Nios® II Processor Reference Guide

ID 683836
Date 8/28/2023
Document Table of Contents

2.1. Processor Implementation

The functional units of the Nios II architecture form the foundation for the Nios II instruction set. However, this does not indicate that any unit is implemented in hardware. The Nios II architecture describes an instruction set, not a particular hardware implementation. A functional unit can be implemented in hardware, emulated in software, or omitted entirely.

A Nios II implementation is a set of design choices embodied by a particular Nios® II processor core. All implementations support the instruction set defined in the Instruction Set Reference chapter.

Each implementation achieves specific objectives, such as smaller core size or higher performance. This flexibility allows the Nios II architecture to adapt to different target applications.

Implementation variables generally fit one of three trade-off patterns: more or less of a feature; inclusion or exclusion of a feature; hardware implementation or software emulation of a feature. An example of each trade-off follows:

  • More or less of a feature—For example, to fine-tune performance, you can increase or decrease the amount of instruction cache memory. A larger cache increases execution speed of large programs, while a smaller cache conserves on-chip memory resources.
  • Inclusion or exclusion of a feature—For example, to reduce cost, you can choose to omit the JTAG debug module. This decision conserves on-chip logic and memory resources, but it eliminates the ability to use a software debugger to debug applications.
  • Hardware implementation or software emulation—For example, in control applications that rarely perform complex arithmetic, you can choose for the division instruction to be emulated in software. Removing the divide hardware conserves on-chip resources but increases the execution time of division operations.

For information about which Nios® II cores supports what features, refer to the Nios® II Core Implementation Details chapter of the Nios® II Processor Reference Handbook.

For complete details about user-selectable parameters for the Nios® II processor, refer to the Instantiating the Nios® II Processor chapter of the Nios® II Processor Reference Handbook.