diff --git a/os/src/mm/frame_allocator.rs b/os/src/mm/frame_allocator.rs
index c17385f1..3cbc5623 100644
--- a/os/src/mm/frame_allocator.rs
+++ b/os/src/mm/frame_allocator.rs
@@ -48,6 +48,7 @@ impl StackFrameAllocator {
     pub fn init(&mut self, l: PhysPageNum, r: PhysPageNum) {
         self.current = l.0;
         self.end = r.0;
+        println!("last {} Physical Frames.", self.end - self.current);
     }
 }
 impl FrameAllocator for StackFrameAllocator {
diff --git a/os/src/mm/memory_set.rs b/os/src/mm/memory_set.rs
index 55c04614..766c7594 100644
--- a/os/src/mm/memory_set.rs
+++ b/os/src/mm/memory_set.rs
@@ -47,10 +47,6 @@ impl MemorySet {
             areas: Vec::new(),
         }
     }
-    #[allow(unused)]
-    pub fn dealloc_all_frames(&mut self) {
-        *self = Self::new_bare();
-    }
     pub fn token(&self) -> usize {
         self.page_table.token()
     }
@@ -217,7 +213,8 @@ impl MemorySet {
         self.page_table.translate(vpn)
     }
     pub fn clear(&mut self) {
-        *self = Self::new_bare();
+        //*self = Self::new_bare();
+        self.areas.clear();
     }
 }
 
diff --git a/os/src/syscall/process.rs b/os/src/syscall/process.rs
index ca5d6c8f..7ace0ffe 100644
--- a/os/src/syscall/process.rs
+++ b/os/src/syscall/process.rs
@@ -5,7 +5,7 @@ use crate::task::{
     current_user_token,
     add_task,
 };
-use crate::timer::get_time;
+use crate::timer::get_time_ms;
 use crate::mm::{
     translated_str,
     translated_refmut,
@@ -14,18 +14,7 @@ use crate::loader::get_app_data_by_name;
 use alloc::sync::Arc;
 
 pub fn sys_exit(exit_code: i32) -> ! {
-    // save exit code
-    let task = current_task().unwrap();
-
-    // ---- hold current PCB lock
-    task.acquire_inner_lock().exit_code = exit_code;
-    // ---- release current PCB lock
-
-    // this function will not return
-    // drop task manually to maintain rc correctly
-    drop(task);
-
-    exit_current_and_run_next();
+    exit_current_and_run_next(exit_code);
     panic!("Unreachable in sys_exit!");
 }
 
@@ -35,7 +24,7 @@ pub fn sys_yield() -> isize {
 }
 
 pub fn sys_get_time() -> isize {
-    get_time() as isize
+    get_time_ms() as isize
 }
 
 pub fn sys_getpid() -> isize {
@@ -68,11 +57,21 @@ pub fn sys_exec(path: *const u8) -> isize {
     }
 }
 
+/// If there is not a child process whose pid is same as given, return -1.
+/// Else if there is a child process but it is still running, return -2.
 pub fn sys_waitpid(pid: isize, exit_code_ptr: *mut i32) -> isize {
     let task = current_task().unwrap();
     // find a child process
+
     // ---- hold current PCB lock
     let mut inner = task.acquire_inner_lock();
+    if inner.children
+        .iter()
+        .find(|p| {pid == -1 || pid as usize == p.getpid()})
+        .is_none() {
+        return -1;
+        // ---- release current PCB lock
+    }
     let pair = inner.children
         .iter()
         .enumerate()
@@ -92,7 +91,7 @@ pub fn sys_waitpid(pid: isize, exit_code_ptr: *mut i32) -> isize {
         *translated_refmut(inner.memory_set.token(), exit_code_ptr) = exit_code;
         found_pid as isize
     } else {
-        -1
+        -2
     }
     // ---- release current PCB lock automatically
 }
\ No newline at end of file
diff --git a/os/src/task/mod.rs b/os/src/task/mod.rs
index fd7fea8d..cebcb3b2 100644
--- a/os/src/task/mod.rs
+++ b/os/src/task/mod.rs
@@ -42,14 +42,17 @@ pub fn suspend_current_and_run_next() {
     schedule(task_cx_ptr);
 }
 
-pub fn exit_current_and_run_next() {
+pub fn exit_current_and_run_next(exit_code: i32) {
     // take from Processor
     let task = take_current_task().unwrap();
     // **** hold current PCB lock
     let mut inner = task.acquire_inner_lock();
     // Change status to Zombie
     inner.task_status = TaskStatus::Zombie;
+    // Record exit code
+    inner.exit_code = exit_code;
     // move any child to its parent
+    // TODO: do not move to its parent but under initproc
 
