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/boards/k210.rs b/os/src/boards/k210.rs
index 4fea2d11..583f05aa 100644
--- a/os/src/boards/k210.rs
+++ b/os/src/boards/k210.rs
@@ -1 +1,3 @@
+//! Constants used in rCore for K210 devel board
+
 pub const CLOCK_FREQ: usize = 403000000 / 62;
diff --git a/os/src/boards/qemu.rs b/os/src/boards/qemu.rs
index 98874fbf..dafcc7b4 100644
--- a/os/src/boards/qemu.rs
+++ b/os/src/boards/qemu.rs
@@ -1 +1,3 @@
+//! Constants used in rCore for K210 devel board
+
 pub const CLOCK_FREQ: usize = 12500000;
diff --git a/os/src/config.rs b/os/src/config.rs
index 4294ff6f..ff7ffbe1 100644
--- a/os/src/config.rs
+++ b/os/src/config.rs
@@ -1,4 +1,6 @@
-pub const USER_STACK_SIZE: usize = 4096 * 2;
+//! Constants used in rCore
+
+pub const USER_STACK_SIZE: usize = 4096;
 pub const KERNEL_STACK_SIZE: usize = 4096 * 2;
 pub const MAX_APP_NUM: usize = 4;
 pub const APP_BASE_ADDRESS: usize = 0x80400000;
diff --git a/os/src/console.rs b/os/src/console.rs
index dda4911a..cc6a91bf 100644
--- a/os/src/console.rs
+++ b/os/src/console.rs
@@ -1,3 +1,5 @@
+//! SBI console driver, for text output
+
 use crate::sbi::console_putchar;
 use core::fmt::{self, Write};
 
@@ -16,6 +18,7 @@ pub fn print(args: fmt::Arguments) {
     Stdout.write_fmt(args).unwrap();
 }
 
+/// print string macro
 #[macro_export]
 macro_rules! print {
     ($fmt: literal $(, $($arg: tt)+)?) => {
@@ -23,6 +26,7 @@ macro_rules! print {
     }
 }
 
+/// println string macro
 #[macro_export]
 macro_rules! println {
     ($fmt: literal $(, $($arg: tt)+)?) => {
diff --git a/os/src/lang_items.rs b/os/src/lang_items.rs
index af3e5152..db902c35 100644
--- a/os/src/lang_items.rs
+++ b/os/src/lang_items.rs
@@ -1,3 +1,5 @@
+//! The panic handler
+
 use crate::sbi::shutdown;
 use core::panic::PanicInfo;
 
diff --git a/os/src/loader.rs b/os/src/loader.rs
index f2ed4f04..af9e508d 100644
--- a/os/src/loader.rs
+++ b/os/src/loader.rs
@@ -1,3 +1,10 @@
+//! Loading user applications into memory
+//!
+//! For chapter 3, user applications are simply part of the data included in the
+//! kernel binary, so we only need to copy them to the space allocated for each
+//! app to load them. We also allocate fixed spaces for each task's
+//! [`KernelStack`] and [`UserStack`].
+
 use crate::config::*;
 use crate::trap::TrapContext;
 use core::arch::asm;
diff --git a/os/src/main.rs b/os/src/main.rs
index 78c41810..55ca76e6 100644
--- a/os/src/main.rs
+++ b/os/src/main.rs
@@ -1,3 +1,22 @@
+//! The main module and entrypoint
+//!
+//! Various facilities of the kernels are implemented as submodules. The most
+//! important ones are:
+//!
+//! - [`trap`]: Handles all cases of switching from userspace to the kernel
+//! - [`task`]: Task management
+//! - [`syscall`]: System call handling and implementation
+//!
+//! The operating system also starts in this module. Kernel code starts
+//! executing from `entry.asm`, after which [`rust_main()`] is called to
+//! initialize various pieces of functionality. (See its source code for
+//! details.)
+//!
+//! 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)]
@@ -25,6 +44,8 @@ mod trap;
 global_asm!(include_str!("entry.asm"));
 global_asm!(include_str!("link_app.S"));
 
+
+/// clear BSS segment
 fn clear_bss() {
     extern "C" {
         fn sbss();
@@ -36,6 +57,7 @@ fn clear_bss() {
     }
 }
 
