Update docs

os/src/sbi.rs

@@ -27,14 +27,17 @@ fn sbi_call(which: usize, arg0: usize, arg1: usize, arg2: usize) -> usize {
     ret
 }
 
+/// 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!");

os/src/syscall/mod.rs

@@ -7,6 +7,7 @@ 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]),

os/src/syscall/process.rs

@@ -1,5 +1,6 @@
 use crate::batch::run_next_app;
 
+/// task exits and submit an exit code
 pub fn sys_exit(exit_code: i32) -> ! {
     println!("[kernel] Application exited with code {}", exit_code);
     run_next_app()

os/src/trap/mod.rs

@@ -11,6 +11,7 @@ use riscv::register::{
 
 global_asm!(include_str!("trap.S"));
 
+/// initialize CSR `stvec` as the entry of `__alltraps`
 pub fn init() {
     extern "C" {
         fn __alltraps();
@@ -21,9 +22,10 @@ pub fn init() {
 }
 
 #[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;