Turning off FLAG(PAGE0) is never necessary within your own code. Adding explicit base reg of 0 (not an index reg of 0) is the technique to indicate you know what you are doing and thus override the flag. You will see that within many macro expansions the need to reference the CVT without requiring the invoker to include the CVT.
Maybe there are some macros that don't do that. If you find any in z/OS, let me know and maybe we can get that improved. Peter Relson z/OS Core Technology Design
