# switching to system mode



## louka (Aug 8, 2012)

Hi,

I've being thinking about this for a while.

Can someone tell me why, in an isr, we switch to system mode? This is specifically from abort/irq/fiq modes on the arm cpu.

regards.


----------



## Maelstorm (Aug 9, 2012)

From my understanding (on IA32/AMD64 platforms), some of the instructions that the ISR requires are privileged instructions which can only be used from RING 0.  As for the ARM, I have no idea, but I suspect it's the same reason.


----------

