diff --git a/crate/memory/src/page_table/mock_page_table.rs b/crate/memory/src/page_table/mock_page_table.rs
index bf5f50d..c4aaea0 100644
--- a/crate/memory/src/page_table/mock_page_table.rs
+++ b/crate/memory/src/page_table/mock_page_table.rs
@@ -5,7 +5,7 @@ pub struct MockPageTable {
     mapped_set: BTreeSet<VirtAddr>,
     accessed_set: BTreeSet<VirtAddr>,
     dirty_set: BTreeSet<VirtAddr>,
-    page_fault_handler: PageFaultHandler,
+    page_fault_handler: Option<PageFaultHandler>,
     capacity: usize,
 }
 
@@ -18,6 +18,12 @@ impl PageTable for MockPageTable {
     fn dirty(&self, addr: VirtAddr) -> bool {
         self.dirty_set.contains(&addr)
     }
+    fn clear_accessed(&mut self, addr: usize) {
+        self.accessed_set.remove(&addr);
+    }
+    fn clear_dirty(&mut self, addr: usize) {
+        self.dirty_set.remove(&addr);
+    }
     /// Map a page, return false if no more space
     fn map(&mut self, addr: VirtAddr) -> bool {
         if self.mapped_set.len() == self.capacity {
@@ -32,30 +38,33 @@ impl PageTable for MockPageTable {
 }
 
 impl MockPageTable {
-    pub fn new(capacity: usize, page_fault_handler: PageFaultHandler) -> Self {
+    pub fn new(capacity: usize) -> Self {
         MockPageTable {
             mapped_set: BTreeSet::<VirtAddr>::new(),
             accessed_set: BTreeSet::<VirtAddr>::new(),
             dirty_set: BTreeSet::<VirtAddr>::new(),
-            page_fault_handler,
+            page_fault_handler: None,
             capacity,
         }
     }
-    /// Read memory, mark accessed, trigger page fault if not present
-    pub fn read(&mut self, addr: VirtAddr) {
+    pub fn set_handler(&mut self, page_fault_handler: PageFaultHandler) {
+        self.page_fault_handler = Some(page_fault_handler);
+    }
+    fn trigger_page_fault_if_not_present(&mut self, addr: VirtAddr) {
         while !self.mapped_set.contains(&addr) {
             let self_mut = unsafe{ &mut *(self as *mut Self) };
-            (self.page_fault_handler)(self_mut, addr);
+            (self.page_fault_handler.as_mut().unwrap())(self_mut, addr);
         }
+    }
+    /// Read memory, mark accessed, trigger page fault if not present
+    pub fn read(&mut self, addr: VirtAddr) {
+        self.trigger_page_fault_if_not_present(addr);
         self.accessed_set.insert(addr);
 
     }
     /// Write memory, mark accessed and dirty, trigger page fault if not present
     pub fn write(&mut self, addr: VirtAddr) {
-        while !self.mapped_set.contains(&addr) {
-            let self_mut = unsafe{ &mut *(self as *mut Self) };
-            (self.page_fault_handler)(self_mut, addr);
-        }
+        self.trigger_page_fault_if_not_present(addr);
         self.accessed_set.insert(addr);
         self.dirty_set.insert(addr);
     }
@@ -71,7 +80,8 @@ mod test {
     fn test() {
         let page_fault_count = Arc::new(RefCell::new(0usize));
 
-        let mut pt = MockPageTable::new(2, Box::new({
+        let mut pt = MockPageTable::new(2);
+        pt.set_handler(Box::new({
             let page_fault_count1 = page_fault_count.clone();
             move |pt: &mut MockPageTable, addr: VirtAddr| {
                 *page_fault_count1.borrow_mut() += 1;
@@ -85,11 +95,17 @@ mod test {
         assert!(pt.accessed(0));
         assert!(!pt.dirty(0));
 
+        pt.clear_accessed(0);
+        assert!(!pt.accessed(0));
+
         pt.write(1);
         assert_eq!(*page_fault_count.borrow(), 1);
         assert!(pt.accessed(1));
         assert!(pt.dirty(1));
 
+        pt.clear_dirty(1);
+        assert!(!pt.dirty(1));
+
         assert_eq!(pt.map(2), false);
 
