The way that Linux handles interrupts has changed quite a bit over the years, due to changes in design and in the hardware it works with. The PC's view of interrupts in the early days was quite simple; there were just 16 interrupt lines and one processor to deal with them. Modern hardware can have many more interrupts, and can also be equipped with fancy advanced programmable interrupt controllers (APICs), which can distribute interrupts across multiple processors in an intelligent (and programmable) way.