Nios II Classic Processor Reference Guide

ID 683620
Date 10/28/2016
Document Table of Contents

3.1.2. User Mode

User mode is available only when the Nios® II processor in your hardware design includes an MMU or MPU. User mode exists solely to support operating systems. Operating systems (that make use of the processor’s user mode) run your application code in user mode. The user mode capabilities of the processor are a subset of the supervisor mode capabilities. Only a subset of the instruction set is available in user mode.

The operating system determines which memory addresses are accessible to user mode applications. Attempts by user mode applications to access memory locations without user access enabled are not permitted and cause an exception. Code running in user mode uses system calls to make requests to the operating system to perform I/O operations, manage memory, and access other system functionality in the supervisor memory.

The Nios II MMU statically divides the 32-bit virtual address space into user and supervisor partitions. Refer to Address Space and Memory Partitions section for more information about the MMU memory partitions. The MMU provides operating systems access permissions on a per-page basis. Refer to Virtual Addressing for more information about MMU pages.

The Nios II MPU supervisor and user memory divisions are determined by the operating system or runtime environment. The MPU provides user access permissions on a region basis. Refer to Memory Regions for more information about MPU regions.