E
82093AA (IOAPIC)
1.0. OVERVIEW
While the standard ISA Compatible interrupt controller (located in the PIIX3) is intended for use in a uni-
processor system, the I/O Advanced Programmable Interrupt Controller (IOAPIC) can be used in either a uni-
processor or multi-processor system. The IOAPIC provides multi-processor interrupt management and
incorporates both static and dynamic symmetric interrupt distribution across all processors. In systems with
multiple I/O subsystems, each subsystem can have its own set of interrupts.
In a uni-processor system, the IOAPIC's dedicated interrupt bus can reduce interrupt latency over the standard
interrupt controller (i.e., the latency associated with the propagation of the interrupt acknowledge cycle across
multiple busses using the standard interrupt controller approach). Interrupts can be controlled by the standard
ISA Compatible interrupt controller in the PIIX3, the IOAPIC unit, or mixed mode where both the standard ISA
Compatible Interrupt Controller and IOAPIC are used. The selection of which controller responds to an interrupt
is determined by how the interrupt controllers are programmed. Note that it is the programmer's responsibility to
make sure that the same interrupt input signal is not handled by both interrupt controllers.
At the system level, APIC consists of two parts (Figure 2.0)—one residing in the I/O subsystem (called the
IOAPIC) and the other in the CPU (called the Local APIC). The local APIC and the IOAPIC communicate over a
dedicated APIC bus. The IOAPIC bus interface consists of two bi-directional data signals (APICD[1:0]) and a
clock input (APICCLK).
The CPU's Local APIC Unit contains the necessary intelligence to determine whether or not its processor should
accept interrupts broadcast on the APIC bus. The Local Unit also provides local pending of interrupts, nesting
and masking of interrupts, and handles all interactions with its local processor (e.g., the INTR/INTA/EOI
protocol). The Local Unit further provides inter-processor interrupts and a timer, to its local processor. The
register level interface of a processor to its local APIC is identical for every processor.
The IOAPIC Unit consists of a set of interrupt input signals, a 24-entry by 64-bit Interrupt Redirection Table,
programmable registers, and a message unit for sending and receiving APIC messages over the APIC bus. I/O
devices inject interrupts into the system by asserting one of the interrupt lines to the IOAPIC. The IOAPIC
selects the corresponding entry in the Redirection Table and uses the information in that entry to format an
interrupt request message. Each entry in the Redirection Table can be individually programmed to indicate
edge/level sensitive interrupt signals, the interrupt vector and priority, the destination processor, and how the
processor is selected (statically or dynamically). The information in the table is used to transmit a message to
other APIC units (via the APIC bus).
The IOAPIC contains a set of programmable registers. Two of the registers (I/O Register Select and I/O Window
Registers) are located in the CPU's memory space and are used to indirectly access the other APIC registers as
described in Section 3.0, Register Description. The Version Register provides the implementation version of the
IOAPIC. The IOAPIC ID Register is programmed with an ID value that serves as a physical name of the IOAPIC.
This ID is loaded into the ARB ID Register when the IOAPIC ID Register is written and is used during bus
arbitration.
NOTE
The interrupt number or the vector does not imply a particular priority for being sent. The IOAPIC continually
polls the 24 interrupts in a rotating fashion, one at a time. The pending interrupt polled first is the one sent.
3
PRELIMINARY
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