Intel® Pentium® Silver and Intel® Celeron® Processors Datasheet, Volume 1


ID 633935
Date 17/06/2021 00:00:00
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Interrupt Mapping

The interrupts associated with the various timers have several interrupt mapping options. When reprogramming the HPET interrupt routing scheme (LEG_​RT_​CNF bit in the General Config Register), a spurious interrupt may occur. This is because the other source of the interrupt (8254 timer) may be asserted. Software should mask interrupts prior to clearing the LEG_​RT_​CNF bit.

Mapping Option #1 (Legacy Replacement Option)

In this case, the Legacy Replacement Rout bit (LEG_​RT_​CNF) is set. This forces the mapping found in the following table.

Legacy Replacement Routing


8259 Mapping

APIC Mapping





In this case, the 8254 timer will not cause any interrupts




In this case, the RTC will not cause any interrupts.

2 and 3

Per IRQ Routing Field.

Per IRQ Routing Field

4, 5, 6, 7

Not available

Not available

Note:The Legacy Option does not preclude delivery of IRQ0/IRQ8 using processor interrupts messages.

Mapping Option #2 (Standard Option)

In this case, the Legacy Replacement Rout bit (LEG_​RT_​CNF) is 0. Each timer has its own routing control. The interrupts can be routed to various interrupts in the 8259 or I/O APIC. A capabilities field indicates which interrupts are valid options for routing. If a timer is set for edge-triggered mode, the timers should not be shared with any legacy interrupts.

For the PCH, the only supported interrupt values are as follows:

Timer 0 and 1: IRQ20, 21, 22, and 23 (I/O APIC only).

Timer 2: IRQ11 (8259 or I/O APIC) and IRQ20, 21, 22, and 23 (I/O APIC only).

Timer 3: IRQ12 (8259 or I/O APIC) and IRQ 20, 21, 22, and 23 (I/O APIC only).

Note:Interrupts from Timer 4, 5, 6, 7 can only be delivered via direct FSB interrupt messages. Note:System architecture changes since the HPET specification 1.0 was released have made some of the terminology used obsolete. In particular the reference to a Front Side Bus (FSB) has no relevance to current platforms, as this interface is no longer in use. For consistency with the HPET specification though, the FSB and specifically the FSB Interrupt Delivery terminology has been maintained. Where the specification refers to FSB, this should be read as ‘processor message interface’; independent of the physical attach mechanism.

Mapping Option #3 (Processor Message Option)

In this case, the interrupts are mapped directly to processor messages without going to the 8259 or I/O (x) APIC. To use this mode, the interrupt must be configured to edge-triggered mode. The Tn_​PROCMSG_​EN_​CNF bit must be set to enable this mode.

When the interrupt is delivered to the processor, the message is delivered to the address indicated in the Tn_​PROCMSG_​INT_​ADDR field. The data value for the write cycle is specified in the Tn_​PROCMSG_​INT_​VAL field.

Note:The FSB interrupt deliver option has HIGHER priority and is mutually exclusive to the standard interrupt delivery option. Thus, if the TIMERn_​FSB_​EN_​CNF bit is set, the interrupts will be delivered via the FSB, rather than via the APIC or 8259.

The FSB interrupt delivery can be used even when the legacy mapping is used.

For the Intel PCH HPET implementation, the direct FSB interrupt delivery mode is supported, besides via 8259 or I/O APIC.