X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fuserprog%2Fpagedir.c;h=c0a015ae831ab20e88311538fb3eb541c0894b25;hb=6dca07ec0e8cde38f64f0c9f543b8e52ba169e94;hp=fa8f244d88220a954e43ebc880e1140d242643f4;hpb=838c30d0075a3ee0413ba4909944b37f4970a10d;p=pintos-anon

diff --git a/src/userprog/pagedir.c b/src/userprog/pagedir.c
index fa8f244..c0a015a 100644
--- 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)));
 }
 
@@ -192,8 +199,11 @@ pagedir_activate (uint32_t *pd)
 static uint32_t *
 active_pd (void) 
 {
+  /* Copy CR3, the page directory base register (PDBR), into
+     `pd'.
+     See [IA32-v2a] "MOV--Move to/from Control Registers" and
+     [IA32-v3] 3.7.5. */
   uint32_t *pd;
-
   asm ("movl %%cr3,%0" : "=r" (pd));
   return pd;
 }