Questions about IO-APIC

Udo Steinberg udo at
Sat Jul 28 12:22:34 CEST 2012

On Fri, 27 Jul 2012 16:34:12 -0800 Chen Tian (CT) wrote:

CT> I have some basic questions about IO-APIC.  In the boot-up message, I can
CT> see this:
CT> IO-APIC[ 0]: pins 23
CT>   PIN[ 0m]: vector=20, del=0, dm=physical, dest=0 (high, edge)
CT>   PIN[ 1m]: vector=21, del=0, dm=physical, dest=0 (high, edge)
CT>   PIN[ 2m]: vector=22, del=0, dm=physical, dest=0 (high, edge)
CT> I understand this info is from IO-APIC redirection table. My question is
CT> what does vector means? Is that the interrupt number CPU is going to see
CT> when an interrupt request is sent from a pin? Or it's an irq number
CT> assigned to an IO-APIC pin?

It is the interrupt vector that the IOAPIC will send to the CPU when there
is interrupt activity on the respective pin. The vector is then used as an
index into the IDT, which tells the CPU where the interrupt handler can be
found. Which is why it starts at 0x20, because vectors 0x0-0x1f are reserved
for exceptions.

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