diff --git a/kernel/uart.c b/kernel/uart.c
index 02b3a77..83846ad 100644
--- a/kernel/uart.c
+++ b/kernel/uart.c
@@ -136,6 +136,7 @@ uartstart()
   while(1){
     if(uart_tx_w == uart_tx_r){
       // transmit buffer is empty.
+      ReadReg(ISR);
       return;
     }