     // ++++++ hold parent PCB lock here
     {
diff --git a/os/src/timer.rs b/os/src/timer.rs
index 7522e708..612d51af 100644
--- a/os/src/timer.rs
+++ b/os/src/timer.rs
@@ -3,11 +3,16 @@ use crate::sbi::set_timer;
 use crate::config::CPU_FREQ;
 
 const TICKS_PER_SEC: usize = 100;
+const MSEC_PER_SEC: usize = 1000;
 
 pub fn get_time() -> usize {
     time::read()
 }
 
+pub fn get_time_ms() -> usize {
+    time::read() / (CPU_FREQ / MSEC_PER_SEC)
+}
+
 pub fn set_next_trigger() {
     set_timer(get_time() + CPU_FREQ / TICKS_PER_SEC);
 }
\ No newline at end of file
diff --git a/os/src/trap/mod.rs b/os/src/trap/mod.rs
index 66f463da..f7b0bc29 100644
--- a/os/src/trap/mod.rs
+++ b/os/src/trap/mod.rs
@@ -67,13 +67,24 @@ pub fn trap_handler() -> ! {
             cx.x[10] = result as usize;
         }
         Trap::Exception(Exception::StoreFault) |
-        Trap::Exception(Exception::StorePageFault) => {
-            println!("[kernel] PageFault in application, bad addr = {:#x}, bad instruction = {:#x}, core dumped.", stval, current_trap_cx().sepc);
-            exit_current_and_run_next();
+        Trap::Exception(Exception::StorePageFault) |
+        Trap::Exception(Exception::InstructionFault) |
+        Trap::Exception(Exception::InstructionPageFault) |
+        Trap::Exception(Exception::LoadFault) |
+        Trap::Exception(Exception::LoadPageFault) => {
+            println!(
+                "[kernel] {:?} in application, bad addr = {:#x}, bad instruction = {:#x}, core dumped.",
+                scause.cause(),
+                stval,
+                current_trap_cx().sepc,
+            );
+            // page fault exit code
+            exit_current_and_run_next(-2);
         }
         Trap::Exception(Exception::IllegalInstruction) => {
             println!("[kernel] IllegalInstruction in application, core dumped.");
-            exit_current_and_run_next();
+            // illegal instruction exit code
+            exit_current_and_run_next(-3);
         }
         Trap::Interrupt(Interrupt::SupervisorTimer) => {
             set_next_trigger();
diff --git a/user/src/bin/exit.rs b/user/src/bin/exit.rs
new file mode 100644
index 00000000..5bde550b
--- /dev/null
+++ b/user/src/bin/exit.rs
@@ -0,0 +1,29 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+use user_lib::{fork, yield_, waitpid, exit, wait};
+
+const MAGIC: i32 = -0x10384;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("I am the parent. Forking the child...");
+    let pid = fork();
+    if pid == 0 {
+        println!("I am the child.");
+        for _ in 0..7 { yield_(); }
+        exit(MAGIC);
+    } else {
+        println!("I am parent, fork a child pid {}", pid);
+    }
+    println!("I am the parent, waiting now..");
+    let mut xstate: i32 = 0;
+    assert!(waitpid(pid as usize, &mut xstate) == pid && xstate == MAGIC);
+    assert!(waitpid(pid as usize, &mut xstate) < 0 && wait(&mut xstate) <= 0);
+    println!("waitpid {} ok.", pid);
+    println!("exit pass.");
+    0
+}
+
diff --git a/user/src/bin/fantastic_text.rs b/user/src/bin/fantastic_text.rs
new file mode 100644
index 00000000..bb51db30
--- /dev/null
+++ b/user/src/bin/fantastic_text.rs
@@ -0,0 +1,44 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+macro_rules! color_text {
+    ($text:expr, $color:expr) => {{
+        format_args!("\x1b[{}m{}\x1b[0m", $color, $text)
+    }};
+}
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!(
+        "{}{}{}{}{} {}{}{}{} {}{}{}{}{}{}",
+        color_text!("H", 31),
+        color_text!("e", 32),
+        color_text!("l", 33),
+        color_text!("l", 34),
+        color_text!("o", 35),
+        color_text!("R", 36),
+        color_text!("u", 37),
+        color_text!("s", 90),
+        color_text!("t", 91),
+        color_text!("u", 92),
+        color_text!("C", 93),
+        color_text!("o", 94),
