Multiprocessing: Functions and Features

The control of interrupts in a SMP system is done by so called APICs (advanced programmable interrupt controller). APICs are a main part of a multiprocessor platform and ensure dynamic distribution of interrupts to single processors. In this way a balanced distribution of interrupts is achieved. A SMP system features two types of APICs. First, there are local APICs, which are integrated in a CPU. Second, there are so called I/O-APICs which take care of external interrupts of peripheral building blocks. Usually they are located in a SMP-capable chipset or in an additional block. Besides other functions, local APICs forward inter-processor interrupts, for example when a processor sends an interrupt to another processor. For example, this is an advantage for control signals.

APICs communicate to each other through the so called ICC bus (interrupt controller communication) and share in this way upcoming controller functions. Since all interrupt messages pass the ICC bus, the memory bus does not receive additional load. This connection also allows a processor to share processing of interrupts with another CPU, which leads to a balanced load.