Nios II Classic Processor Reference Guide

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

3.6.2. Handling ECC Errors

ECC error exceptions occur when unrecoverable ECC errors are detected. The software’s ability to recover from the ECC error depends on the nature of the error.

Typically, software can recover from an unrecoverable MMU TLB ECC error (2 bit error) because the TLB is a software-managed cache of the operating system page tables stored in the main memory (e.g., SDRAM). Software can invalid the TLB entry, return to the instruction that took the ECC error exception, and execute the TLB’s mishandled code to load a TLB entry from the page tables.

In general, software cannot recover from a register file ECC error (2 bit error) because the correct value of a register is not known. If the exception handler reads a register that has a 2 bit ECC error associated with it, another ECC error occurs and an exception handler loop can occur.

Exception handler loops occur when an ECC error exception occurs in the exception handler before it is ready to handle nested exceptions. To minimize the occurrence or exception handler loops, locate the ECC error exception handler code in normal cacheable memory, ensure that all data accesses are to non-cacheable memory, and minimize register reading.

The ECC error signals (ecc_event_bus) provide the EEH signal for external logic to detect a possible exception handler loop and reset the Nios® II processor.