Você encontrará mais informações neste artigo: Por que interromper a afinidade com múltiplos núcleos não é uma coisa tão boa
Every x86 motherboard has a chip called IO-APIC. This is a device that controls interrupt delivery within your system. It knows how many CPUs are in your system and can direct various interrupts to various CPUs. It uses so called local APIC-ID as an identifier of the processor.
It has two modes of operation. In one mode it sends interrupts from certain device to single, predefined core. This mode of operation called fixed/physical mode. In another mode, it can deliver interrupts from certain device to multiple cores. The later mode called logical/low priority interrupt delivery mode.
Veja também esta patente da IBM: Interromper o método de envio em ambiente multi-core e processador multi-core