diff --git a/os/build.rs b/os/build.rs
index 3794b21a..ba1ff3de 100644
--- a/os/build.rs
+++ b/os/build.rs
@@ -2,11 +2,11 @@ use std::io::{Result, Write};
 use std::fs::{File, read_dir};
 
 fn main() {
-    println!("cargo:rerun-if-changed=../user");
+    println!("cargo:rerun-if-changed=../user/src/bin/");
     insert_app_data().unwrap();
 }
 
-static TARGET_PATH: &'static str = "../user/target/riscv64gc-unknown-none-elf/release/";
+static TARGET_PATH: &str = "../user/target/riscv64gc-unknown-none-elf/release/";
 
 fn insert_app_data() -> Result<()> {
     let mut f = File::create("src/link_app.S").unwrap();
diff --git a/os/src/batch.rs b/os/src/batch.rs
index 567b4b30..9e6bc278 100644
--- a/os/src/batch.rs
+++ b/os/src/batch.rs
@@ -43,7 +43,7 @@ struct AppManager {
 }
 struct AppManagerInner {
     num_app: usize,
-    next_app: usize,
+    current_app: usize,
     app_start: [usize; MAX_APP_NUM + 1],
 }
 unsafe impl Sync for AppManager {}
@@ -57,6 +57,10 @@ impl AppManagerInner {
     }
 
     unsafe fn load_app(&self, app_id: usize) {
+        if app_id >= self.num_app {
+            panic!("All applications completed!");
+        }
+        println!("[kernel] Loading app_{}", app_id);
         // clear app area
         (APP_BASE_ADDRESS..APP_BASE_ADDRESS + APP_SIZE_LIMIT).for_each(|addr| {
             (addr as *mut u8).write_volatile(0);
@@ -72,15 +76,10 @@ impl AppManagerInner {
         app_dst.copy_from_slice(app_src);
     }
 
-    pub fn run_next_app(&mut self) {
-        unsafe { self.load_app(self.next_app); }
-        self.next_app += 1;
-        extern "C" { fn __restore(cx: &mut TrapContext); }
-        unsafe {
-            __restore(KERNEL_STACK.push_context(
-                TrapContext::app_init_context(APP_BASE_ADDRESS, USER_STACK.get_sp())
-            ));
-        }
+    pub fn get_current_app(&self) -> usize { self.current_app }
+
+    pub fn move_to_next_app(&mut self) {
+        self.current_app += 1;
     }
 }
 
@@ -94,10 +93,10 @@ lazy_static! {
             let app_start_raw: &[usize] = unsafe {
                 core::slice::from_raw_parts(num_app_ptr.add(1), num_app + 1)
             };
-            &app_start[..=num_app].copy_from_slice(app_start_raw);
+            app_start[..=num_app].copy_from_slice(app_start_raw);
             AppManagerInner {
                 num_app,
-                next_app: 0,
+                current_app: 0,
                 app_start,
             }
         }),
@@ -112,6 +111,17 @@ pub fn print_app_info() {
     APP_MANAGER.inner.borrow().print_app_info();
 }
 
