The Local APIC uses two register values to determine in what order to dispatch user-defined interrupts to the processor's core:
The Task Priority Register (TPR).
The Processor Priority Register (PPR).
The TPR is written to by the OS while the PPR derives its contents from the TPR and, if a user-defined interrupt is currently being serviced, the priority of that interrupt. In order to determine the eligibility of a user-defined interrupt to interrupt the executing program or the currently executing user-defined interrupt handler, the Local APIC compares the user-defined interrupt's priority to the processor's priority. The following sections describe the TPR, the PPR and the eligibility test.