Nios® V Processor Reference Manual

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ID 683632
Date 5/26/2023
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Document Table of Contents
Document Table of Contents x
1. Overview 2. Nios® V/m Processor 3. Nios® V/g Processor 4. Nios V Processor Reference Manual Archives 5. Document Revision History for the Nios® V Processor Reference Manual
1. Overview x
1.1. Intel® Quartus® Prime Software Support
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 2.3.7.2. Address Map
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
2.3.8. 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
3. Nios® V/g Processor x
3.1. Processor Performance Benchmarks 3.2. Processor Pipeline 3.3. Processor Architecture 3.4. Programming Model 3.5. Core Implementation
3.3. Processor Architecture x
3.3.1. General-Purpose Register File 3.3.2. Arithmetic Logic Unit 3.3.3. Multipy and Divide Units 3.3.4. Custom Instruction 3.3.5. Reset and Debug Signals 3.3.6. Control and Status Registers 3.3.7. Exception Controller 3.3.8. Interrupt Controller 3.3.9. Memory and I/O Organization 3.3.10. RISC-V based Debug Module
3.3.8. Interrupt Controller x
3.3.8.1. Timer and Software Interrupt Module
3.3.9. Memory and I/O Organization x
3.3.9.1. Instruction and Data Buses 3.3.9.2. Address Map 3.3.9.3. Cache Memory
3.3.9.1. Instruction and Data Buses x
3.3.9.1.1. Instruction Manager Port 3.3.9.1.2. Data Manager Port
3.3.9.3. Cache Memory x
3.3.9.3.1. Instruction Cache 3.3.9.3.2. Data Cache 3.3.9.3.3. Configurable Cache Memory 3.3.9.3.4. Bypassing Cache (Peripheral Region) 3.3.9.3.5. Using Cache Memory Effectively
3.3.10. RISC-V based Debug Module x
3.3.10.1. Debug Mode 3.3.10.2. Halt from Debug Module 3.3.10.3. Trigger 3.3.10.4. Abstract Commands in Debug Mode 3.3.10.5. Hardware/Software Interface
3.4. Programming Model x
3.4.1. Privilege Levels 3.4.2. Control and Status Registers (CSR) Mapping
3.4.1. Privilege Levels x
3.4.1.1. Machine Mode (M-mode) 3.4.1.2. Debug Mode (D-mode)
3.4.2. Control and Status Registers (CSR) Mapping x
3.4.2.1. Control and Status Register Field
3.5. Core Implementation x
3.5.1. Instruction Set Reference
1. Overview
2. Nios® V/m Processor
3. Nios® V/g Processor
4. Nios V Processor Reference Manual Archives
5. Document Revision History for the Nios® V Processor Reference Manual

3.3.9.3.5. Using Cache Memory Effectively

The effectiveness of cache memory to improve performance is based on the following conditions:
  • Regular memory is located off-chip and has a longer access time than on-chip memory.
  • The largest, performance-critical instruction loop is smaller than the instruction cache.
  • The largest block of performance-critical data is smaller than the data cache.

The optimal cache configuration is application-specific, but you can define a configuration that works for a variety of applications. Refer to the following examples:

  • If a Nios® V/g processor system only has fast on-chip memory and never accesses slow off-chip memory, an instruction or data cache is unlikely to boost the performance.
  • If a program's critical loop is 2 KB but the instruction cache is 1 KB, an instruction cache does not improve execution speed. In this case, an instruction cache can actually degrade performance.

Mixing cached and uncached accesses to the same cache line can result in invalid data reads. For example, the following sequences of events causes cache incoherency.

  1. Nios® V processor writes data to cache, creating a dirty data cache line.
  2. Nios® V processor reads data from the same address, but bypassed the cache.

If it is necessary to mix cached and uncached data accesses, flush the corresponding line of data cache after completing the cached accesses and before performing the uncached accesses.

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