Advanced Programmable Interrupt Controller

In computing, Intel's Advanced Programmable Interrupt Controller (APIC) is a family of interrupt controllers. As its name suggests, the APIC is more advanced than Intel's 8259 Programmable Interrupt Controller (PIC), particularly enabling the construction of multiprocessor systems. It is one of several architectural designs intended to solve interrupt routing efficiency issues in multiprocessor computer systems. In computing, Intel's Advanced Programmable Interrupt Controller (APIC) is a family of interrupt controllers. As its name suggests, the APIC is more advanced than Intel's 8259 Programmable Interrupt Controller (PIC), particularly enabling the construction of multiprocessor systems. It is one of several architectural designs intended to solve interrupt routing efficiency issues in multiprocessor computer systems. The APIC is a split architecture design, with a local component (LAPIC) usually integrated into the processor itself, and an optional I/O APIC on a system bus. The first APIC was the 82489DX – it was a discrete chip that functioned both as local and I/O APIC. The 82489DX enabled construction of symmetric multiprocessor (SMP) systems with the Intel 486 and early Pentium processors; for example, the reference two-way 486 SMP system used three 82489DX chips, two as local APICs and one as I/O APIC. Starting with the P54C processor, the local APIC functionality was integrated into the Intel processors' silicon. The first dedicated I/O APIC was the Intel 82093AA, which was intended for PIIX3-based systems. There are two components in the Intel APIC system, the local APIC (LAPIC) and the I/O APIC. There is one LAPIC in each CPU in the system. In the very first implementation (82489DX), the LAPIC was a discrete circuit opposed to its thereafter implementation in Intel processors' silicon. There is typically one I/O APIC for each peripheral bus in the system. In original system designs, LAPICs and I/O APICs were connected by a dedicated APIC bus. Newer systems use the system bus for communication between all APIC components. Each APIC, whether a discrete chip or integrated in a CPU, has a version register containing a four-bit version number for its specific APIC implementation. For example, the 82489DX has an APIC version number of 0, while version 1 was assigned to the first generation of local APICs integrated in the Pentium 90 and 100 processors. In systems containing an 8259 PIC, the 8259 may be connected to the LAPIC in the system's bootstrap processor (BSP), or to one of the system's I/O APICs, or both. Logically, however, the 8259 is only connected once at any given time. The first-generation Intel APIC chip, the 82489DX, which was meant to be used with Intel 80486 and early Pentium processors, is actually an external local and I/O APIC in one circuit. The Intel MP 1.4 specification refers to it as 'discrete APIC' 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. 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. In single-processor systems, the major advantage of the local APIC is that 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.