diff --git a/c_os/devices/fonts/Font_8x16.h b/c_os/devices/fonts/Font_8x16.h
index 197717d..bc2d572 100644
--- a/c_os/devices/fonts/Font_8x16.h
+++ b/c_os/devices/fonts/Font_8x16.h
@@ -6,9 +6,9 @@
 /*                                            */
 /**********************************************/
 
-constexpr unsigned int FONTDATAMAX_8x16 = 4096;
+constexpr const unsigned int FONTDATAMAX_8x16 = 4096;
 
-constexpr unsigned char fontdata_8x16[FONTDATAMAX_8x16] = {
+constexpr const unsigned char fontdata_8x16[FONTDATAMAX_8x16] = {
 
     /* 0 0x00 '^@' */
     0x00, /* 00000000 */
diff --git a/c_os/devices/fonts/Font_8x8.h b/c_os/devices/fonts/Font_8x8.h
index 6b13c1d..03645c7 100644
--- a/c_os/devices/fonts/Font_8x8.h
+++ b/c_os/devices/fonts/Font_8x8.h
@@ -6,9 +6,9 @@
 /*                                            */
 /**********************************************/
 
-constexpr unsigned int FONTDATAMAX_8x8 = 2048;
+constexpr const unsigned int FONTDATAMAX_8x8 = 2048;
 
-constexpr unsigned char fontdata_8x8[FONTDATAMAX_8x8] = {
+constexpr const unsigned char fontdata_8x8[FONTDATAMAX_8x8] = {
 
     /* 0 0x00 '^@' */
     0x00, /* 00000000 */
diff --git a/c_os/devices/fonts/Font_acorn_8x8.h b/c_os/devices/fonts/Font_acorn_8x8.h
index 30bc307..664ba16 100644
--- a/c_os/devices/fonts/Font_acorn_8x8.h
+++ b/c_os/devices/fonts/Font_acorn_8x8.h
@@ -2,7 +2,7 @@
 
 /* Acorn-like font definition, with PC graphics characters */
 
-constexpr unsigned char acorndata_8x8[] = {
+constexpr const unsigned char acorndata_8x8[] = {
     /* 00 */  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /* ^@ */
     /* 01 */  0x7e, 0x81, 0xa5, 0x81, 0xbd, 0x99, 0x81, 0x7e, /* ^A */
     /* 02 */  0x7e, 0xff, 0xbd, 0xff, 0xc3, 0xe7, 0xff, 0x7e, /* ^B */