diff --git a/ch3-coop/user/src/bin/00write_a.rs b/ch3-coop/user/src/bin/00write_a.rs
index 8bebc9d..3303899 100644
--- a/ch3-coop/user/src/bin/00write_a.rs
+++ b/ch3-coop/user/src/bin/00write_a.rs
@@ -1,10 +1,7 @@
 #![no_std]
 #![no_main]
 
-#[macro_use]
-extern crate user_lib;
-
-use user_lib::yield_;
+use user_lib::{syscall::*, *};
 
 const WIDTH: usize = 10;
 const HEIGHT: usize = 5;
@@ -16,7 +13,7 @@ fn main() -> i32 {
             print!("A");
         }
         println!(" [{}/{}]", i + 1, HEIGHT);
-        yield_();
+        sys_yield();
     }
     println!("Test write_a OK!");
     0
diff --git a/ch3-coop/user/src/bin/01write_b.rs b/ch3-coop/user/src/bin/01write_b.rs
index 652d5dd..64bd665 100644
--- a/ch3-coop/user/src/bin/01write_b.rs
+++ b/ch3-coop/user/src/bin/01write_b.rs
@@ -1,10 +1,7 @@
 #![no_std]
 #![no_main]
 
-#[macro_use]
-extern crate user_lib;
-
-use user_lib::yield_;
+use user_lib::{syscall::*, *};
 
 const WIDTH: usize = 10;
 const HEIGHT: usize = 2;
@@ -16,7 +13,7 @@ fn main() -> i32 {
             print!("B");
         }
         println!(" [{}/{}]", i + 1, HEIGHT);
-        yield_();
+        sys_yield();
     }
     println!("Test write_b OK!");
     0
diff --git a/ch3-coop/user/src/bin/02write_c.rs b/ch3-coop/user/src/bin/02write_c.rs
index 9bed4da..25464bf 100644
--- a/ch3-coop/user/src/bin/02write_c.rs
+++ b/ch3-coop/user/src/bin/02write_c.rs
@@ -1,10 +1,7 @@
 #![no_std]
 #![no_main]
 
-#[macro_use]
-extern crate user_lib;
-
-use user_lib::yield_;
+use user_lib::{syscall::*, *};
 
 const WIDTH: usize = 10;
 const HEIGHT: usize = 3;
@@ -16,7 +13,7 @@ fn main() -> i32 {
             print!("C");
         }
         println!(" [{}/{}]", i + 1, HEIGHT);
-        yield_();
+        sys_yield();
     }
     println!("Test write_c OK!");
     0
diff --git a/ch3-coop/user/src/syscall.rs b/ch3-coop/user/src/syscall.rs
index 1604d1b..0c9f3e1 100644
--- a/ch3-coop/user/src/syscall.rs
+++ b/ch3-coop/user/src/syscall.rs
@@ -2,6 +2,7 @@ use core::arch::asm;
 
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
+const SYSCALL_YIELD: usize = 124;
 
 
 fn syscall(id: usize, args: [usize; 3]) -> isize {
@@ -28,3 +29,7 @@ pub fn sys_exit(exit_code: i32) -> isize {
     syscall(SYSCALL_EXIT, [exit_code as usize, 0, 0])
 }
 
+pub fn sys_yield() {
+    syscall(SYSCALL_YIELD, [0, 0, 0]);
+}
+