Nios II Custom Instruction User Guide

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ID 683242
Date 4/14/2025
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Document Table of Contents
Document Table of Contents x
1. Nios II Custom Instruction Overview 2. Custom Instruction Hardware Interface 3. Custom Instruction Software Interface 4. Design Example: Cyclic Redundancy Check 5. Introduction to Nios® II Floating Point Custom Instructions 6. Nios II Floating Point Hardware 2 Component 7. Nios® II Floating Point Hardware (FPH1) Component 8. Document Revision History for Nios II Custom Instruction User Guide
1. Nios II Custom Instruction Overview x
1.1. Custom Instruction Implementation
1.1. Custom Instruction Implementation x
1.1.1. Custom Instruction Hardware Implementation 1.1.2. Custom Instruction Software Implementation
2. Custom Instruction Hardware Interface x
2.1. Custom Instruction Types
2.1. Custom Instruction Types x
2.1.1. Combinational Custom Instructions 2.1.2. Multicycle Custom Instructions 2.1.3. Extended Custom Instructions 2.1.4. Internal Register File Custom Instructions 2.1.5. External Interface Custom Instructions
2.1.1. Combinational Custom Instructions x
2.1.1.1. Combinational Custom Instruction Ports 2.1.1.2. Combinational Custom Instruction Timing
2.1.2. Multicycle Custom Instructions x
2.1.2.1. Multicycle Custom Instruction Ports 2.1.2.2. Multicycle Custom Instruction Timing
2.1.3. Extended Custom Instructions x
2.1.3.1. Extended Custom Instruction Timing
2.1.4. Internal Register File Custom Instructions x
2.1.4.1. Internal Register File Custom Instruction Example 2.1.4.2. Internal Register File Custom Instruction Ports
3. Custom Instruction Software Interface x
3.1. Custom Instruction Software Examples 3.2. Built-in Functions and User-defined Macros 3.3. Custom Instruction Assembly Language Interface
3.2. Built-in Functions and User-defined Macros x
3.2.1. Built-in Functions with No Return Value 3.2.2. Built-in Functions that Return a Value of Type Int 3.2.3. Built-in Functions that Return a Value of Type Float 3.2.4. Built-in Functions that Return a Pointer Value
3.3. Custom Instruction Assembly Language Interface x
3.3.1. Custom Instruction Assembly Language Syntax 3.3.2. Custom Instruction Assembly Language Examples 3.3.3. Custom Instruction Word Format
3.3.3. Custom Instruction Word Format x
3.3.3.1. Select Index Field (N)
4. Design Example: Cyclic Redundancy Check x
4.1. Building the CRC Example Hardware 4.2. Building the CRC Example Software
4.1. Building the CRC Example Hardware x
4.1.1. Setting up the Environment for the CRC Example Design 4.1.2. Opening the Component Editor 4.1.3. Specifying the Custom Instruction Component Type 4.1.4. Displaying the Custom Instruction Block Symbol 4.1.5. Adding the CRC Custom Instruction HDL Files 4.1.6. Configuring the Custom Instruction Parameter Type 4.1.7. Setting Up the CRC Custom Instruction Interfaces 4.1.8. Configuring the Custom Instruction Signal Type 4.1.9. Saving and Adding the CRC Custom Instruction 4.1.10. Generating and Compiling the CRC Example System
4.1.7. Setting Up the CRC Custom Instruction Interfaces x
4.1.7.1. Specifying Additional Interfaces
4.2. Building the CRC Example Software x
4.2.1. Running and Analyzing the CRC Example Software 4.2.2. Using the User-defined Custom Instruction Macro
4.2.1. Running and Analyzing the CRC Example Software x
4.2.1.1. Output of the CRC Design Example Software Run on a Cyclone V E FPGA Development Kit using the Quartus® Prime Software v15.1.
5. Introduction to Nios® II Floating Point Custom Instructions x
5.1. Floating Point Background 5.2. IEEE 754 Format 5.3. Rounding Schemes 5.4. Special Floating Point Cases
5.2. IEEE 754 Format x
5.2.1. Unit in the Last Place 5.2.2. Floating Point Value Encoding
5.3. Rounding Schemes x
5.3.1. Nearest Rounding 5.3.2. Truncation Rounding 5.3.3. Faithful Rounding 5.3.4. Rounding Examples
6. Nios II Floating Point Hardware 2 Component x
6.1. Overview of the Floating Point Hardware 2 Component 6.2. Floating Point Hardware 2 IEEE 754 Compliance 6.3. IEEE 754 Exception Conditions with FPH2 6.4. Floating Point Hardware 2 Operations 6.5. Building the FPH2 Example Hardware 6.6. Building the FPH2 Example Software 6.7. FPH2 Implementation of GCC Options 6.8. Nios II FPH2 and the Newlib Library 6.9. C Macros for round(), fmins(), and fmaxs()
6.6. Building the FPH2 Example Software x
6.6.1. FPH2 and Nios II GCC 6.6.2. Floating Point Hardware 2 Conversions 6.6.3. Nios II FPH2 Software Options
6.6.3. Nios II FPH2 Software Options x
6.6.3.1. -mcustom-<operation> 6.6.3.2. Nios II FPH2 Pragmas 6.6.3.3. -mcustom-fpu-cfg
6.7. FPH2 Implementation of GCC Options x
6.7.1. -fno-math-errno 6.7.2. -fsingle-precision-constant 6.7.3. -funsafe-math-optimizations 6.7.4. -ffinite-math-only 6.7.5. -fno-trapping-math 6.7.6. -frounding-math
7. Nios® II Floating Point Hardware (FPH1) Component x
7.1. Creating the FPH1 Example Hardware 7.2. Adding FPH1 to the Design and Configuring the Device 7.3. Building the FPH1 Example Software 7.4. Nios II FPH1 and the Newlib Library 7.5. Assessing Your Floating Point Optimization Needs 7.6. Hardware Divide Considerations with FPH1
7.3. Building the FPH1 Example Software x
7.3.1. Creating the FPH1 Software Project 7.3.2. Running and Analyzing the FPH1 Example Software 7.3.3. Software Implementation for FPH1
1. Nios II Custom Instruction Overview
2. Custom Instruction Hardware Interface
3. Custom Instruction Software Interface
4. Design Example: Cyclic Redundancy Check
5. Introduction to Nios® II Floating Point Custom Instructions
6. Nios II Floating Point Hardware 2 Component
7. Nios® II Floating Point Hardware (FPH1) Component
8. Document Revision History for Nios II Custom Instruction User Guide

6.7.2. -fsingle-precision-constant

From the GCC documentation: 
“Treat floating-point constants as single-precision constants instead of implicitly converting them to double-precision constants.”

For FPH2, the Nios II SBT omits -fsingle-precision-constant from the makefile GCC command line by default. This behavior contrasts with SBT support for FPH1, which sets this option with -mcustom-fpu-cfg. The SBT does not use -fsingle-precision-constant for FPH2 because it can cause problems for double-precision code.

You can enable -fsingle-precision-constant if you are sure it will not cause problems for your code. In general, it is better to cast floating point constants to the float type, or use the 'f' suffix (for example 3.14f), because these approaches are localized and independent of compiler options.

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