Intel ARCHITECTURE IA-32 User Manual Page 346

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8-22 Vol. 3A
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
8.5.5 Local Interrupt Acceptance
When a local interrupt is sent to the processor core, it is subject to the acceptance criteria spec-
ified in the interrupt acceptance flow chart in Figure 8-17. If the interrupt is accepted, it is logged
into the IRR register and handled by the processor according to its priority (see Section 8.8.4,
“Interrupt Acceptance for Fixed Interrupts”). If the interrupt is not accepted, it is sent back to
the local APIC and retried.
8.6 ISSUING INTERPROCESSOR INTERRUPTS
The following sections describe the local APIC facilities that are provided for issuing interpro-
cessor interrupts (IPIs) from software. The primary local APIC facility for issuing IPIs is the
interrupt command register (ICR). The ICR can be used for the following functions:
To send an interrupt to another processor.
To allow a processor to forward an interrupt that it received but did not service to another
processor for servicing.
To direct the processor to interrupt itself (perform a self interrupt).
To deliver special IPIs, such as the start-up IPI (SIPI) message, to other processors.
Interrupts generated with this facility are delivered to the other processors in the system through
the system bus (for Pentium 4 and Intel Xeon processors) or the APIC bus (for P6 family and
Pentium processors). The ability for a processor to send a lowest priority IPI is model specific
and should be avoided by BIOS and operating system software.
8.6.1 Interrupt Command Register (ICR)
The interrupt command register (ICR) is a 64-bit local APIC register (see Figure 8-12) that
allows software running on the processor to specify and send interprocessor interrupts (IPIs) to
other IA-32 processors in the system.
To send an IPI, software must set up the ICR to indicate the type of IPI message to be sent and
the destination processor or processors. (All fields of the ICR are read-write by software with
the exception of the delivery status field, which is read-only.) The act of writing to the low
doubleword of the ICR causes the IPI to be sent.
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