Intel D845GEBV2 BLKD845GERG2-10PK User Manual

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This section describes interrupt sharing and how the interrupt signals are connected between the 
PCI bus connectors and onboard PCI devices.  The PCI specification specifies how interrupts can 
be shared between devices attached to the PCI bus.  In most cases, the small amount of latency 
added by interrupt sharing does not affect the operation or throughput of the devices.  In some 
special cases where maximum performance is needed from a device, a PCI device should not share 
an interrupt with other PCI devices.  Use the following information to avoid sharing an interrupt 
with a PCI add-in card. 

PCI devices are categorized as follows to specify their interrupt grouping: 
•  INTA:  By default, all add-in cards that require only one interrupt are in this category.  For 

almost all cards that require more than one interrupt, the first interrupt on the card is also 
classified as INTA.   

•  INTB:  Generally, the second interrupt on add-in cards that require two or more interrupts is 

classified as INTB.  (This is not an absolute requirement.) 

•  INTC and INTD:  Generally, a third interrupt on add-in cards is classified as INTC and a 

fourth interrupt is classified as INTD. 

The ICH4 has eight Programmable Interrupt Request (PIRQ) input signals.  All PCI interrupt 
sources either onboard or from a PCI add-in card connect to one of these PIRQ signals.  Some PCI 
interrupt sources are electrically tied together on the Desktop Boards D845GEBV2 and 
D845GERG2 and therefore share the same interrupt.  Table 24 shows an example of how the 
PIRQ signals are routed.   

For example, using Table 24 as a reference, assume an add-in card using INTA is plugged into PCI 
bus connector 3.  In PCI bus connector 3, INTA is connected to PIRQC, which is already 
connected to the ICH4 USB.  The add-in card in PCI bus connector 3 now shares an interrupt with 
the onboard interrupt source.