Nios® V Processor Reference Manual

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ID 683632
Date 8/01/2022
Public

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Document Table of Contents
Document Table of Contents x
1. Overview 2. Nios® V/m Processor 3. Nios V Processor Reference Manual Archives 4. Document Revision History for the Nios® V Processor Reference Manual
2. Nios® V/m Processor x
2.1. Processor Performance Benchmarks 2.2. Processor Pipeline 2.3. Processor Architecture 2.4. Programming Model 2.5. Core Implementation
2.3. Processor Architecture x
2.3.1. General-Purpose Register File 2.3.2. Arithmetic Logic Unit 2.3.3. Reset and Debug Signals 2.3.4. Control and Status Registers 2.3.5. Exception Controller 2.3.6. Interrupt Controller 2.3.7. Memory and I/O Organization 2.3.8. RISC-V based Debug Module
2.3.6. Interrupt Controller x
2.3.6.1. Timer and Software Interrupt Module
2.3.7. Memory and I/O Organization x
2.3.7.1. Instruction and Data Buses
2.3.7.1. Instruction and Data Buses x
2.3.7.1.1. Instruction Manager Port 2.3.7.1.2. Data Manager Port 2.3.7.1.3. Address Map
2.3.8. RISC-V based Debug Module x
2.3.8.1. Debug Mode 2.3.8.2. Halt from Debug Module 2.3.8.3. Trigger 2.3.8.4. Abstract Commands in Debug Mode 2.3.8.5. Hardware/Software Interface
2.4. Programming Model x
2.4.1. Privilege Levels 2.4.2. Control and Status Registers (CSR) Mapping
2.4.1. Privilege Levels x
2.4.1.1. Machine Mode (M-mode) 2.4.1.2. Debug Mode (D-mode)
2.4.2. Control and Status Registers (CSR) Mapping x
2.4.2.1. Control and Status Register Field
2.5. Core Implementation x
2.5.1. Instruction Set Reference
1. Overview
2. Nios® V/m Processor
3. Nios V Processor Reference Manual Archives
4. Document Revision History for the Nios® V Processor Reference Manual

2.3.8.1. Debug Mode

You can enter the Debug Mode, as specified in the RISC-V architecture specification, in the following ways:

  1. Halt from Debug Module
  2. Software breakpoints
  3. Trigger
Upon entering Debug Mode, Nios® V processor completes the instruction in W-stage. By the order of priority, instruction in M-stage, E-stage, D-stage or F-stage takes the interrupt.

  • When there is a valid instruction, Program Counter writes to the Debug Program Counter, .dpc.

  • If the instruction in M-stage is not valid, then instruction in E-stage takes the interrupt and so on and so forth.

  • If there is no valid instruction in the pipeline, the Program Counter for the next instruction writes to the Debug Program Counter, .dpc.

Note: For branches, the next Program Counter depends on whether a branch was taken or not taken, and whether the branch prediction (if any) was correct.
Debug Module selects Hardware Thread (Hart); which can be in one of the following states:
  1. Non-existent: Debug Module probes a hart which does not exist.
  2. Unavailable: Reset or temporary shutdown.
  3. Running: Normal operation outside of debug.
  4. Halted: Hart is said to be halted when it is in debug mode.
Figure 3. Debug Module Block Diagram
Level Two Title