MVECEN = 0
When MVECEN = 0, the address location pointed to by IVTBASE has a
GOTO instruction for a high-priority interrupt. Similarly, the
corresponding low-priority vector also has a GOTO instruction, which is
executed in case of a low-priority interrupt.
MVECEN = 1
When MVECEN = 1, the value in the vector table of each
interrupt points to the address location of the first instruction of the Interrupt
Service Routine, hence: ISR Location = Interrupt Vector Table entry << 2.