diff --git a/op000.py b/op000.py
index eec3519..e6896e5 100644
--- a/op000.py
+++ b/op000.py
@@ -52,7 +52,7 @@ def op_rtt(cpu, inst):
 
     # this knows  KERNEL < SUPERVISOR < USER
     newpsw_d['curmode'] = max(cpu.psw_curmode, newpsw_d['curmode'])
-    newpsw_d['prevmode'] = max(cpu.psw_prevmode, newpsw_d['prevmode'])
+    newpsw_d['prevmode'] = max(cpu.psw_curmode, newpsw_d['prevmode'])
 
     # if not in kernel mode, cannot change pri
     if cpu.psw_curmode != cpu.KERNEL: