diff --git a/Makefile b/Makefile
index 8c1e1f7..d11cc62 100644
--- a/Makefile
+++ b/Makefile
@@ -70,7 +70,7 @@ USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
 
 
 
-USER_TARGET 	:= $(OBJ_DIR)/app_naive_fork
+USER_TARGET 	:= $(OBJ_DIR)/app_yield
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
diff --git a/kernel/syscall.c b/kernel/syscall.c
index b5ee586..d2b1fd1 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -68,6 +68,19 @@ ssize_t sys_user_fork() {
   return do_fork( current );
 }
 
+//
+// kerenl entry point of yield
+//
+ssize_t sys_user_yield() {
+  // TODO (lab3_2): implment the syscall of yield.
+  // hint: the functionality of yield is to give up the processor. therefore,
+  // we should set the status of currently running process to READY, insert it in 
+  // the rear of ready queue, and finally, schedule a READY process to run.
+  panic( "You need to implement the yield syscall in lab3_2.\n" );
+
+  return 0;
+}
+
 //
 // [a0]: the syscall number; [a1] ... [a7]: arguments to the syscalls.
 // returns the code of success, (e.g., 0 means success, fail for otherwise)
@@ -84,6 +97,8 @@ long do_syscall(long a0, long a1, long a2, long a3, long a4, long a5, long a6, l
       return sys_user_free_page(a1);
     case SYS_user_fork:
       return sys_user_fork();
+    case SYS_user_yield:
+      return sys_user_yield();
     default:
       panic("Unknown syscall %ld \n", a0);
   }
diff --git a/kernel/syscall.h b/kernel/syscall.h
index 13332b7..e4e02fe 100644
--- a/kernel/syscall.h
+++ b/kernel/syscall.h
@@ -11,6 +11,7 @@
 #define SYS_user_allocate_page (SYS_user_base + 2)
 #define SYS_user_free_page (SYS_user_base + 3)
 #define SYS_user_fork (SYS_user_base + 4)
+#define SYS_user_yield (SYS_user_base + 5)
 
 long do_syscall(long a0, long a1, long a2, long a3, long a4, long a5, long a6, long a7);
 
diff --git a/user/app_naive_fork.c b/user/app_naive_fork.c
deleted file mode 100644
index 06ed1b9..0000000
--- a/user/app_naive_fork.c
+++ /dev/null
@@ -1,18 +0,0 @@
-/*
- * The application of lab3_1.
- * it simply forks a child process.
- */
-
-#include "user/user_lib.h"
-#include "util/types.h"
-
-int main(void) {
-  uint64 pid = fork();
-  if (pid == 0) {
-    printu("Child: Hello world!\n");
-  } else {
-    printu("Parent: Hello world! child id %ld\n", pid);
-  }
-
-  exit(0);
-}
diff --git a/user/app_yield.c b/user/app_yield.c
new file mode 100644
index 0000000..bf2349c
--- /dev/null
+++ b/user/app_yield.c
@@ -0,0 +1,32 @@
+/*
+ * The application of lab3_2.
+ * parent and child processes intermittently give up their processors.
+ */
+
+#include "user/user_lib.h"
+#include "util/types.h"
+
+int main(void) {
+  uint64 pid = fork();
+  uint64 rounds = 0xffff;
+  if (pid == 0) {
+    printu("Child: Hello world! \n");
+    for (uint64 i = 0; i < rounds; ++i) {
+      if (i % 10000 == 0) {
+        printu("Child running %ld \n", i);
+        yield();
+      }
+    }
+  } else {
+    printu("Parent: Hello world! \n");
+    for (uint64 i = 0; i < rounds; ++i) {
+      if (i % 10000 == 0) {
+        printu("Parent running %ld \n", i);
+        yield();
+      }
+    }
+  }
+
+  exit(0);
+  return 0;
+}
diff --git a/user/user_lib.c b/user/user_lib.c
index f3756d8..3fcb85d 100644
--- a/user/user_lib.c
+++ b/user/user_lib.c
@@ -69,3 +69,10 @@ void naive_free(void* va) {
 int fork() {
   return do_user_call(SYS_user_fork, 0, 0, 0, 0, 0, 0, 0);
 }
+
+//
+// lib call to yield
+//
+void yield() {
+  do_user_call(SYS_user_yield, 0, 0, 0, 0, 0, 0, 0);
+}
diff --git a/user/user_lib.h b/user/user_lib.h
index e1c4207..63e2e25 100644
--- a/user/user_lib.h
+++ b/user/user_lib.h
@@ -7,3 +7,4 @@ int exit(int code);
 void* naive_malloc();
 void naive_free(void* va);
 int fork();
+void yield();