Hui
If you have a GPIO configured to generate an interrupt on a falling or rising edge (eg. PORTC_PCR8 is originally 0x000a0103 for falling edge sensitive interrupt with pull-up enabled) you can change its direction from input to output (in GPIOC_PDDR bit 0 to 1) and it will drive the value in GPIOC_PDOR to the pin.
At the same time each change of the pin value from '1' to '0' will still trigger the original falling edge interrupt.
This allows generating a port interrupt by controlling the value on a GPIO output pin.
Regards
Mark