         pt.unmap(0);
diff --git a/crate/memory/src/page_table/mod.rs b/crate/memory/src/page_table/mod.rs
index 245311a..d10f652 100644
--- a/crate/memory/src/page_table/mod.rs
+++ b/crate/memory/src/page_table/mod.rs
@@ -5,6 +5,8 @@ pub mod mock_page_table;
 pub trait PageTable {
     fn accessed(&self, addr: VirtAddr) -> bool;
     fn dirty(&self, addr: VirtAddr) -> bool;
+    fn clear_accessed(&mut self, addr: VirtAddr);
+    fn clear_dirty(&mut self, addr: VirtAddr);
     fn map(&mut self, addr: VirtAddr) -> bool;
     fn unmap(&mut self, addr: VirtAddr);
 }
\ No newline at end of file
diff --git a/crate/memory/src/swap/enhanced_clock.rs b/crate/memory/src/swap/enhanced_clock.rs
new file mode 100644
index 0000000..ea782a5
--- /dev/null
+++ b/crate/memory/src/swap/enhanced_clock.rs
@@ -0,0 +1,135 @@
+use alloc::vec_deque::VecDeque;
+use super::*;
+
+pub struct EnhancedClockSwapManager<T: 'static + SwappablePageTable> {
+    page_table: &'static mut T,
+    clock_ptr: usize,
+    deque: VecDeque<VirtAddr>,
+}
+
+impl<T: 'static + SwappablePageTable> SwapManager for EnhancedClockSwapManager<T> {
+    fn tick(&mut self) {
+    }
+
+    fn push(&mut self, addr: usize) {
+        let pos = if self.clock_ptr == 0 {self.deque.len()} else {self.clock_ptr};
+        self.deque.insert(pos, addr);
+    }
+
+    fn remove(&mut self, addr: usize) {
+        let id = self.deque.iter()
+            .position(|&x| x == addr)
+            .expect("address not found");
+        if id < self.clock_ptr {
+            self.clock_ptr -= 1;
+        }
+        self.deque.remove(id);
+    }
+
+    fn pop(&mut self) -> Option<usize> {
+        loop {
+            let addr = self.deque[self.clock_ptr];
+            let accessed = self.page_table.accessed(addr);
+            let dirty = self.page_table.dirty(addr);
+
+            match (accessed, dirty) {
+                (true, _) => {
+                    self.page_table.clear_accessed(addr);
+
+                },
+                (false, true) => {
+                    if self.page_table.swap_out(addr).is_ok() {
+                        self.page_table.clear_dirty(addr);
+                    }
+                },
+                _ => {
+                    return self.remove_current();
+                }
+            }
+            self.move_next();
+        }
+    }
+}
+
+
+impl<T: 'static + SwappablePageTable> EnhancedClockSwapManager<T> {
+    pub fn new(page_table: &'static mut T) -> Self {
+        EnhancedClockSwapManager {
+            page_table,
+            clock_ptr: 0,
+            deque: VecDeque::<VirtAddr>::new()
+        }
+    }
+    fn remove_current(&mut self) -> Option<VirtAddr> {
+        let addr = self.deque.remove(self.clock_ptr);
+        if self.clock_ptr == self.deque.len() {
+            self.clock_ptr = 0;
+        }
+        return addr;
+    }
+    fn move_next(&mut self) {
+        self.clock_ptr += 1;
+        if self.clock_ptr == self.deque.len() {
+            self.clock_ptr = 0;
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use alloc::{arc::Arc, boxed::Box};
+    use core::mem::uninitialized;
+    use core::cell::RefCell;
+    use page_table::mock_page_table::MockPageTable;
+
+    impl SwappablePageTable for MockPageTable {
+        fn swap_out(&mut self, addr: usize) -> Result<(), ()> {
+            Ok(())
+        }
+    }
+
+    enum MemOp {
+        R(usize), W(usize)
+    }
+
+    #[test]
+    fn test() {
+        use self::MemOp::{R, W};
+        let page_fault_count = Arc::new(RefCell::new(0usize));
+
+        let mut pt = Box::new(MockPageTable::new(4));
+        let static_pt = unsafe{ &mut *(pt.as_mut() as *mut MockPageTable) };
+        pt.set_handler(Box::new({
+            let page_fault_count1 = page_fault_count.clone();
+            let mut clock = EnhancedClockSwapManager::new(static_pt);
+
+            move |pt: &mut MockPageTable, addr: VirtAddr| {
+                *page_fault_count1.borrow_mut() += 1;
+
+                if !pt.map(addr) {  // is full?
+                    pt.unmap(clock.pop().unwrap());
+                    pt.map(addr);
+                }
+                clock.push(addr);
+            }
+        }));
+
+
+        let op_seq = [
+            R(1), R(2), R(3), R(4),
+            R(3), W(1), R(4), W(2), R(5),
+            R(2), W(1), R(2), R(3), R(4)];
+        let pgfault_count = [
+            1, 2, 3, 4,
+            4, 4, 4, 4, 5,
+            5, 5, 5, 6, 7];
+        for (op, &count) in op_seq.iter().zip(pgfault_count.iter()) {
+            match op {
+                R(addr) => pt.read(*addr),
+                W(addr) => pt.write(*addr),
+            }
+            assert_eq!(*(*page_fault_count).borrow(), count);
+        }
+    }
+}
diff --git a/crate/memory/src/swap/fifo.rs b/crate/memory/src/swap/fifo.rs
index daa796f..ed63980 100644
--- a/crate/memory/src/swap/fifo.rs
+++ b/crate/memory/src/swap/fifo.rs
@@ -1,7 +1,7 @@
 use alloc::vec_deque::VecDeque;
 use super::*;
 
-struct FifoSwapManager {
+pub struct FifoSwapManager {
     deque: VecDeque<VirtAddr>,
 }
 
