The exception that proves the rule

To clean up a final point, the interrupt_id() and interrupt_eoi() routines aren't called as normal routines. Instead, for performance reasons, the kernel intermixes these routines directly with kernel code — the normal function-calling conventions aren't followed. The callout_interrupt_*.s files in the startup library will have a description of what registers are used to pass values into and out of these callouts for your particular CPU. Note also that you can't return from the middle of the routine as you normally would. Instead, you're required to "fall off the end" of the code.