diff --git a/kernel/src/arch/mipsel/interrupt.rs b/kernel/src/arch/mipsel/interrupt.rs
index f6bea8a..b6957bf 100644
--- a/kernel/src/arch/mipsel/interrupt.rs
+++ b/kernel/src/arch/mipsel/interrupt.rs
@@ -74,9 +74,6 @@ pub extern fn rust_trap(tf: &mut TrapFrame) {
 
 fn external() {
     // TODO
-    #[cfg(feature = "board_u540")]
-    unsafe { super::board::handle_external_interrupt(); }
-
     // true means handled, false otherwise
     let handlers = [try_process_serial, try_process_drivers];
     for handler in handlers.iter() {
@@ -87,7 +84,6 @@ fn external() {
 }
 
 fn try_process_serial() -> bool {
-    // TODO
     match super::io::getchar_option() {
         Some(ch) => {
             crate::trap::serial(ch);
diff --git a/kernel/src/arch/mipsel/io.rs b/kernel/src/arch/mipsel/io.rs
index e14c4ec..50c224d 100644
--- a/kernel/src/arch/mipsel/io.rs
+++ b/kernel/src/arch/mipsel/io.rs
@@ -59,8 +59,7 @@ pub fn putfmt(fmt: Arguments) {
     SerialPort.write_fmt(fmt).unwrap();
 }
 
-pub fn init(serial_base_addr: usize) {
-    COM1 = serial_base_addr;
+pub fn init() {
     // Turn off the FIFO
     write(COM1 + COM_FCR, 0 as u8);
     // Set speed; requires DLAB latch
@@ -77,7 +76,8 @@ pub fn init(serial_base_addr: usize) {
     write(COM1 + COM_IER, COM_IER_RDI);
 }
 
-static mut COM1: usize = 0;
+#[cfg(feature = "board_malta")]
+const COM1          :usize = 0xbf000900; // 16550 Base Address
 
 const COM_RX        :usize = 0;   // In:  Receive buffer (DLAB=0)
 const COM_TX        :usize = 0;   // Out: Transmit buffer (DLAB=0)