System BIOS
BIOS Addresses
Chapter 3
76
Three major interrupt modes are available:
• PIC mode: This mode uses only legacy interrupt controllers, so the system can
support only one processor. You can select this mode when you install Windows NT.
• Virtual wire mode: This mode, which is implemented using the 82C59 interrupt
and the I/O APIC controller, is used during boot time. The virtual wire mode allows
the transition to the symmetric I/O mode. In the virtual wire mode, only one
processor executes operations.
• Symmetric I/O mode: This mode is implemented using the I/O APIC controller and
allows for multiple processor operations.
NOTE In PIC mode and virtual wire mode, PCI interrupts are routed to the INT line. In
symmetric I/O mode, PCI interrupts are routed to the I/O APIC controllers and
forwarded over an APIC bus to the processors.
PCI IRQ Lines
PCI devices generate IRQs using up to four PCI IRQ lines (INTA#, INTB#, INTC#, and
INTD#).
PCI interrupts can be shared; several devices can use the same interrupt. However,
optimal system performance is reached when minimizing the sharing of interrupts.
Refer to page 60 for a table of the PCI device interrupts.
INTIN18 PCINTC
INTIN19 PCINTD
INTIN20 TFPC IRQ
INTIN21 SCI IRQ
INTIN22 not
connected
INTIN23 ICH SMI
(not used)
I/O APIC Input IRQ IRQ Description