+/// the rust entry-point of os
 #[no_mangle]
 pub fn rust_main() -> ! {
     clear_bss();
diff --git a/os/src/sbi.rs b/os/src/sbi.rs
index 37efee33..404004d1 100644
--- a/os/src/sbi.rs
+++ b/os/src/sbi.rs
@@ -1,17 +1,18 @@
-#![allow(unused)]
+//! SBI call wrappers
 
 use core::arch::asm;
 
 const SBI_SET_TIMER: usize = 0;
 const SBI_CONSOLE_PUTCHAR: usize = 1;
-const SBI_CONSOLE_GETCHAR: usize = 2;
-const SBI_CLEAR_IPI: usize = 3;
-const SBI_SEND_IPI: usize = 4;
-const SBI_REMOTE_FENCE_I: usize = 5;
-const SBI_REMOTE_SFENCE_VMA: usize = 6;
-const SBI_REMOTE_SFENCE_VMA_ASID: usize = 7;
 const SBI_SHUTDOWN: usize = 8;
+// const SBI_CONSOLE_GETCHAR: usize = 2;
+// const SBI_CLEAR_IPI: usize = 3;
+// const SBI_SEND_IPI: usize = 4;
+// const SBI_REMOTE_FENCE_I: usize = 5;
+// const SBI_REMOTE_SFENCE_VMA: usize = 6;
+// const SBI_REMOTE_SFENCE_VMA_ASID: usize = 7;
 
+///  handle SBI call with `which` SBI_id and other arguments
 #[inline(always)]
 fn sbi_call(which: usize, arg0: usize, arg1: usize, arg2: usize) -> usize {
     let mut ret;
@@ -38,9 +39,9 @@ pub fn console_putchar(c: usize) {
 }
 
 /// use sbi call to getchar from console (qemu uart handler)
-pub fn console_getchar() -> usize {
-    sbi_call(SBI_CONSOLE_GETCHAR, 0, 0, 0)
-}
+// pub fn console_getchar() -> usize {
+//     sbi_call(SBI_CONSOLE_GETCHAR, 0, 0, 0)
+// }
 
 /// use sbi call to shutdown the kernel
 pub fn shutdown() -> ! {
diff --git a/os/src/sync/mod.rs b/os/src/sync/mod.rs
index d1ce5bcf..4743e31e 100644
--- a/os/src/sync/mod.rs
+++ b/os/src/sync/mod.rs
@@ -1,3 +1,5 @@
+//! Synchronization and interior mutability primitives
+
 mod up;
 
 pub use up::UPSafeCell;
diff --git a/os/src/sync/up.rs b/os/src/sync/up.rs
index c7b2c9ee..e8ba20c8 100644
--- a/os/src/sync/up.rs
+++ b/os/src/sync/up.rs
@@ -1,3 +1,5 @@
+//! Uniprocessor interior mutability primitives
+
 use core::cell::{RefCell, RefMut};
 
 /// Wrap a static data structure inside it so that we are
@@ -22,7 +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 3e38eae7..2174f26e 100644
--- a/os/src/syscall/fs.rs
+++ b/os/src/syscall/fs.rs
@@ -1,5 +1,8 @@
+//! File and filesystem-related syscalls
+
 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/mod.rs b/os/src/syscall/mod.rs
index 17d17b73..ee52f5b4 100644
--- a/os/src/syscall/mod.rs
+++ b/os/src/syscall/mod.rs
@@ -1,3 +1,15 @@
+//! Implementation of syscalls
+//!
+//! The single entry point to all system calls, [`syscall()`], is called
+//! whenever userspace wishes to perform a system call using the `ecall`
+//! instruction. In this case, the processor raises an 'Environment call from
+//! U-mode' exception, which is handled as one of the cases in
+//! [`crate::trap::trap_handler`].
+//!
+//! For clarity, each single syscall is implemented as its own function, named
+//! `sys_` then the name of the syscall. You can find functions like this in
+//! submodules, and you should also implement syscalls this way.
+
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
 const SYSCALL_YIELD: usize = 124;
diff --git a/os/src/syscall/process.rs b/os/src/syscall/process.rs
index 85314ea2..bdf5c8bd 100644
--- a/os/src/syscall/process.rs
+++ b/os/src/syscall/process.rs
@@ -1,3 +1,4 @@
+//! Process management syscalls
 use crate::task::{exit_current_and_run_next, suspend_current_and_run_next};
 use crate::timer::get_time_ms;
 
