diff --git a/.gitignore b/.gitignore
index 770aba77..9fc90c58 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,3 +15,4 @@ easy-fs-fuse/Cargo.lock
 easy-fs-fuse/target/*
 tools/
 pushall.sh
+*.bak
diff --git a/os/src/main.rs b/os/src/main.rs
index 5ad63749..6494e1d0 100644
--- a/os/src/main.rs
+++ b/os/src/main.rs
@@ -15,6 +15,8 @@
 //! We then call [`task::run_first_task()`] and for the first time go to
 //! userspace.
 
+#![deny(missing_docs)]
+#![deny(warnings)]
 #![no_std]
 #![no_main]
 #![feature(panic_info_message)]
diff --git a/os/src/sync/up.rs b/os/src/sync/up.rs
index 039b0397..e8ba20c8 100644
--- a/os/src/sync/up.rs
+++ b/os/src/sync/up.rs
@@ -24,7 +24,7 @@ impl<T> UPSafeCell<T> {
             inner: RefCell::new(value),
         }
     }
-    /// Panic if the data has been borrowed.
+    /// Exclusive access inner data in UPSafeCell. Panic if the data has been borrowed.
     pub fn exclusive_access(&self) -> RefMut<'_, T> {
         self.inner.borrow_mut()
     }
diff --git a/os/src/syscall/fs.rs b/os/src/syscall/fs.rs
index 0cbd0ca0..2fe02bc6 100644
--- a/os/src/syscall/fs.rs
+++ b/os/src/syscall/fs.rs
@@ -5,6 +5,7 @@ use crate::task::current_user_token;
 
 const FD_STDOUT: usize = 1;
 
+/// write buf of length `len`  to a file with `fd`
 pub fn sys_write(fd: usize, buf: *const u8, len: usize) -> isize {
     match fd {
         FD_STDOUT => {
diff --git a/os/src/syscall/process.rs b/os/src/syscall/process.rs
index a26fb3fe..e5b4ad42 100644
--- a/os/src/syscall/process.rs
+++ b/os/src/syscall/process.rs
@@ -3,17 +3,20 @@
 use crate::task::{exit_current_and_run_next, suspend_current_and_run_next};
 use crate::timer::get_time_ms;
 
+/// task exits and submit an exit code
 pub fn sys_exit(exit_code: i32) -> ! {
     println!("[kernel] Application exited with code {}", exit_code);
     exit_current_and_run_next();
     panic!("Unreachable in sys_exit!");
 }
 
+/// current task gives up resources for other tasks
 pub fn sys_yield() -> isize {
     suspend_current_and_run_next();
     0
 }
 
+/// get current time 
 pub fn sys_get_time() -> isize {
     get_time_ms() as isize
 }
diff --git a/os/src/task/context.rs b/os/src/task/context.rs
index 8e4f5473..d945c5e0 100644
--- a/os/src/task/context.rs
+++ b/os/src/task/context.rs
@@ -4,12 +4,16 @@ use crate::trap::trap_return;
 #[repr(C)]
 /// task context structure containing some registers
 pub struct TaskContext {
+    /// return address ( e.g. __restore ) of __switch ASM function
     ra: usize,
+    /// kernel stack pointer of app
     sp: usize,
+    /// callee saved registers:  s 0..11
     s: [usize; 12],
 }
 
 impl TaskContext {
+    /// init task context
     pub fn zero_init() -> Self {
         Self {
             ra: 0,
@@ -17,6 +21,7 @@ impl TaskContext {
             s: [0; 12],
         }
     }
+    /// set Task Context{__restore ASM funciton: trap_return, sp: kstack_ptr, s: s_0..12}
     pub fn goto_trap_return(kstack_ptr: usize) -> Self {
         Self {
             ra: trap_return as usize,
diff --git a/os/src/task/mod.rs b/os/src/task/mod.rs
index ee36241b..0f548d0a 100644
--- a/os/src/task/mod.rs
+++ b/os/src/task/mod.rs
@@ -6,7 +6,7 @@
 //! A single global instance of [`TaskManager`] called `TASK_MANAGER` controls
 //! all the tasks in the operating system.
 //!
-//! Be careful when you see [`__switch`]. Control flow around this function
+//! Be careful when you see `__switch` ASM function in `switch.S`. Control flow around this function
 //! might not be what you expect.
 
