An Add-On Instruction is executed just like any other routine belonging to a
particular task. Because another task can preempt a task containing an Add-On
Instruction being executed, it is not guaranteed that an Add-On Instruction
will always be executed to its completion. You can use the User Interrupt
Disable / Enable (UID/UIE) instructions to block a task switch if you want to
be sure the Add-On Instruction executes to completion before switching to
another task.