-pub fn run_next_app() {
-    APP_MANAGER.inner.borrow_mut().run_next_app();
+pub fn run_next_app() -> ! {
+    let current_app = APP_MANAGER.inner.borrow().get_current_app();
+    unsafe {
+        APP_MANAGER.inner.borrow().load_app(current_app);
+    }
+    APP_MANAGER.inner.borrow_mut().move_to_next_app();
+    extern "C" { fn __restore(cx_addr: usize); }
+    unsafe {
+        __restore(KERNEL_STACK.push_context(
+            TrapContext::app_init_context(APP_BASE_ADDRESS, USER_STACK.get_sp())
+        ) as *const _ as usize);
+    }
+    panic!("Unreachable in batch::run_current_app!");
 }
diff --git a/os/src/lang_items.rs b/os/src/lang_items.rs
index 655f6c4f..3f5462ab 100644
--- a/os/src/lang_items.rs
+++ b/os/src/lang_items.rs
@@ -4,9 +4,9 @@ use crate::sbi::shutdown;
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     if let Some(location) = info.location() {
-        println!("Panicked at {}:{} {}", location.file(), location.line(), info.message().unwrap());
+        println!("[kernel] Panicked at {}:{} {}", location.file(), location.line(), info.message().unwrap());
     } else {
-        println!("Panicked: {}", info.message().unwrap());
+        println!("[kernel] Panicked: {}", info.message().unwrap());
     }
     shutdown()
 }
diff --git a/os/src/main.rs b/os/src/main.rs
index 67b32312..8f8fdeb7 100644
--- a/os/src/main.rs
+++ b/os/src/main.rs
@@ -28,8 +28,8 @@ fn clear_bss() {
 #[no_mangle]
 pub fn rust_main() -> ! {
     clear_bss();
-    println!("Hello, world!");
+    println!("[kernel] Hello, world!");
+    trap::init();
     batch::init();
     batch::run_next_app();
-    panic!("Shutdown machine!");
 }
\ No newline at end of file
diff --git a/os/src/syscall/fs.rs b/os/src/syscall/fs.rs
new file mode 100644
index 00000000..b8c46591
--- /dev/null
+++ b/os/src/syscall/fs.rs
@@ -0,0 +1,15 @@
+const FD_STDOUT: usize = 1;
+
+pub fn sys_write(fd: usize, buf: *const u8, len: usize) -> isize {
+    match fd {
+        FD_STDOUT => {
+            let slice = unsafe { core::slice::from_raw_parts(buf, len) };
+            let str = core::str::from_utf8(slice).unwrap();
+            print!("{}", str);
+            len as isize
+        },
+        _ => {
+            panic!("Unsupported fd in sys_write!");
+        }
+    }
+}
\ No newline at end of file
diff --git a/os/src/syscall/mod.rs b/os/src/syscall/mod.rs
index 5a1a1246..51dea1e7 100644
--- a/os/src/syscall/mod.rs
+++ b/os/src/syscall/mod.rs
@@ -1,2 +1,17 @@
 const SYSCALL_WRITE: usize = 64;
+const SYSCALL_EXIT: usize = 93;
+
+mod fs;
+mod process;
+
+use fs::*;
+use process::*;
+
+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]),
+        SYSCALL_EXIT => sys_exit(args[0] as i32),
+        _ => panic!("Unsupported syscall_id: {}", syscall_id),
+    }
+}
 
diff --git a/os/src/syscall/process.rs b/os/src/syscall/process.rs
new file mode 100644
index 00000000..ef56607a
--- /dev/null
+++ b/os/src/syscall/process.rs
@@ -0,0 +1,6 @@
+use crate::batch::run_next_app;
+
+pub fn sys_exit(xstate: i32) -> ! {
+    println!("[kernel] Application exited with code {}", xstate);
+    run_next_app()
+}
\ No newline at end of file
diff --git a/os/src/trap/context.rs b/os/src/trap/context.rs
index b4245227..e2575de9 100644
--- a/os/src/trap/context.rs
+++ b/os/src/trap/context.rs
@@ -2,9 +2,9 @@ use riscv::register::sstatus::{Sstatus, self, SPP};
 
 #[repr(C)]
 pub struct TrapContext {
-    x: [usize; 32],
-    sstatus: Sstatus,
-    sepc: usize,
+    pub x: [usize; 32],
+    pub sstatus: Sstatus,
+    pub sepc: usize,
 }
 
 impl TrapContext {
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index 2d7db829..25cd9405 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -10,6 +10,7 @@ use riscv::register::{
     },
     stval,
 };
+use crate::syscall::syscall;
 
 global_asm!(include_str!("trap.S"));
 
@@ -23,10 +24,11 @@ pub fn init() {
 #[no_mangle]
 pub fn trap_handler(cx: &mut TrapContext) -> &mut TrapContext {
     let scause = scause::read();
-    let stval = stval::read();
+    let _stval = stval::read();
     match scause.cause() {
         Trap::Exception(Exception::UserEnvCall) => {
-            panic!("Triggered UserEnvCall!");
+            cx.sepc += 4;
+            cx.x[10] = syscall(cx.x[17], [cx.x[10], cx.x[11], cx.x[12]]) as usize;
         }
         _ => {
             panic!("Unsupported trap!");
diff --git a/user/src/bin/power.rs b/user/src/bin/power.rs
index cc9cb32d..86154c71 100644
--- a/user/src/bin/power.rs
+++ b/user/src/bin/power.rs
@@ -6,7 +6,7 @@ extern crate user_lib;
 
 const SIZE: usize = 10;
 const P: u32 = 3;
-const STEP: usize = 10000000;
+const STEP: usize = 100000;
 const MOD: u32 = 10007;
 
 #[no_mangle]