in contrast with the "integrated APIC" found in most of the Pentium processors. The 82489DX had 16 interrupt lines; it also had a quirk that it could lose some ISA interrupts. In a multiprocessor 486 system, each CPU had to be paired with its own 82489DX; additionally a supplementary 82489DX had to be used as I/O APIC. The 82489DX could not emulate the 8259A (XT-PIC) so these also had to be included as physical chips for backwards compatibility. The 82489DX was a packaged as a 132-pin PQFP. Integrated local APICs 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 interrupts (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. 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 processors and later ones. The Message Signaled Interrupts (MSI) feature of the PCI 2.2 and later specifications cannot be used without the local APIC being enabled. 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. APIC timer Another advantage of the local APIC is that it also provides a high-resolution (on the order of one microsecond or better) timer that can be used in both interval and one-off mode. The APIC timer had its initial acceptance woes. A Microsoft document from 2002 (which advocated for the adoption of High Precision Event Timer instead) criticized the LAPIC timer for having "poor resolution" and stating that "the clocks silicon is sometimes very buggy". Nevertheless, the APIC timer is used for example by Windows 7 when profiling is enabled, and by Windows 8 in all circumstances. (Before Windows 8 claimed exclusive rights to this timer, it was also used by some programs like CPU-Z.) Under Microsoft Windows the APIC timer is not a shareable resource. The aperiodic interrupts offered by the APIC timer are used by the Linux kernel tickless kernel feature. This optional but default feature is new with 2.6.18. When enabled on a computer with