It takes different time for the GPIO interrupt and external interrupt to find trigger sources. Therefore, external interrupts are used in special urgent events, and a set of GPIO interrupt is shared in common events.
In the M051, for example, the 16 pins of P0 [7: 0] and P1 [7: 0] share the GP01_INT interrupt handler. Polling is needed to find the trigger source when an interrupt occurs, which takes more time. EINT0 is only triggered by P3.2 and thus does not require polling, which relatively saves time.
Table 1 M051Interrupt Mapping Table