diff --git a/src/userprog/pagedir.c b/src/userprog/pagedir.c
new file mode 100644
index 0000000000000000000000000000000000000000..f03afa0c5d958cde8c333e77b4e099e10f17cba6
--- /dev/null
+++ b/src/userprog/pagedir.c
@@ -0,0 +1,263 @@
+#include "userprog/pagedir.h"
+#include <stdbool.h>
+#include <stddef.h>
+#include <string.h>
+#include "threads/init.h"
+#include "threads/pte.h"
+#include "threads/palloc.h"
+
+static uint32_t *active_pd (void);
+static void invalidate_pagedir (uint32_t *);
+
+/* Creates a new page directory that has mappings for kernel
+   virtual addresses, but none for user virtual addresses.
+   Returns the new page directory, or a null pointer if memory
+   allocation fails. */
+uint32_t *
+pagedir_create (void)
+{
+  uint32_t *pd = palloc_get_page (0);
+  if (pd != NULL)
+    memcpy (pd, init_page_dir, PGSIZE);
+  return pd;
+}
+
+/* Destroys page directory PD, freeing all the pages it
+   references. */
+void
+pagedir_destroy (uint32_t *pd)
+{
+  uint32_t *pde;
+
+  if (pd == NULL)
+    return;
+
+  ASSERT (pd != init_page_dir);
+  for (pde = pd; pde < pd + pd_no (PHYS_BASE); pde++)
+    if (*pde & PTE_P)
+      {
+        uint32_t *pt = pde_get_pt (*pde);
+        uint32_t *pte;
+
+        for (pte = pt; pte < pt + PGSIZE / sizeof *pte; pte++)
+          if (*pte & PTE_P)
+            palloc_free_page (pte_get_page (*pte));
+        palloc_free_page (pt);
+      }
+  palloc_free_page (pd);
+}
+
+/* Returns the address of the page table entry for virtual
+   address VADDR in page directory PD.
+   If PD does not have a page table for VADDR, behavior depends
+   on CREATE.  If CREATE is true, then a new page table is
+   created and a pointer into it is returned.  Otherwise, a null
+   pointer is returned. */
+static uint32_t *
+lookup_page (uint32_t *pd, const void *vaddr, bool create)
+{
+  uint32_t *pt, *pde;
+
+  ASSERT (pd != NULL);
+
+  /* Shouldn't create new kernel virtual mappings. */
+  ASSERT (!create || is_user_vaddr (vaddr));
+
+  /* Check for a page table for VADDR.
+     If one is missing, create one if requested. */
+  pde = pd + pd_no (vaddr);
+  if (*pde == 0)
+    {
+      if (create)
+        {
+          pt = palloc_get_page (PAL_ZERO);
+          if (pt == NULL)
+            return NULL;
+
+          *pde = pde_create (pt);
+        }
+      else
+        return NULL;
+    }
+
+  /* Return the page table entry. */
+  pt = pde_get_pt (*pde);
+  return &pt[pt_no (vaddr)];
+}
+
+/* Adds a mapping in page directory PD from user virtual page
+   UPAGE to the physical frame identified by kernel virtual
+   address KPAGE.
+   UPAGE must not already be mapped.
+   KPAGE should probably be a page obtained from the user pool
+   with palloc_get_page().
+   If WRITABLE is true, the new page is read/write;
+   otherwise it is read-only.
+   Returns true if successful, false if memory allocation
+   failed. */
+bool
+pagedir_set_page (uint32_t *pd, void *upage, void *kpage, bool writable)
+{
+  uint32_t *pte;
+
+  ASSERT (pg_ofs (upage) == 0);
+  ASSERT (pg_ofs (kpage) == 0);
+  ASSERT (is_user_vaddr (upage));
+  ASSERT (vtop (kpage) >> PTSHIFT < init_ram_pages);
+  ASSERT (pd != init_page_dir);
+
+  pte = lookup_page (pd, upage, true);
+
+  if (pte != NULL)
+    {
+      ASSERT ((*pte & PTE_P) == 0);
+      *pte = pte_create_user (kpage, writable);
+      return true;
+    }
+  else
+    return false;
+}
+
+/* Looks up the physical address that corresponds to user virtual
+   address UADDR in PD.  Returns the kernel virtual address
+   corresponding to that physical address, or a null pointer if
+   UADDR is unmapped. */
+void *
+pagedir_get_page (uint32_t *pd, const void *uaddr)
+{
+  uint32_t *pte;
+
+  ASSERT (is_user_vaddr (uaddr));
+
+  pte = lookup_page (pd, uaddr, false);
+  if (pte != NULL && (*pte & PTE_P) != 0)
+    return pte_get_page (*pte) + pg_ofs (uaddr);
+  else
+    return NULL;
+}
+
+/* Marks user virtual page UPAGE "not present" in page
+   directory PD.  Later accesses to the page will fault.  Other
+   bits in the page table entry are preserved.
