diff --git a/sim/rx/ChangeLog b/sim/rx/ChangeLog
index 6b4b3d9aed..c67b8d5fe7 100644
--- a/sim/rx/ChangeLog
+++ b/sim/rx/ChangeLog
@@ -1,3 +1,8 @@
+2021-02-08  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* reg.c (trace_register_changes): Use PRIx64 in printf format
+	string.
+
 2021-02-08  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* syscalls.c (rx_syscall): Move declaration of some variables to
diff --git a/sim/rx/reg.c b/sim/rx/reg.c
index ce9be12649..bdf2debac6 100644
--- a/sim/rx/reg.c
+++ b/sim/rx/reg.c
@@ -548,8 +548,8 @@ trace_register_changes (void)
   if (oldregs.r_acc != regs.r_acc)
     {
       if (tag) { printf ("%s", tag); tag = 0; }
-      printf("  acc %016llx:", oldregs.r_acc);
-      printf("%016llx", regs.r_acc);
+      printf("  acc %016" PRIx64 ":", oldregs.r_acc);
+      printf("%016" PRIx64, regs.r_acc);
       oldregs.r_acc = regs.r_acc;
     }