Nios II Custom Instruction User Guide

ID 683242
Date 4/27/2020
Document Table of Contents

3.3.3. Custom Instruction Word Format

Custom instructions are R-type instructions.

The instruction word specifies the 8-bit custom instruction selection index and register usage.

Figure 10. Custom Instruction Word Format
Table 5.  Custom Instruction Fields
Field Name Purpose Corresponding Signal
A Register address of input operand A  
B Register address of input operand B  
C Register address of output operand C  
readra Register file selector for input operand A readra
readrb Register file selector for input operand B readrb
writerc Register file selector for ouput operand C writerc
N Custom instruction select index (optionally includes an extension index)  
OP custom opcode, 0x32 n/a

The register file selectors determine whether the custom instruction component accesses Nios II processor registers or custom registers, as follows:

Table 6.  Register File Selection
Register File Selector Value Register File
0 Custom instruction component internal register file
1 Nios II processor register file