diff --git a/os/src/loader.rs b/os/src/loader.rs
index 0f08a174..f2ed4f04 100644
--- a/os/src/loader.rs
+++ b/os/src/loader.rs
@@ -41,10 +41,12 @@ impl UserStack {
     }
 }
 
+/// Get base address of app i.
 fn get_base_i(app_id: usize) -> usize {
     APP_BASE_ADDRESS + app_id * APP_SIZE_LIMIT
 }
 
+/// Get the total number of applications.
 pub fn get_num_app() -> usize {
     extern "C" {
         fn _num_app();
@@ -52,6 +54,8 @@ pub fn get_num_app() -> usize {
     unsafe { (_num_app as usize as *const usize).read_volatile() }
 }
 
+/// Load nth user app at
+/// [APP_BASE_ADDRESS + n * APP_SIZE_LIMIT, APP_BASE_ADDRESS + (n+1) * APP_SIZE_LIMIT).
 pub fn load_apps() {
     extern "C" {
         fn _num_app();
@@ -78,6 +82,7 @@ pub fn load_apps() {
     }
 }
 
+/// get app info with entry and sp and save `TrapContext` in kernel stack
 pub fn init_app_cx(app_id: usize) -> usize {
     KERNEL_STACK[app_id].push_context(TrapContext::app_init_context(
         get_base_i(app_id),
diff --git a/os/src/sbi.rs b/os/src/sbi.rs
index 2e2804b2..37efee33 100644
--- a/os/src/sbi.rs
+++ b/os/src/sbi.rs
@@ -27,18 +27,22 @@ fn sbi_call(which: usize, arg0: usize, arg1: usize, arg2: usize) -> usize {
     ret
 }
 
+/// use sbi call to set timer
 pub fn set_timer(timer: usize) {
     sbi_call(SBI_SET_TIMER, timer, 0, 0);
 }
 
+/// use sbi call to putchar in console (qemu uart handler)
 pub fn console_putchar(c: usize) {
     sbi_call(SBI_CONSOLE_PUTCHAR, c, 0, 0);
 }
 
+/// use sbi call to getchar from console (qemu uart handler)
 pub fn console_getchar() -> usize {
     sbi_call(SBI_CONSOLE_GETCHAR, 0, 0, 0)
 }
 
+/// use sbi call to shutdown the kernel
 pub fn shutdown() -> ! {
     sbi_call(SBI_SHUTDOWN, 0, 0, 0);
     panic!("It should shutdown!");
diff --git a/os/src/syscall/mod.rs b/os/src/syscall/mod.rs
index 572b081a..17d17b73 100644
--- a/os/src/syscall/mod.rs
+++ b/os/src/syscall/mod.rs
@@ -9,6 +9,7 @@ mod process;
 use fs::*;
 use process::*;
 
+/// handle syscall exception with `syscall_id` and other arguments
 pub fn syscall(syscall_id: usize, args: [usize; 3]) -> isize {
     match syscall_id {
         SYSCALL_WRITE => sys_write(args[0], args[1] as *const u8, args[2]),
diff --git a/os/src/syscall/process.rs b/os/src/syscall/process.rs
index 55856e70..85314ea2 100644
--- a/os/src/syscall/process.rs
+++ b/os/src/syscall/process.rs
@@ -1,17 +1,20 @@
 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 time in milliseconds
 pub fn sys_get_time() -> isize {
     get_time_ms() as isize
 }
diff --git a/os/src/task/mod.rs b/os/src/task/mod.rs
index ca3ca2f6..453d6a7d 100644
--- a/os/src/task/mod.rs
+++ b/os/src/task/mod.rs
@@ -13,12 +13,16 @@ use task::{TaskControlBlock, TaskStatus};
 pub use context::TaskContext;
 
 pub struct TaskManager {
+    /// total number of tasks
     num_app: usize,
+    /// use inner value to get mutable access
     inner: UPSafeCell<TaskManagerInner>,
 }
 
 struct TaskManagerInner {
+    /// task list
     tasks: [TaskControlBlock; MAX_APP_NUM],
+    /// id of current `Running` task
     current_task: usize,
 }
 
@@ -46,6 +50,10 @@ lazy_static! {
 }
 
 impl TaskManager {
+    /// Run the first task in task list.
+    ///
+    /// Generally, the first task in task list is an idle task (we call it zero process later).
+    /// But in ch3, we load apps statically, so the first task is a real app.
     fn run_first_task(&self) -> ! {
         let mut inner = self.inner.exclusive_access();
         let task0 = &mut inner.tasks[0];
@@ -60,18 +68,23 @@ impl TaskManager {
         panic!("unreachable in run_first_task!");
     }
 
+    /// Change the status of current `Running` task into `Ready`.
     fn mark_current_suspended(&self) {
         let mut inner = self.inner.exclusive_access();
         let current = inner.current_task;
         inner.tasks[current].task_status = TaskStatus::Ready;
     }
 
+    /// Change the status of current `Running` task into `Exited`.
     fn mark_current_exited(&self) {
         let mut inner = self.inner.exclusive_access();
         let current = inner.current_task;
         inner.tasks[current].task_status = TaskStatus::Exited;
     }
 
+    /// Find next task to run and return app id.
+    ///
+    /// In this case, we only return the first `Ready` task in task list.
     fn find_next_task(&self) -> Option<usize> {
         let inner = self.inner.exclusive_access();
         let current = inner.current_task;
@@ -80,6 +93,8 @@ impl TaskManager {
             .find(|id| inner.tasks[*id].task_status == TaskStatus::Ready)
     }
 
+    /// Switch current `Running` task to the task we have found,
+    /// or there is no `Ready` task and we can exit with all applications completed
     fn run_next_task(&self) {
         if let Some(next) = self.find_next_task() {
             let mut inner = self.inner.exclusive_access();
diff --git a/os/src/timer.rs b/os/src/timer.rs
index 048aa3b7..1d101d43 100644
--- a/os/src/timer.rs
+++ b/os/src/timer.rs
@@ -5,14 +5,17 @@ use riscv::register::time;
 const TICKS_PER_SEC: usize = 100;
 const MSEC_PER_SEC: usize = 1000;
 
+/// read the `mtime` register
 pub fn get_time() -> usize {
     time::read()
 }
 
+/// get current time in milliseconds
 pub fn get_time_ms() -> usize {
     time::read() / (CLOCK_FREQ / MSEC_PER_SEC)
 }
 
+/// set the next timer interrupt
 pub fn set_next_trigger() {
     set_timer(get_time() + CLOCK_FREQ / TICKS_PER_SEC);
 }
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index 527d6b4b..dcb4c2bb 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -12,6 +12,7 @@ use riscv::register::{
 
 global_asm!(include_str!("trap.S"));
 
+/// initialize CSR `stvec` as the entry of `__alltraps`
 pub fn init() {
     extern "C" {
         fn __alltraps();
@@ -21,6 +22,7 @@ pub fn init() {
     }
 }
 
+/// timer interrupt enabled
 pub fn enable_timer_interrupt() {
     unsafe {
         sie::set_stimer();
@@ -28,9 +30,10 @@ pub fn enable_timer_interrupt() {
 }
 
 #[no_mangle]
+/// handle an interrupt, exception, or system call from user space
 pub fn trap_handler(cx: &mut TrapContext) -> &mut TrapContext {
-    let scause = scause::read();
-    let stval = stval::read();
+    let scause = scause::read(); // get trap cause
+    let stval = stval::read(); // get extra value
     match scause.cause() {
         Trap::Exception(Exception::UserEnvCall) => {
             cx.sepc += 4;