Nios II Custom Instruction User Guide

ID 683242
Date 4/27/2020
Document Table of Contents
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2.1.4. Internal Register File Custom Instructions

The Nios II processor allows custom instruction logic to access its own internal register file.

Internal register file access gives you the flexibility to specify whether the custom instruction reads its operands from the Nios II processor’s register file or from the custom instruction’s own internal register file. In addition, a custom instruction can write its results to the local register file rather than to the Nios II processor’s register file.

Custom instructions containing internal register files use readra, readrb, and writerc signals to determine if the custom instruction should use the internal register file or the dataa, datab, and result signals. Ports a, b, and c specify the internal registers from which to read or to which to write. For example, if readra is deasserted (specifying a read operation from the internal register), the a signal value provides the register number in the internal register file. Ports a, b, and c are five bits each, allowing you to address as many as 32 registers.