ARM, RISC-V контроллеры

А как вы думаете, почему это исключение назвали Fast? Невнимательно читали TRM:

The Interrupt Request (IRQ) exception is a normal interrupt caused by a LOW level on the nIRQ input. IRQ has a lower priority than FIQ, and is masked on entry to an FIQ sequence. As with the nFIQ input, nIRQ passes into the core through a synchronizer.