All Novos environments use the same two-level interrupt servicing model. The higher level is called the Immediate Interrupt Handler (IIH) and its purpose is to service the physical device that is causing the interrupt in order to clear it. Other than invoking the lower level of the ISR, the Deferred Interrupt Handler (DIH), the IIH does not involve and Novos services. The DIH is a Foreground execution entity with a priority higher than those of EARs and FG tasks. Multiple DIH priority levels are supported. The DIH can perform any operation it needs and invoke any supported Novos service. There is no restriction on its runtime. The DIH is interruptible and pre-emptible by higher priority DIHs.