AM1806: Is the timing delay between the Rising Edge of the source signal and the processing first instruction of the FIQ ISR function a CONSTANT value or a VOLATILE value?

Hi,

I'm trying to figure out the timing delay between the trigger's rising edge and the the first instruction of its ISR function on AM1806. The porgram is simple:

main() : Initialization followed by while(1)

FIQ ISR: Tirggered by an external signal's Rising edge （once per second ) and Pull Up and Pull Down a GPIO Output Signal.

I observed the Rising Edge of the Trigger and the Gpio Output Signal and found that the timing delay between them was changing every time. The difference might be larger than 1us.

I want to know whether the timing delay between the trigger and the ISR is a CONSTANT value or a VOLATILE value?

Thank you!