PSW.IPL can be changed only in supervisor mode.
Because tasks are always run in user mode,
Please implement it as an interrupt handler to be in supervisor mode.
The interrupt handler can be called using CC-RX's built-in function int_exception.
Interrupt vector numbers 1 to 8 are reserved for RI600V4, so use other numbers.
* In handlers, PSW.IPL should not be lower than when processing starts.