diff --git a/os/src/task/context.rs b/os/src/task/context.rs
index 375042d8..5784a6fc 100644
--- a/os/src/task/context.rs
+++ b/os/src/task/context.rs
@@ -1,3 +1,5 @@
+//! Implementation of [`TaskContext`]
+
 #[derive(Copy, Clone)]
 #[repr(C)]
 pub struct TaskContext {
diff --git a/os/src/task/mod.rs b/os/src/task/mod.rs
index 453d6a7d..3ea4f989 100644
--- a/os/src/task/mod.rs
+++ b/os/src/task/mod.rs
@@ -1,3 +1,14 @@
+//! Task management implementation
+//!
+//! Everything about task management, like starting and switching tasks is
+//! implemented here.
+//!
+//! 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
+//! might not be what you expect.
+
 mod context;
 mod switch;
 #[allow(clippy::module_inception)]
@@ -12,6 +23,15 @@ use task::{TaskControlBlock, TaskStatus};
 
 pub use context::TaskContext;
 
+/// The task manager, where all the tasks are managed.
+///
+/// Functions implemented on `TaskManager` deals with all task state transitions
+/// and task context switching. For convenience, you can find wrappers around it
+/// in the module level.
+///
+/// Most of `TaskManager` are hidden behind the field `inner`, to defer
+/// borrowing checks to runtime. You can see examples on how to use `inner` in
+/// existing functions on `TaskManager`.
 pub struct TaskManager {
     /// total number of tasks
     num_app: usize,
@@ -82,7 +102,7 @@ impl TaskManager {
         inner.tasks[current].task_status = TaskStatus::Exited;
     }
 
-    /// Find next task to run and return app id.
+    /// Find next task to run and return task id.
     ///
     /// In this case, we only return the first `Ready` task in task list.
     fn find_next_task(&self) -> Option<usize> {
diff --git a/os/src/task/switch.rs b/os/src/task/switch.rs
index 59f8b1a0..3535a5ca 100644
--- a/os/src/task/switch.rs
+++ b/os/src/task/switch.rs
@@ -1,8 +1,17 @@
+//! Rust wrapper around `__switch`.
+//!
+//! Switching to a different task's context happens here. The actual
+//! implementation must not be in Rust and (essentially) has to be in assembly
+//! language (Do you know why?), so this module really is just a wrapper around
+//! `switch.S`.
+
 use super::TaskContext;
 use core::arch::global_asm;
 
 global_asm!(include_str!("switch.S"));
 
 extern "C" {
+    /// Switch to the context of `next_task_cx_ptr`, saving the current context
+    /// in `current_task_cx_ptr`.
     pub fn __switch(current_task_cx_ptr: *mut TaskContext, next_task_cx_ptr: *const TaskContext);
 }
diff --git a/os/src/task/task.rs b/os/src/task/task.rs
index 43c4f9c2..d1c6dd83 100644
--- a/os/src/task/task.rs
+++ b/os/src/task/task.rs
@@ -1,9 +1,12 @@
+//! Types related to task management
+
 use super::TaskContext;
 
 #[derive(Copy, Clone)]
 pub struct TaskControlBlock {
     pub task_status: TaskStatus,
     pub task_cx: TaskContext,
+    // LAB1: Add whatever you need about the Task.
 }
 
 #[derive(Copy, Clone, PartialEq)]
diff --git a/os/src/timer.rs b/os/src/timer.rs
index 1d101d43..b5ce0be8 100644
--- a/os/src/timer.rs
+++ b/os/src/timer.rs
@@ -1,3 +1,5 @@
+//! RISC-V timer-related functionality
+
 use crate::config::CLOCK_FREQ;
 use crate::sbi::set_timer;
 use riscv::register::time;
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index dcb4c2bb..2ac6e034 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -1,3 +1,17 @@
+//! Trap handling functionality
+//!
+//! For rCore, we have a single trap entry point, namely `__alltraps`. At
+//! initialization in [`init()`], we set the `stvec` CSR to point to it.
+//!
+//! All traps go through `__alltraps`, which is defined in `trap.S`. The
+//! assembly language code does just enough work restore the kernel space
+//! context, ensuring that Rust code safely runs, and transfers control to
+//! [`trap_handler()`].
+//!
+//! It then calls different functionality based on what exactly the exception
+//! was. For example, timer interrupts trigger task preemption, and syscalls go
+//! to [`syscall()`].
+
 mod context;
 
 use crate::syscall::syscall;