+        color_text!("r", 95),
+        color_text!("e", 96),
+        color_text!("!", 97),
+    );
+
+    let text =
+        "reguler \x1b[4munderline\x1b[24m \x1b[7mreverse\x1b[27m \x1b[9mstrikethrough\x1b[29m";
+    println!("\x1b[47m{}\x1b[0m", color_text!(text, 30));
+    for i in 31..38 {
+        println!("{}", color_text!(text, i));
+    }
+    for i in 90..98 {
+        println!("{}", color_text!(text, i));
+    }
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/forktest.rs b/user/src/bin/forktest.rs
new file mode 100644
index 00000000..dcf8859e
--- /dev/null
+++ b/user/src/bin/forktest.rs
@@ -0,0 +1,34 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{fork, wait, exit};
+
+const MAX_CHILD: usize = 20;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    for i in 0..MAX_CHILD {
+        let pid = fork();
+        if pid == 0 {
+            println!("I am child {}", i);
+            exit(0);
+        } else {
+            println!("forked child pid = {}", pid);
+        }
+        assert!(pid > 0);
+    }
+    let mut exit_code: i32 = 0;
+    for _ in 0..MAX_CHILD {
+        if wait(&mut exit_code) <= 0 {
+            panic!("wait stopped early");
+        }
+    }
+    if wait(&mut exit_code) > 0 {
+        panic!("wait got too many");
+    }
+    println!("forktest pass.");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/forktest2.rs b/user/src/bin/forktest2.rs
new file mode 100644
index 00000000..95d5ca75
--- /dev/null
+++ b/user/src/bin/forktest2.rs
@@ -0,0 +1,35 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{fork, wait, get_time, getpid, exit, yield_,};
+
+static NUM: usize = 13;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    for _ in 0..NUM {
+        let pid = fork();
+        if pid == 0 {
+            let current_time = get_time();
+            let sleep_length = (current_time as i32 as isize) * (current_time as i32 as isize) % 1000 + 1000;
+            println!("Subprocess {} sleep for {} ms", getpid(), sleep_length);
+            while get_time() < current_time + sleep_length {
+                yield_();
+            }
+            println!("Subprocess {} OK!", getpid());
+            exit(0);
+        }
+    }
+
+    let mut xstate: i32 = 0;
+    for _ in 0..NUM {
+        assert!(wait(&mut xstate) > 0);
+        assert_eq!(xstate, 0);
+    }
+    assert!(wait(&mut xstate) < 0);
+    println!("r_forktest2 test passed!");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/forktest_simple.rs b/user/src/bin/forktest_simple.rs
new file mode 100644
index 00000000..1851aea0
--- /dev/null
+++ b/user/src/bin/forktest_simple.rs
@@ -0,0 +1,28 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{fork, getpid, wait};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    assert_eq!(wait(&mut 0i32), -1);
+    println!("sys_wait without child process test passed!");
+    println!("parent start, pid = {}!", getpid());
+    let pid = fork();
+    if pid == 0 {
+        // child process
+        println!("hello child process!");
+        100
+    } else {
+        // parent process
+        let mut xstate: i32 = 0;
+        println!("ready waiting on parent process!");
+        assert_eq!(pid, wait(&mut xstate));
+        assert_eq!(xstate, 100);
+        println!("child process pid = {}, exit code = {}", pid, xstate);
+        0
+    }
+}
\ No newline at end of file
diff --git a/user/src/bin/hello_world.rs b/user/src/bin/hello_world.rs
new file mode 100644
index 00000000..de4a6a92
--- /dev/null
+++ b/user/src/bin/hello_world.rs
@@ -0,0 +1,11 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("Hello world from user mode program!");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/matrix.rs b/user/src/bin/matrix.rs
index 729625e0..d5f946b8 100644
--- a/user/src/bin/matrix.rs
+++ b/user/src/bin/matrix.rs
@@ -4,9 +4,9 @@
 #[macro_use]
 extern crate user_lib;
 
