diff --git a/lib/tms9918.h b/lib/tms9918.h
index ec09194..f97733a 100644
--- a/lib/tms9918.h
+++ b/lib/tms9918.h
@@ -15,11 +15,11 @@
 #include "utils.h"
 
 #ifdef APPLE1
-   const byte *VDP_DATA = (byte *) 0xCC00;       // TMS9918 data port (VRAM)
-   const byte *VDP_REG  = (byte *) 0xCC01;       // TMS9918 register port (write) or status (read)
+   byte *const VDP_DATA = (byte *) 0xCC00;       // TMS9918 data port (VRAM)
+   byte *const VDP_REG  = (byte *) 0xCC01;       // TMS9918 register port (write) or status (read)
 #else
-   const byte *VDP_DATA = (byte *) 0xA000;       // TMS9918 data port (VRAM)
-   const byte *VDP_REG  = (byte *) 0xA001;       // TMS9918 register port (write) or status (read)
+   byte *const VDP_DATA = (byte *) 0xA000;       // TMS9918 data port (VRAM)
+   byte *const VDP_REG  = (byte *) 0xA001;       // TMS9918 register port (write) or status (read)
 #endif
 
 // control port bits