 mod context;
@@ -49,7 +49,7 @@ struct TaskManagerInner {
 }
 
 lazy_static! {
-    /// a `TaskManager` instance through lazy_static!
+    /// a `TaskManager` global instance through lazy_static!
     pub static ref TASK_MANAGER: TaskManager = {
         println!("init TASK_MANAGER");
         let num_app = get_num_app();
diff --git a/os/src/trap/context.rs b/os/src/trap/context.rs
index 420c016b..527d1b61 100644
--- a/os/src/trap/context.rs
+++ b/os/src/trap/context.rs
@@ -5,18 +5,26 @@ use riscv::register::sstatus::{self, Sstatus, SPP};
 #[repr(C)]
 /// trap context structure containing sstatus, sepc and registers
 pub struct TrapContext {
+    /// general regs[0..31]
     pub x: [usize; 32],
+    /// CSR sstatus      
     pub sstatus: Sstatus,
+    /// CSR sepc
     pub sepc: usize,
+    /// Addr of Page Table
     pub kernel_satp: usize,
+    /// kernel stack
     pub kernel_sp: usize,
+    /// Addr of trap_handler function
     pub trap_handler: usize,
 }
 
 impl TrapContext {
+    /// set stack pointer to x_2 reg (sp)
     pub fn set_sp(&mut self, sp: usize) {
         self.x[2] = sp;
     }
+    /// init app context
     pub fn app_init_context(
         entry: usize,
         sp: usize,
@@ -24,17 +32,17 @@ impl TrapContext {
         kernel_sp: usize,
         trap_handler: usize,
     ) -> Self {
-        let mut sstatus = sstatus::read();
-        sstatus.set_spp(SPP::User);
+        let mut sstatus = sstatus::read(); // CSR sstatus
+        sstatus.set_spp(SPP::User); //previous privilege mode: user mode
         let mut cx = Self {
             x: [0; 32],
             sstatus,
-            sepc: entry,
-            kernel_satp,
-            kernel_sp,
-            trap_handler,
+            sepc: entry, // entry point of app
+            kernel_satp, // addr of page table
+            kernel_sp,   // kernel stack
+            trap_handler,// addr of trap_handler function
         };
-        cx.set_sp(sp);
-        cx
+        cx.set_sp(sp); // app's user stack pointer
+        cx // return initial Trap Context of app
     }
 }
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index 56aec31a..14fb76b2 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -28,6 +28,7 @@ use riscv::register::{
 
 global_asm!(include_str!("trap.S"));
 
+/// initialize CSR `stvec` as the entry of `__alltraps`
 pub fn init() {
     set_kernel_trap_entry();
 }
@@ -44,6 +45,7 @@ fn set_user_trap_entry() {
     }
 }
 
+/// enable timer interrupt in sie CSR
 pub fn enable_timer_interrupt() {
     unsafe {
         sie::set_stimer();
@@ -51,6 +53,7 @@ pub fn enable_timer_interrupt() {
 }
 
 #[no_mangle]
+/// handle an interrupt, exception, or system call from user space
 pub fn trap_handler() -> ! {
     set_kernel_trap_entry();
     let cx = current_trap_cx();
@@ -86,6 +89,9 @@ pub fn trap_handler() -> ! {
 }
 
 #[no_mangle]
+/// set the new addr of __restore asm function in TRAMPOLINE page,
+/// set the reg a0 = trap_cx_ptr, reg a1 = phy addr of usr page table,
+/// finally, jump to new addr of __restore asm function
 pub fn trap_return() -> ! {
     set_user_trap_entry();
     let trap_cx_ptr = TRAP_CONTEXT;
@@ -98,16 +104,18 @@ pub fn trap_return() -> ! {
     unsafe {
         asm!(
             "fence.i",
-            "jr {restore_va}",
+            "jr {restore_va}",             // jump to new addr of __restore asm function
             restore_va = in(reg) restore_va,
-            in("a0") trap_cx_ptr,
-            in("a1") user_satp,
+            in("a0") trap_cx_ptr,      // a0 = virt addr of Trap Context
+            in("a1") user_satp,        // a1 = phy addr of usr page table
             options(noreturn)
         );
     }
 }
 
 #[no_mangle]
+/// Unimplement: traps/interrupts/exceptions from kernel mode
+/// Todo: Chapter 9: I/O device 
 pub fn trap_from_kernel() -> ! {
     panic!("a trap from kernel!");
 }