Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
Enabled 1 0 If zero, this processor is unusable, and the operating system
support will not attempt to use it.
Reserved 31 1 Must be zero.
5.2.12.3 I/O APIC Structure
In an APIC implementation, there are one or more I/O APICs. Each I/O APIC has a series of
interrupt inputs, referred to as INTIn, where the value of n is from 0 to the number of the last
interrupt input on the I/O APIC. The I/O APIC structure declares which global system interrupts are
uniquely associated with the I/O APIC interrupt inputs. There is one I/O APIC structure for each I/O
APIC in the system. For more information on global system interrupts see Section 5.2.13, “Global
System Interrupts.”
Table 5-49 I/O APIC Structure
Field
Byte
Length
Byte
Offset
Description
Type 1 0 1 I/O APIC structure
Length 1 1 12
I/O APIC ID 1 2 The I/O APIC’s ID.
Reserved 1 3 0
I/O APIC Address 4 4 The 32-bit physical address to access this I/O APIC. Each I/O APIC
resides at a unique address.
Global System
Interrupt Base
4 8 The global system interrupt number where this I/O APIC’s interrupt
inputs start. The number of interrupt inputs is determined by the I/O
APIC’s Max Redir Entry register.
5.2.12.4 Platforms with APIC and Dual 8259 Support
Systems that support both APIC and dual 8259 interrupt models must map global system interrupts
0-15 to the 8259 IRQs 0-15, except where Interrupt Source Overrides are provided (see
Section 5.2.12.5, “Interrupt Source Override Structure” below). This means that I/O APIC interrupt
inputs 0-15 must be mapped to global system interrupts 0-15 and have identical sources as the 8259
IRQs 0-15 unless overrides are used. This allows a platform to support OSPM implementations that
use the APIC model as well as OSPM implementations that use the 8259 model (OSPM will only
use one model; it will not mix models).
When OSPM supports the 8259 model, it will assume that all interrupt descriptors reporting global
system interrupts 0-15 correspond to 8259 IRQs. In the 8259 model all global system interrupts
greater than 15 are ignored. If OSPM implements APIC support, it will enable the APIC as described
by the APIC specification and will use all reported global system interrupts that fall within the limits
of the interrupt inputs defined by the I/O APIC structures. For more information on hardware
resource configuration see Section 6, “Configuration.”
5.2.12.5 Interrupt Source Override Structure
Interrupt Source Overrides are necessary to describe variances between the IA-PC standard dual
8259 interrupt definition and the platform’s implementation.
Version 6.0 141