Nios II Classic Processor Reference Guide

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

3.3. Memory Protection Unit

The Nios® II processor provides an MPU for operating systems and runtime environments that desire memory protection but do not require virtual memory management. For information about memory protection with virtual memory management, refer to the Memory Management Unit section.

When present and enabled, the MPU monitors all Nios II instruction fetches and data memory accesses to protect against errant software execution. The MPU is a hardware facility that system software uses to define memory regions and their associated access permissions. The MPU triggers an exception if software attempts to access a memory region in violation of its permissions, allowing you to intervene and handle the exception as appropriate. The precise exception effectively prevents the illegal access to memory.

The MPU extends the Nios® II processor to support user mode and supervisor mode. Typically, system software runs in supervisor mode and end-user applications run in user mode, although all software can run in supervisor mode if desired. System software defines which MPU regions belong to supervisor mode and which belong to user mode.

MPU protects user application. Therefore for interrupt service, the system must have access to the regions that may cause potential violation because MPU generates exception post access and does not prevent access to the memory region in hardware.