Local APICs (LAPICs) manage all external interrupts for some specific processor in an SMP system. In addition, they are able to accept and generate [[inter-processor interrupt]]s (IPIs) between LAPICs. LAPICs may support up to 224 usable [[interrupt]] vectors from an I/O APIC. Vector numbers 0 to 31, out of 0 to 255, are reserved for exception handling by x86 processors.
All Intel processors starting with the P5 microarchitecture ([[P54C]]) have a built-in local APIC.<ref name="Mueller2011" /><ref name="timer" /> However, if the local APIC is disabled in a P5 processor, it cannot be re-enabled by software; this limitation no longer exists in the [[P6 (microarchitecture)|P6 processors]] and later ones.<ref name="timer" /> In single-processor systems, the major advantage of the local APIC is that [[Peripheral Component Interconnect|PCI]] cards no longer need to have their interrupts mapped to ISA interrupts, but can use virtual PCI IRQs above 15, resulting in fewer conflicts and better performance.<ref name="Mueller2011" />
In the Microsoft family of operating systems, [[Windows XP]] was the first to properly make use of virtual [[Interrupt request (PC architecture)|IRQ]]s provided by the local APIC, with some partial support present in [[Windows 2000]].<ref name="Mueller2011">{{cite book|author=Scott M. Mueller|title=Upgrading and Repairing PCs|year=2011|publisher=Que Publishing|isbn=978-0-13-268218-3|page=242|edition=20th}}</ref> In Windows 2000, the spreading of PCI card interrupts to virtual IRQ happens only if the machine also has an I/O APIC. Microsoft blamed their Windows 2000 implementation on information they had received from Intel supposedly promising that all future systems would contain an I/O APIC as well, something that in retrospect did not happen.<ref name="up">{{cite web|url=http://msdn.microsoft.com/en-us/windows/hardware/gg462964.aspx|title=Windows Hardware Dev Center|website=msdn.microsoft.com}}</ref> (A Microsoft document from that era even claimed that "without an I/O APIC in the system, the local APICs are useless. In such a situation, Windows 2000 has to revert to using the 8259 PIC."<ref>{{cite web|url=http://www.microsoft.com/whdc/archive/io-apic.mspx|title=Content not found|website=www.microsoft.com}}</ref>) The sudden widespread use of the local APIC with Windows XP did expose a number of bugs in various BIOS implementations. In particular, Microsoft was forced to retain the PCI IRQ stacking behavior for any machine with a [[CardBus]] controller (and without an I/O APIC).<ref name="up"/>
The [[Message Signaled Interrupts]] (MSI) feature of the PCI 2.2 and later specifications cannot be used without the local APIC being enabled.<ref name="up"/> Use of MSI obviates the need for an I/O APIC. Additionally, up to 224 interrupts are supported in MSI mode, and IRQ sharing is not allowed.<ref name="msi"/>
