diff --git a/trap.c b/trap.c
index e17f8fe..5f5d4a0 100644
--- a/trap.c
+++ b/trap.c
@@ -36,6 +36,8 @@ trapinit(void)
 void
 usertrap(void)
 {
+  int which_dev = 0;
+  
   if((r_sstatus() & SSTATUS_SPP) != 0)
     panic("usertrap: not from user mode");
 
@@ -62,7 +64,7 @@ usertrap(void)
     p->tf->epc += 4;
 
     syscall();
-  } else if(devintr()){
+  } else if((which_dev = devintr()) != 0){
     // ok
   } else {
     printf("usertrap(): unexpected scause 0x%p pid=%d\n", r_scause(), p->pid);
@@ -73,6 +75,10 @@ usertrap(void)
   if(p->killed)
     exit();
 
+  // give up the CPU if this is a timer interrupt.
+  if(which_dev == 2)
+    yield();
+
   usertrapret();
 }
 
@@ -146,7 +152,9 @@ kerneltrap()
 
 // check if it's an external interrupt or software interrupt,
 // and handle it.
-// returns 1 if handled, 0 if not recognized.
+// returns 2 if timer interrupt,
+// 1 if other device,
+// 0 if not recognized.
 int
 devintr()
 {
@@ -176,7 +184,7 @@ devintr()
     // acknowledge.
     w_sip(r_sip() & ~2);
 
-    return 1;
+    return 2;
   } else {
     return 0;
   }