ACPI floods CPUs with interrupts

[Artem Koutchine]

I have installed FreeBSd 5.2-RC2 on ASUS P2B-DS
with 3ware raid controller and DUAL PIII (slot2) 550Mhz.
Updated motherboard bios to
the latest bios and after patching the twe driver, so it does not
show the "PCI parity erro" several times a second. (I think
it is related to the fact that 3ware 7506 controller is pci 2.2
while p2b is pci 2.1). After that i did top and saw that 
50% of cpu power is wasted on interrupts. I did
sysstat and :vmstat. It showed that 46700 interrupts per
second were generated by acpi on irq 20. So, i booted without
acpi and got 100% cpu idle. The question is:
How ACPI can generate IRQs? I thought it is just some kind
of interface and protocol standard, not a real hardware part which
can rise irq. Why does it generate so many irqs? May it be related
to the PCI parity error on twe?

Artem