Tweak memory allocation size for portability

Close #161

Author: ChinYikMing

src/io.c

@@ -16,9 +16,19 @@
 
 #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()
 {