Free translation from osdev.ru forum: http://osdev.ru/viewtopic.php?f=7&t=542&start=10
220.127.116.11 Platforms with APIC and Dual 8259 Support писал(а):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 18.104.22.168, “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).
In Bochs 2.5 as in may other systems, PIT is connected to entry 0 of 8259 (ISA IRQ 0) but not to entry 0 of I/O APIC but to entry 2 instead.
According to ACPI, in this case in the MADT table must be entry "Interrupt Source Override" which tell that IRQ ISA 0 is connected to entry 2 of I/O APIC. In real hardware this entry looks like:
[0A4h 0164 1] Subtable Type : 02
[0A5h 0165 1] Length : 0A
[0A6h 0166 1] Bus : 00
[0A7h 0167 1] Source : 00
[0A8h 0168 4] Interrupt : 00000002
[0ACh 0172 2] Flags (decoded below) : 0000
Polarity : 0
Trigger Mode : 0
In Bochs BIOS this table in missed.