Short for Advanced Programmable Interrupt Controller, APIC is a programmable interrupt controller (PIC) with advanced interrupt management. It was first developed by Intel and replaces the 8259 interrupt controllers. APIC is necessary for dual processing or multiple-processor support and has been available since the original Intel Pentium. However, the motherboard chipset, BIOS, and operating system (Windows 2000 and later) must also support APIC before it can be used.
Although APIC adds support for multiple processors, it is still used with single-processor computers since it provides support for virtual PCI IRQs above 15. Windows XP is one of the first versions of Windows that takes full advantage of APIC if enabled and assigns more devices to IRQs above 15.
Note: Windows XP Professional x64 edition and Windows Server 2003 for x64 cannot be installed unless APIC is enabled in the system BIOS.