diff --git a/kernel/uart.c b/kernel/uart.c
index d586ea4..3c1dc34 100644
--- a/kernel/uart.c
+++ b/kernel/uart.c
@@ -42,8 +42,8 @@
 struct spinlock uart_tx_lock;
 #define UART_TX_BUF_SIZE 32
 char uart_tx_buf[UART_TX_BUF_SIZE];
-int uart_tx_w; // write next to uart_tx_buf[uart_tx_w++]
-int uart_tx_r; // read next from uart_tx_buf[uar_tx_r++]
+uint64 uart_tx_w; // write next to uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE]
+uint64 uart_tx_r; // read next from uart_tx_buf[uar_tx_r % UART_TX_BUF_SIZE]
 
 extern volatile int panicked; // from printf.c
 
@@ -94,13 +94,13 @@ uartputc(int c)
   }
 
   while(1){
-    if(((uart_tx_w + 1) % UART_TX_BUF_SIZE) == uart_tx_r){
+    if(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
       // buffer is full.
       // wait for uartstart() to open up space in the buffer.
       sleep(&uart_tx_r, &uart_tx_lock);
     } else {
-      uart_tx_buf[uart_tx_w] = c;
-      uart_tx_w = (uart_tx_w + 1) % UART_TX_BUF_SIZE;
+      uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
+      uart_tx_w += 1;
       uartstart();
       release(&uart_tx_lock);
       return;
@@ -150,8 +150,8 @@ uartstart()
       return;
     }
     
-    int c = uart_tx_buf[uart_tx_r];
-    uart_tx_r = (uart_tx_r + 1) % UART_TX_BUF_SIZE;
+    int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];
+    uart_tx_r += 1;
     
     // maybe uartputc() is waiting for space in the buffer.
     wakeup(&uart_tx_r);