Linux/init IRQ()
From XOmB wiki
Defined for x86_64 in: /arch/x86_64/kernel/i8259.c
[edit] Purpose
Used to interface with the PIC 8259 controller.
It seems they set up IRQ0 even when they intend to init the IO-APIC. Therefore, they really wish to use the PIC timer? Probably to calibrate something else.