diff --git a/start.c b/start.c
index 46c6bfc..3b923f8 100644
--- a/start.c
+++ b/start.c
@@ -5,7 +5,7 @@
 #define NCPU 3
 
 /* QEMU memory maps a UART device here. */
-#define UART_BASE ((char *)0x10000000)
+#define UART_BASE ((volatile char *)0x10000000)
 
 /** Send a single character to the UART device */
 void uart_putc(char c) { *UART_BASE = c; }