diff --git a/src/userprog/syscall.c b/src/userprog/syscall.c
index 6a1997d..931d177 100644
--- a/src/userprog/syscall.c
+++ b/src/userprog/syscall.c
@@ -18,7 +18,9 @@ static struct lock filesys_lock;
 
 static int get_byte (const uint8_t *uaddr);
 static uint32_t get_word (const uint32_t *uaddr);
-static void validate_ptr (const uint8_t *uaddr);
+static bool put_byte (uint8_t *udst, uint8_t byte);
+static void validate_ptr_read (const uint8_t *uaddr, unsigned);
+static void validate_ptr_write (uint8_t *udst, unsigned size);
 
 static void syscall_handler (struct intr_frame *);
 
@@ -42,53 +44,6 @@ syscall_init (void)
   intr_register_int (0x30, 3, INTR_ON, syscall_handler, "syscall");
 }
 
-/* Reads a byte at user virtual address UADDR.
-   Returns the byte value if successful, -1 if a segfault
-   occured or UADDR is not in the user space. */
-static int
-get_byte (const uint8_t *uaddr)
-{
-  if (!is_user_vaddr (uaddr))
-    return -1;
-  int result;
-  asm ("movl $1f, %0; movzbl %1, %0; 1:"
-       : "=&a" (result) : "m" (*uaddr));
-  return result;
-}
-
-/* Reads a word at user virtual address ADDR.
-   Returns the word value if successful, calls exit system
-   call if not. */
-static uint32_t
-get_word (const uint32_t *uaddr)
-{
-  uint32_t res;
-  int byte;
-  int i;
-  for (i = 0; i < 4; i++)
-    {
-      byte = get_byte ((uint8_t *) uaddr + i);
-      if (byte == -1)
-        {
-          syscall_exit (-1);
-          NOT_REACHED ();
-        }
-      *((uint8_t *) &res + i) = (uint8_t) byte;
-    }
-  return res;
-}
-
-/* Validates a given user virtual address. */
-static void
-validate_ptr (const uint8_t *uaddr)
-{
-  if (get_byte (uaddr) == -1)
-    {
-      syscall_exit (-1);
-      NOT_REACHED ();
-    }
-}
-
 /* Handler which matches the appropriate system call. */
 static void
 syscall_handler (struct intr_frame *f UNUSED)
@@ -181,7 +136,7 @@ static pid_t
 syscall_exec (const char *cmd_line)
 {
   /* Check the validity. */
-  validate_ptr (cmd_line);
+  validate_ptr_read (cmd_line, 1);
 
   /* Create a new process. */
   pid_t pid = process_execute (cmd_line);
@@ -216,7 +171,7 @@ static bool
 syscall_create (const char *file, unsigned init_size)
 {
   /* Check the validity. */
-  validate_ptr (file);
+  validate_ptr_read (file, 1);
 
   /* Create a new file. */
   lock_acquire (&filesys_lock);
@@ -230,7 +185,7 @@ static bool
 syscall_remove (const char *file)
 {
   /* Check the validity. */
-  validate_ptr (file);
+  validate_ptr_read (file, 1);
 
   /* Create a new file. */
   lock_acquire (&filesys_lock);
@@ -245,7 +200,7 @@ static int
 syscall_open (const char *file)
 {
   /* Check the validity. */
-  validate_ptr (file);
+  validate_ptr_read (file, 1);
 
   /* Open the file. */
   lock_acquire (&filesys_lock);
@@ -292,8 +247,9 @@ static int
 syscall_read (int fd, void *buffer, unsigned size)
 {
   /* Check the validity. */
-  validate_ptr (buffer);
   uint8_t *bf = (uint8_t *) buffer;
+  validate_ptr_read (bf, size);
+  validate_ptr_write (bf, size);
 
   /* Read from STDIN. */
   unsigned bytes = 0;
@@ -331,7 +287,7 @@ static int
 syscall_write (int fd, void *buffer, unsigned size)
 {
   /* Check the validity. */
-  validate_ptr (buffer);
+  validate_ptr_read (buffer, size);
 
   /* Write to STDOUT. */
   if (fd == STDOUT_FILENO)
@@ -421,3 +377,87 @@ syscall_close (int fd)
     }
   lock_release (&filesys_lock);
 }
+
+/* Reads a byte at user virtual address UADDR.
+   Returns the byte value if successful, -1 if a segfault
+   occured or UADDR is not in the user space. */
+static int
+get_byte (const uint8_t *uaddr)
+{
+  if (!is_user_vaddr (uaddr))
+    return -1;
+  int result;
+  asm ("movl $1f, %0; movzbl %1, %0; 1:"
+       : "=&a" (result) : "m" (*uaddr));
+  return result;
+}
+
+/* Reads a word at user virtual address ADDR.
+   Returns the word value if successful, calls exit system
+   call if not. */
+static uint32_t
+get_word (const uint32_t *uaddr)
+{
+  uint32_t res;
+  int byte;
+  int i;
+  for (i = 0; i < 4; i++)
+    {
+      byte = get_byte ((uint8_t *) uaddr + i);
+      if (byte == -1)
+        {
+          syscall_exit (-1);
+          NOT_REACHED ();
+        }
+      *((uint8_t *) &res + i) = (uint8_t) byte;
+    }
+  return res;
+}
+
+/* Writes BYTE to user address UDST.
+   Returns true if successful, false if a segfault occurred
+   or UADDR is not in the user space. */
+static bool
+put_byte (uint8_t *udst, uint8_t byte)
+{
+  if (!is_user_vaddr (udst))
+    return false;
+  int error_code;
+  asm ("movl $1f, %0; movb %b2, %1; 1:"
+       : "=&a" (error_code), "=m" (*udst) : "r" (byte));
+  return error_code != -1;
+}
+
+/* Validates reading to a given user virtual address UADDR up to
+   size SIZE. */
+static void
+validate_ptr_read (const uint8_t *uaddr, size_t size)
+{
+  uint8_t *ptr;
+  for (ptr = pg_round_down (uaddr); ptr < uaddr + size; ptr += PGSIZE)
+    {
+      if (get_byte (ptr) == -1)
+        {
+          syscall_exit (-1);
+          NOT_REACHED ();
+        }
+    }
+}
+
+/* Validates writing to a given user virtual address UADDR up to
+   size SIZE.
+
+   Use this method after validate_ptr_read(). */
+static void
+validate_ptr_write (uint8_t *udst, unsigned size)
+{
+  uint8_t *ptr;
+  for (ptr = pg_round_down (udst); ptr < udst + size; ptr += PGSIZE)
+    {
+      if (!put_byte (ptr, get_byte (ptr)))
+        {
+          syscall_exit (-1);
+          NOT_REACHED ();
+        }
+    }
+}