Intel IA-32 - Manual (Page 342)

8-18 Vol. 3A
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
Remote IRR Flag (Read Only)
For fixed mode, level-triggered interrupts; this flag is set when the
local APIC accepts the interrupt for servicing and is reset when an
EOI command is received from the processor. The meaning of this
flag is undefined for edge-triggered interrupts and other delivery
modes.
Trigger Mode Selects the trigger mode for the local LINT0 and LINT1 pins: (0)
edge sensitive and (1) level sensitive. This flag is only used when the
delivery mode is Fixed. When the delivery mode is NMI, SMI, or
INIT, the trigger mode is always edge sensitive. When the delivery
mode is ExtINT, the trigger mode is always level sensitive. The timer
and error interrupts are always treated as edge sensitive.
If the local APIC is not used in conjunction with an I/O APIC and
fixed delivery mode is selected; the Pentium 4, Intel Xeon, and P6
family processors will always use level-sensitive triggering, regard-
less if edge-sensitive triggering is selected.
Mask Interrupt mask: (0) enables reception of the interrupt and (1) inhibits
reception of the interrupt. When the local APIC handles a perfor-
mance-monitoring counters interrupt, it automatically sets the mask
flag in the corresponding LVT entry. This flag will remain set until
software clears it.
Timer Mode Selects the timer mode: (0) one-shot and (1) periodic (see Section
8.5.4, “APIC Timer”).
8.5.2 Valid Interrupt Vectors
The IA-32 architecture defines 256 vector numbers, ranging from 0 through 255 (see Section 5.2,
“Exception and Interrupt Vectors”). Local and I/O APICs support 240 of these vectors (in the
range of 16 to 255) as valid interrupts.
When an interrupt vector in the range of 0 to 15 is sent or received through the local APIC, the
APIC indicates an illegal vector in its Error Status Register (see Section 8.5.3, “Error
Handling”). The IA-32 architecture reserves vectors 16 through 31 for predefined interrupts,
exceptions, and Intel-reserved encodings (see Table 5-1). However, the local APIC does not
treat vectors in this range as illegal.
When an illegal vector value (0
to 15) is written to an LVT entry and the delivery mode is Fixed
(bits 8-11
equal 0), the APIC may signal an illegal vector error, without regard to whether the
mask bit is set or whether an interrupt is actually seen on the input.