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