diff --git a/vm/mm.c b/vm/mm.c
index 14e68ac..dabf812 100644
--- a/vm/mm.c
+++ b/vm/mm.c
@@ -21,11 +21,6 @@
 #include <stdlib.h>
 #include "mm.h"
 
-#ifndef MEM_SIZE
-/* Set a default memory size. (2^8) */
-#define MEM_SIZE 256
-#endif
-
 // Get a WORD aligned (16-bit) address.
 // Casts the 8-bit pointer (mm_base_addr) to 16-bit (WORD) and add offset.
 #define addr_align_word(offset) \
diff --git a/vm/mm.h b/vm/mm.h
index 772c7c3..f136bf9 100644
--- a/vm/mm.h
+++ b/vm/mm.h
@@ -22,6 +22,11 @@
 
 #include <stdint.h>
 
+#ifndef MEM_SIZE
+/* Set a default memory size. (2^8) */
+#define MEM_SIZE 256
+#endif
+
 extern uint8_t *mm_base_addr;
 
 void mm_init();