+   UPAGE need not be mapped. */
+void
+pagedir_clear_page (uint32_t *pd, void *upage)
+{
+  uint32_t *pte;
+
+  ASSERT (pg_ofs (upage) == 0);
+  ASSERT (is_user_vaddr (upage));
+
+  pte = lookup_page (pd, upage, false);
+  if (pte != NULL && (*pte & PTE_P) != 0)
+    {
+      *pte &= ~PTE_P;
+      invalidate_pagedir (pd);
+    }
+}
+
+/* Returns true if the PTE for virtual page VPAGE in PD is dirty,
+   that is, if the page has been modified since the PTE was
+   installed.
+   Returns false if PD contains no PTE for VPAGE. */
+bool
+pagedir_is_dirty (uint32_t *pd, const void *vpage)
+{
+  uint32_t *pte = lookup_page (pd, vpage, false);
+  return pte != NULL && (*pte & PTE_D) != 0;
+}
+
+/* Set the dirty bit to DIRTY in the PTE for virtual page VPAGE
+   in PD. */
+void
+pagedir_set_dirty (uint32_t *pd, const void *vpage, bool dirty)
+{
+  uint32_t *pte = lookup_page (pd, vpage, false);
+  if (pte != NULL)
+    {
+      if (dirty)
+        *pte |= PTE_D;
+      else
+        {
+          *pte &= ~(uint32_t) PTE_D;
+          invalidate_pagedir (pd);
+        }
+    }
+}
+
+/* Returns true if the PTE for virtual page VPAGE in PD has been
+   accessed recently, that is, between the time the PTE was
+   installed and the last time it was cleared.  Returns false if
+   PD contains no PTE for VPAGE. */
+bool
+pagedir_is_accessed (uint32_t *pd, const void *vpage)
+{
+  uint32_t *pte = lookup_page (pd, vpage, false);
+  return pte != NULL && (*pte & PTE_A) != 0;
+}
+
+/* Sets the accessed bit to ACCESSED in the PTE for virtual page
+   VPAGE in PD. */
+void
+pagedir_set_accessed (uint32_t *pd, const void *vpage, bool accessed)
+{
+  uint32_t *pte = lookup_page (pd, vpage, false);
+  if (pte != NULL)
+    {
+      if (accessed)
+        *pte |= PTE_A;
+      else
+        {
+          *pte &= ~(uint32_t) PTE_A;
+          invalidate_pagedir (pd);
+        }
+    }
+}
+
+/* Loads page directory PD into the CPU's page directory base
+   register. */
+void
+pagedir_activate (uint32_t *pd)
+{
+  if (pd == NULL)
+    pd = init_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-v3a] 3.7.5 "Base
+     Address of the Page Directory". */
+  asm volatile ("movl %0, %%cr3" : : "r" (vtop (pd)) : "memory");
+}
+
+/* Returns the currently active page directory. */
+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-v3a] 3.7.5 "Base Address of the Page Directory". */
+  uintptr_t pd;
+  asm volatile ("movl %%cr3, %0" : "=r" (pd));
+  return ptov (pd);
+}
+
+/* Seom page table changes can cause the CPU's translation
+   lookaside buffer (TLB) to become out-of-sync with the page
+   table.  When this happens, we have to "invalidate" the TLB by
+   re-activating it.
+
+   This function invalidates the TLB if PD is the active page
+   directory.  (If PD is not active then its entries are not in
+   the TLB, so there is no need to invalidate anything.) */
+static void
+invalidate_pagedir (uint32_t *pd)
+{
+  if (active_pd () == pd)
+    {
+      /* Re-activating PD clears the TLB.  See [IA32-v3a] 3.12
+         "Translation Lookaside Buffers (TLBs)". */
+      pagedir_activate (pd);
+    }
+}