Nios II Classic Processor Reference Guide

ID 683620
Date 10/28/2016
Document Table of Contents Requested Register Set

If shadow register sets are implemented on the Nios II core, the EIC specifies a register set when it asserts an interrupt request. When it takes the interrupt, the Nios® II processor switches to the requested register set. When an interrupt has a dedicated register set, the interrupt handler avoids the overhead of saving registers.

The method of assigning register sets to interrupts depends on the specific EIC implementation. Register set assignments can be software-configurable.

Multiple interrupts can be configured to share a register set. In this case, the interrupt handlers must be written so as to avoid register corruption. For example, one of the following conditions must be true:

  • The interrupts cannot pre-empt one another. For example, all interrupts are at the same level.
  • Registers are saved in software. For example, each interrupt handler saves its own registers on entry, and restores them on exit.

Typically, the Nios® II processor is configured so that when it takes an interrupt, other interrupts in the same register set are disabled. If interrupt preemption within a register set is desired, the interrupt handler can re-enable interrupts in its register set.

By default, the Nios® II processor disables maskable interrupts when it takes an interrupt request. To enable nested interrupts, system software or the ISR itself must re-enable interrupts after the interrupt is taken.