IDU is a simple multiplexer which routes N input IRQ lines to M attached CPU cores.
See chapter "26.10 Interrupt Distribution Unit" of ARC HS databook.

Note:

• It might be used on UP (Uni-Processor) systems as well, in that corner case we're just forwarding M inputs to M outputs. So implementation and testing might be done on simple UP QEMU at any point.
• It might not be obvious, but IRQ lines out of IDU are connected to ARC cores starting from IRQ24, i. e. IDU's IRQ line 0 is wired to CPU's IRQ line 24. As of today this is hard-coded setting and so we may just implement it in that simple way.