| APIC is the Advanced Peripheral Interrupt Controller... It's a standard that's slowly replacing the old PICs.
Basically, when IBM invented the XT PC, they put two PICs in there to handle the interrupts. Each PIC can handle 8 IRQs, giving a total of 15. Why 15, rather than 16? Well, the output from the second PIC is connected to IRQ2 on the first PIC, using one up. However, the PIC design is slow and old, originally designed to be on the ISA bus. Although computers have moved forwards, the PIC interface still needs to look like it's ISA.
APIC splits up the way the PIC works, and provides the possibilities for more IRQ lines. Basically, the processor has part of the APIC onboard, and the chipset has another part of APIC. These communicate over a special serial bus.
Unfortunately, APIC mode doesn't always work as well as it should do, and some drivers can't handle interrupts above 15.
__________________
|