Register r9 is the static base register, sb. It must point to the base address of the appropriate static data segments whenever you call any externally visible routine.

You can use r9 for other purposes in a routine that does not use sb. If you do this you must save the contents of sb on entry to your routine and restore these before exit. You must also restore the contents before any call to an external routine.

In all other respects the usage of registers is the same with or without RWPI.