11.7 Returning from Interrupt Service Routine (ISR)
The Return from Interrupt (RETFIE) instruction is used to mark the end
            of an ISR. 
When the RETFIE 1 instruction is executed, the PC is loaded
            with the saved PC value from the top of the PC stack. Saved context is also restored
            with the execution of this instruction. Thus, execution returns to the state of
            operation that existed before the interrupt occurred. 
When the RETFIE 0 instruction is executed, the saved
            context is not restored back to the registers.
