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APIC
APIC stands for
Advanced Programmable Interrupt Controller and is an
interrupt controller integrated in the
CPU.
An I/O APIC provides more than 16 interrupt requests (IRQs), thus IRQ sharing is not required anymore which improves reliability.
An APIC is accessed through memory-mapped registers and provides interrupt service to each CPU, used for IPIs and local
interrupts.
APIC is programmed through the LVT (local vector timer) register.
IO-APIC
An I/O APIC is an SMP-capable replacement for PC-Style interrupt controllers. Most (all?) SMP systems and many recent uniprocessor systems have one.
Misc
Apparently, Windows installs different kernels depending on the availability of APIC.
See also
I find it confusing that
ACPI has the same letters like APIC.