in interrupt controllers in struct pic, and try to keep as much
common code as possible. At the lowest (asm) level, this is done
with CPP macros.
The main structure is now struct intrsource, describing an established
interrupt line, of any kind (soft/hard local apic/legacy apic/IO apic).
For quick masking, there may be a maximum of 32 sources per CPU.
Sources can be assigned to any CPU in the MP case, though currently they
all go to the boot CPU.
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