diff --git a/src/io.c b/src/io.c
index ad78c50f..98182fe7 100644
--- a/src/io.c
+++ b/src/io.c
@@ -17,8 +17,17 @@
 #include "io.h"
 
 static uint8_t *data_memory_base;
-/* set memory size to 2^32 bytes */
-#define MEM_SIZE 0x100000000ULL
+
+/*
+ * set memory size to 2^32 - 1 bytes
+ *
+ * The memory size is set to 2^32 - 1 bytes in order
+ * to make rv32emu portable for both 32-bit and 64-bit platforms.
+ * As a result, rv32emu can access any segment of the memory in
+ * either platform. Furthermore, it is safe because most of the 
+ * test cases' data memory usage will not exceed this memory size.
+ */
+#define MEM_SIZE 0xFFFFFFFFULL
 
 memory_t *memory_new()
 {