@@ -27,7 +27,7 @@ impl SwapManager for FifoSwapManager {
 }
 
 impl FifoSwapManager {
-    fn new() -> Self {
+    pub fn new() -> Self {
         FifoSwapManager {
             deque: VecDeque::<VirtAddr>::new()
         }
@@ -50,7 +50,8 @@ mod test {
         use self::MemOp::{R, W};
         let page_fault_count = Arc::new(RefCell::new(0usize));
 
-        let mut pt = MockPageTable::new(4, Box::new({
+        let mut pt = MockPageTable::new(4);
+        pt.set_handler(Box::new({
             let page_fault_count1 = page_fault_count.clone();
             let mut fifo = FifoSwapManager::new();
 
diff --git a/crate/memory/src/swap/mock_swapper.rs b/crate/memory/src/swap/mock_swapper.rs
index af8d8ba..f1b9704 100644
--- a/crate/memory/src/swap/mock_swapper.rs
+++ b/crate/memory/src/swap/mock_swapper.rs
@@ -6,14 +6,25 @@ pub struct MockSwapper {
 }
 
 impl Swapper for MockSwapper {
-    fn swap_out(&mut self, data: &[u8; 4096]) -> usize {
+    fn swap_out(&mut self, data: &[u8; 4096]) -> Result<usize, ()> {
         let id = self.alloc_id();
         self.map.insert(id, data.clone());
-        id
+        Ok(id)
     }
 
-    fn swap_in(&mut self, token: usize, data: &mut [u8; 4096]) {
-        *data = self.map.remove(&token).unwrap();
+    fn swap_update(&mut self, token: usize, data: &[u8; 4096]) -> Result<(), ()> {
+        if !self.map.contains_key(&token) {
+            return Err(());
+        }
+        self.map.insert(token, data.clone());
+        Ok(())
+    }
+    fn swap_in(&mut self, token: usize, data: &mut [u8; 4096]) -> Result<(), ()> {
+        match self.map.remove(&token) {
+            Some(d) => *data = d,
+            None => return Err(()),
+        }
+        Ok(())
     }
 }
 
@@ -31,16 +42,25 @@ mod test {
     use super::*;
     use core::mem::uninitialized;
 
+    fn assert_data_eq(data1: &[u8; 4096], data2: &[u8; 4096]) {
+        for (&a, &b) in data2.iter().zip(data1.iter()) {
+            assert_eq!(a, b);
+        }
+    }
+
     #[test]
     fn test() {
         let mut swapper = MockSwapper::new();
-        let data: [u8; 4096] = unsafe{ uninitialized() };
-        let mut data1: [u8; 4096] = unsafe{ uninitialized() };
-        let token = swapper.swap_out(&data);
-        swapper.swap_in(token, &mut data1);
-        for (&a, &b) in data.iter().zip(data1.iter()) {
-            assert_eq!(a, b);
-        }
+        let mut data: [u8; 4096] = unsafe{ uninitialized() };
+        let data1: [u8; 4096] = unsafe{ uninitialized() };
+        let token = swapper.swap_out(&data1).unwrap();
+        swapper.swap_in(token, &mut data);
+        assert_data_eq(&data, &data1);
+
+        let data2: [u8; 4096] = unsafe{ uninitialized() };
+        swapper.swap_update(token, &data2);
+        swapper.swap_in(token, &mut data);
+        assert_data_eq(&data, &data2);
     }
 
     #[test]
diff --git a/crate/memory/src/swap/mod.rs b/crate/memory/src/swap/mod.rs
index 6adaa6b..5b9c108 100644
--- a/crate/memory/src/swap/mod.rs
+++ b/crate/memory/src/swap/mod.rs
@@ -1,10 +1,14 @@
+pub use self::fifo::FifoSwapManager;
+pub use self::enhanced_clock::EnhancedClockSwapManager;
+
 use super::*;
 use super::page_table::PageTable;
 
-pub mod fifo;
+mod fifo;
+mod enhanced_clock;
 mod mock_swapper;
 
-trait SwapManager {
+pub trait SwapManager {
     /// Called when tick interrupt occured
     fn tick(&mut self);
     /// Called when map a swappable page into the memory
@@ -15,7 +19,12 @@ trait SwapManager {
     fn pop(&mut self) -> Option<VirtAddr>;
 }
 
-trait Swapper {
-    fn swap_out(&mut self, data: &[u8; 4096]) -> usize;
-    fn swap_in(&mut self, token: usize, data: &mut [u8; 4096]);
+pub trait Swapper {
+    fn swap_out(&mut self, data: &[u8; 4096]) -> Result<usize, ()>;
+    fn swap_update(&mut self, token: usize, data: &[u8; 4096]) -> Result<(), ()>;
+    fn swap_in(&mut self, token: usize, data: &mut [u8; 4096]) -> Result<(), ()>;
+}
+
+pub trait SwappablePageTable: PageTable {
+    fn swap_out(&mut self, addr: VirtAddr) -> Result<(), ()>;
 }
\ No newline at end of file