-use user_lib::{fork, wait, yield_, exit, getpid, get_time, wait_once};
+use user_lib::{fork, wait, yield_, exit, getpid, get_time};
 
-static NUM: usize = 13;
+static NUM: usize = 20;
 const N: usize = 10;
 static P: i32 = 10007;
 type Arr = [[i32; N]; N];
@@ -50,7 +50,7 @@ pub fn main() -> i32 {
         if pid == 0 {
             let current_time = get_time();
             let times = (current_time as i32 as isize) * (current_time as i32 as isize) % 1000;
-            work(times * 40);
+            work(times * 10);
         }
     }
 
@@ -62,7 +62,7 @@ pub fn main() -> i32 {
             panic!("wait failed.");
         }
     }
-    assert!(wait_once(&mut xstate) < 0);
+    assert!(wait(&mut xstate) < 0);
     println!("matrix passed.");
     0
 }
\ No newline at end of file
diff --git a/user/src/bin/sleep.rs b/user/src/bin/sleep.rs
new file mode 100644
index 00000000..439e8a65
--- /dev/null
+++ b/user/src/bin/sleep.rs
@@ -0,0 +1,30 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{sleep, exit, get_time, fork, waitpid};
+
+fn sleepy() {
+    let time: usize = 100;
+    for i in 0..5 {
+        sleep(time);
+        println!("sleep {} x {} msecs.", i + 1, time);
+    }
+    exit(0);
+}
+
+#[no_mangle]
+pub fn main() -> i32 {
+    let current_time = get_time();
+    let pid = fork();
+    let mut xstate: i32 = 0;
+    if pid == 0 {
+        sleepy();
+    }
+    assert!(waitpid(pid as usize, &mut xstate) == pid && xstate == 0);
+    println!("use {} msecs.", get_time() - current_time);
+    println!("sleep pass.");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/sleep_simple.rs b/user/src/bin/sleep_simple.rs
new file mode 100644
index 00000000..4c058f87
--- /dev/null
+++ b/user/src/bin/sleep_simple.rs
@@ -0,0 +1,19 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{get_time, sleep};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("into sleep test!");
+    let start = get_time();
+    println!("current time_msec = {}", start);
+    sleep(100);
+    let end = get_time();
+    println!("time_msec = {} after sleeping 100 ticks, delta = {}ms!", end, end - start);
+    println!("r_sleep passed!");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/stack_overflow.rs b/user/src/bin/stack_overflow.rs
new file mode 100644
index 00000000..e0ea471d
--- /dev/null
+++ b/user/src/bin/stack_overflow.rs
@@ -0,0 +1,17 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+fn f(d: usize) {
+    println!("d = {}",d);
+    f(d + 1);
+}
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("It should trigger segmentation fault!");
+    f(0);
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/user_shell.rs b/user/src/bin/user_shell.rs
index b871ea74..6aa8e31f 100644
--- a/user/src/bin/user_shell.rs
+++ b/user/src/bin/user_shell.rs
@@ -53,7 +53,7 @@ pub fn main() -> i32 {
                 }
                 print!(">> ");
             }
-            DL => {
+            BS | DL => {
                 if !line.is_empty() {
                     print!("{}", BS as char);
                     print!(" ");
diff --git a/user/src/bin/usertests.rs b/user/src/bin/usertests.rs
new file mode 100644
index 00000000..d5d0d7d2
--- /dev/null
+++ b/user/src/bin/usertests.rs
@@ -0,0 +1,40 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+static TESTS: &[&str] = &[
+    "exit\0",
+    "fantastic_text\0",
+    "forktest\0",
+    "forktest2\0",
+    "forktest_simple\0",
+    "hello_world\0",
+    "matrix\0",
+    "sleep\0",
+    "sleep_simple\0",
+    "stack_overflow\0",
+    "yield\0",
+];
+
+use user_lib::{exec, fork, waitpid};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    for test in TESTS {
+        println!("Usertests: Running {}", test);
+        let pid = fork();
+        if pid == 0 {
+            exec(*test);
+            panic!("unreachable!");
+        } else {
+            let mut xstate: i32 = Default::default();
+            let wait_pid = waitpid(pid as usize, &mut xstate);
+            assert_eq!(pid, wait_pid);
+            println!("\x1b[32mUsertests: Test {} in Process {} exited with code {}\x1b[0m", test, pid, xstate);
+        }
+    }
+    println!("Usertests passed!");
+    0
+}
\ No newline at end of file
diff --git a/user/src/bin/yield.rs b/user/src/bin/yield.rs
new file mode 100644
index 00000000..55032e40
--- /dev/null
+++ b/user/src/bin/yield.rs
@@ -0,0 +1,17 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+use user_lib::{getpid, yield_};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("Hello, I am process {}.", getpid());
+    for i in 0..5 {
+        yield_();
+        println!("Back in process {}, iteration {}.", getpid(), i);
+    }
+    println!("yield pass.");
+    0
+}
\ No newline at end of file
diff --git a/user/src/lib.rs b/user/src/lib.rs
index 8f3c77da..5c434e6d 100644
--- a/user/src/lib.rs
+++ b/user/src/lib.rs
@@ -51,19 +51,25 @@ pub fn exec(path: &str) -> isize { sys_exec(path) }
 pub fn wait(exit_code: &mut i32) -> isize {
     loop {
         match sys_waitpid(-1, exit_code as *mut _) {
-            -1 => { yield_(); }
+            -2 => { yield_(); }
+            // -1 or a real pid
             exit_pid => return exit_pid,
         }
     }
 }
-pub fn wait_once(exit_code: &mut i32) -> isize {
-    sys_waitpid(-1, exit_code as *mut _)
-}
+
 pub fn waitpid(pid: usize, exit_code: &mut i32) -> isize {
     loop {
         match sys_waitpid(pid as isize, exit_code as *mut _) {
-            -1 => { yield_(); }
+            -2 => { yield_(); }
+            // -1 or a real pid
             exit_pid => return exit_pid,
         }
     }
+}
+pub fn sleep(period_ms: usize) {
+    let start = sys_get_time();
+    while sys_get_time() < start + period_ms as isize {
+        sys_yield();
+    }
 }
\ No newline at end of file