4.3.3 I/O APIC Entries
The configuration table contains one or more entries for I/O APICs. Figure 4-6 shows the format
of each I/O APIC entry, and Table 4-9 explains each field.
Table 4-9. I/O APIC Entry Fields
I/O APIC ID
I/O APIC VERSION #
I/O APIC FLAGS: EN
I/O APIC ADDRESS
4.3.4 I/O Interrupt Assignment Entries
These entries indicate which interrupt source is connected to each I/O APIC interrupt input. There
is one entry for each I/O APIC interrupt input that is connected. Figure 4-7 shows the format of
each entry, and Table 4-10 explains each field. Appendix D provides the semantics for encoding
The MP specification enables significantly more interrupt sources than the standard AT
architecture by using I/O APICs. When using I/O APICs, it is preferable that the buses do not
share interrupts with the other buses. Bus implementations that share interrupts, such as the PCI
and VL local buses, support their bus interrupts by overloading them into another bus space. These
buses can be supported in one of the following two ways:
1. Interrupt Assignment Entries for each of the bus interrupts are listed in the MP configuration
table. Each interrupt destination matches the destination of another interrupt source interrupt
that this interrupt shares. For example, if
ISA-IRQ2, then both Interrupt Assignment Entries for these vectors would refer to the same
destination I/O APIC and INTIN#.
MEMORY-MAPPED ADDRESS OF I/O APIC
I/O APIC FLAGS
Figure 4-6. I/O APIC Entry
I/O APIC ID
A value of 2 identifies an I/O APIC entry.
The ID of this I/O APIC.
Bits 0–7 of the I/O APIC's version register.
If zero, this I/O APIC is unusable, and the
operating system should not attempt to access
this I/O APIC.
At least one I/O APIC must be enabled.
Base address for this I/O APIC.
has the same vector as