diff --git a/crate/process/src/process_manager.rs b/crate/process/src/process_manager.rs
index dd8dab5..37a9652 100644
--- a/crate/process/src/process_manager.rs
+++ b/crate/process/src/process_manager.rs
@@ -71,6 +71,11 @@ impl ProcessManager {
         self.scheduler.lock().tick(pid)
     }
 
+    /// Set the priority of process `pid`
+    pub fn set_priority(&self, pid: Pid, priority: u8) {
+        self.scheduler.lock().set_priority(pid, priority);
+    }
+
     /// Called by Processor to get a process to run.
     /// The manager first mark it `Running`,
     /// then take out and return its Context.
diff --git a/crate/process/src/processor.rs b/crate/process/src/processor.rs
index 4fc3e18..c1432ca 100644
--- a/crate/process/src/processor.rs
+++ b/crate/process/src/processor.rs
@@ -78,6 +78,10 @@ impl Processor {
         self.inner().proc.as_ref().unwrap().0
     }
 
+    pub fn context(&self) -> &Context {
+        &*self.inner().proc.as_ref().unwrap().1
+    }
+
     pub fn manager(&self) -> &ProcessManager {
         &*self.inner().manager
     }
diff --git a/kernel/src/process/context.rs b/kernel/src/process/context.rs
index e078219..dcc53f5 100644
--- a/kernel/src/process/context.rs
+++ b/kernel/src/process/context.rs
@@ -12,13 +12,8 @@ pub struct ContextImpl {
 
 impl Context for ContextImpl {
     unsafe fn switch_to(&mut self, target: &mut Context) {
-        use core::raw::TraitObject;
         use core::mem::transmute;
-
-        // Cast &mut Context -> &mut ContextImpl
-        let raw: TraitObject = transmute(target);
-        let target = &mut *(raw.data as *mut ContextImpl);
-
+        let (target, _): (&mut ContextImpl, *const ()) = transmute(target);
         self.arch.switch(&mut target.arch);
     }
 }
diff --git a/kernel/src/process/mod.rs b/kernel/src/process/mod.rs
index 0e13b33..3f143da 100644
--- a/kernel/src/process/mod.rs
+++ b/kernel/src/process/mod.rs
@@ -19,7 +19,7 @@ pub fn init() {
     extern fn idle(_arg: usize) -> ! {
         loop { cpu::halt(); }
     }
-    for i in 0..MAX_CPU_NUM {
+    for i in 0..4 {
         manager.add(ContextImpl::new_kernel(idle, i));
     }
 
diff --git a/kernel/src/syscall.rs b/kernel/src/syscall.rs
index 5c48ecf..6dbd056 100644
--- a/kernel/src/syscall.rs
+++ b/kernel/src/syscall.rs
@@ -58,7 +58,11 @@ fn sys_close(fd: usize) -> i32 {
 
 /// Fork the current process. Return the child's PID.
 fn sys_fork(tf: &TrapFrame) -> i32 {
-    unimplemented!();
+    use core::mem::transmute;
+    let (context, _): (&ContextImpl, *const ()) = unsafe { transmute(processor().context()) };
+    let pid = processor().manager().add(context.fork(tf));
+    info!("fork: {} -> {}", thread::current().id(), pid);
+    pid as i32
 }
 
 /// Wait the process exit.
@@ -85,7 +89,7 @@ fn sys_getpid() -> i32 {
 
 /// Exit the current process
 fn sys_exit(exit_code: usize) -> i32 {
-    let pid = processor().pid();
+    let pid = thread::current().id();
     processor().manager().set_status(pid, Status::Exited(exit_code));
     processor().yield_now();
     0
@@ -102,7 +106,8 @@ fn sys_get_time() -> i32 {
 }
 
 fn sys_lab6_set_priority(priority: usize) -> i32 {
-    unimplemented!();
+    let pid = thread::current().id();
+    processor().manager().set_priority(pid, priority as u8);
     0
 }