Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

8-36 Vol. 3A
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
4. When interrupts are pending in the IRR and ISR register, the local APIC
dispatches them to the processor one at a time, based on their priority and the
current task and processor priorities in the TPR and PPR (see Section 8.8.3.1,
“Task and Processor Priorities”).
5. When a fixed interrupt has been dispatched to the processor core for handling,
the completion of the handler routine is indicated with an instruction in the
instruction handler code that writes to the end-of-interrupt (EOI) register in the
local APIC (see Section 8.8.5, “Signaling Interrupt Servicing Completion”). The
act of writing to the EOI register causes the local APIC to delete the interrupt
from its ISR queue and (for level-triggered interrupts) send a message on the
bus indicating that the interrupt handling has been completed. (A write to the EOI
register must not be included in the handler routine for an NMI, SMI, INIT,
ExtINT, or SIPI.)
8.8.2 Interrupt Handling with the P6 Family and Pentium
Processors
With the P6 family and Pentium processors, the local APIC handles the local inter-
rupts, interrupt messages, and IPIs it receives as follows (see Figure 8-17).