Nios II Custom Instruction User Guide

ID 683242
Date 4/27/2020
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2.1.2. Multicycle Custom Instructions

Multicycle (sequential) custom instructions consist of a logic block that requires two or more clock cycles to complete an operation.
Figure 5. Multicycle Custom Instruction Block Diagram

Multicycle custom instructions complete in either a fixed or variable number of clock cycles. For a custom instruction that completes in a fixed number of clock cycles, you specify the required number of clock cycles at system generation. For a custom instruction that requires a variable number of clock cycles, you instantiate the start and done ports. These ports participate in a handshaking scheme to determine when the custom instruction execution is complete.

A basic multicycle custom instruction block, with the required ports shown in "Custom Instruction Types", implements a single custom operation. This operation has a selection index determined when the instruction is instantiated in the system using Platform Designer.

You can further optimize multicycle custom instructions by implementing the extended internal register file, or by creating external interface custom instructions.