+     aka PDBR (page directory base register).  This activates our
+     new page tables immediately.  See [IA32-v2a] "MOV--Move
+     to/from Control Registers" and [IA32-v3] 3.7.5. */
   asm volatile ("movl %0,%%cr3" :: "r" (vtop (pd)));
 }
 
@@ -194,6 +201,9 @@ active_pd (void)
 {
   uint32_t *pd;
 
+  /* Copy CR3, the page directory base register (PDBR), into `pd'
+     for us to exmaine.  See [IA32-v2a] "MOV--Move to/from
+     Control Registers" and [IA32-v3] 3.7.5. */
   asm ("movl %%cr3,%0" : "=r" (pd));
   return pd;
 }