Intel IA-32 - Manual (Page 355)

Vol. 3A 8-31
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
8.6.2.3 Broadcast/Self Delivery Mode
The destination shorthand field of the ICR allows the delivery mode to be by-passed in favor of
broadcasting the IPI to all the processors on the system bus and/or back to itself (see Section
8.6.1, “Interrupt Command Register (ICR)”). Three destination shorthands are supported: self,
all excluding self, and all including self. The destination mode is ignored when a destination
shorthand is used.
8.6.2.4 Lowest Priority Delivery Mode
With lowest priority delivery mode, the ICR is programmed to send an IPI to several processors
on the system bus, using the logical or shorthand destination mechanism for selecting the
processor. The selected processors then arbitrate with one another over the system bus or the
APIC bus, with the lowest-priority processor accepting the IPI.
For systems based on the Intel Xeon processor, the chipset bus controller accepts messages from
the I/O APIC agents in the system and directs interrupts to the processors on the system bus.
When using the lowest priority delivery mode, the chipset chooses a target processor to receive
the interrupt out of the set of possible targets. The Pentium 4 processor provides a special bus
cycle on the system bus that informs the chipset of the current task priority for each logical
processor in the system. The chipset saves this information and uses it to choose the lowest
priority processor when an interrupt is received.
For systems based on P6 family processors, the processor priority used in lowest-priority arbi-
tration is contained in the arbitration priority register (APR) in each local APIC. Figure 8-15
shows the layout of the APR.
The APR value is computed as follows:
IF (TPR[7:4]
≥
IRRV[7:4]) AND (TPR[7:4]
>
ISRV[7:4])
THEN
APR[7:0]
←
TPR[7:0]
ELSE
APR[7:4]
←
max(TPR[7:4] AND ISRV[7:4], IRRV[7:4])
APR[3:0]
←
0.
Figure 8-15. Arbitration Priority Register (APR)
31 0 7 8
Reserved
Address: FEE0 0090H
Value after reset: 0H
Arbitration Priority Sub-Class
Arbitration Priority
4 3