Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
It is assumed that the ISA interrupts will be identity-mapped into the first I/O APIC sources. Most
existing APIC designs, however, will contain at least one exception to this assumption. The Interrupt
Source Override Structure is provided in order to describe these exceptions. It is not necessary to
provide an Interrupt Source Override for every ISA interrupt. Only those that are not identitymapped
onto the APIC interrupt inputs need be described.
Note: This specification only supports overriding ISA interrupt sources.
For example, if your machine has the ISA Programmable Interrupt Timer (PIT) connected to ISA
IRQ 0, but in APIC mode, it is connected to I/O APIC interrupt input 2, then you would need an
Interrupt Source Override where the source entry is ‘0’ and the Global System Interrupt is ‘2.’
Table 5-50 Interrupt Source Override Structure
Field
Byte
Length
Byte
Offset
Description
Type 1 0 2 Interrupt Source Override
Length 1 1 10
Bus 1 2 0 Constant, meaning ISA
Source 1 3 Bus-relative interrupt source (IRQ)
Global System
Interrupt
4 4 The Global System Interrupt that this bus-relative interrupt source
will signal.
Flags 2 8 MPS INTI flags. See Table 5-51 for a description of this field.
The MPS INTI flags listed in Table 5-51 are identical to the flags used in Table 4-10 of the MPS
version 1.4 specifications. The Polarity flags are the PO bits and the Trigger Mode flags are the EL
bits.
Table 5-51 MPS INTI Flags
Local APIC -
Flags
Bit
Length
Bit
Offset
Description
Polarity 2 0 Polarity of the APIC I/O input signals:
00 Conforms to the specifications of the bus
(For example, EISA is active-low for level-triggered interrupts)
01 Active high
10 Reserved
11 Active low
Trigger Mode 2 2 Trigger mode of the APIC I/O Input signals:
00 Conforms to specifications of the bus
(For example, ISA is edge-triggered)
01 Edge-triggered
10 Reserved
11 Level-triggered
Reserved 12 4 Must be zero.
142 April, 2015 Version 6.0