Memory Partitioning & Protection (MPU) - AUTOSAR Expert

OsApplication Memory Regions

Each non-trusted OsApplication has dedicated linker-section-backed memory regions configured in the AUTOSAR OS. The Cortex-M MPU is reprogrammed on every context switch to enforce the current task's OsApplication boundaries.

XMLOs_MemoryRegions.arxml


  OsApp_EPS_Safety
  FALSE
  
    
    
      EPS_Data_RW
      
        OS-APPLICATION-MEMORY-REGION-ACCESS-TYPE-RW
      
      
        __EPS_DATA_START__
      
      0x400
    
    
    
      EPS_Code_RX
      
        OS-APPLICATION-MEMORY-REGION-ACCESS-TYPE-RX
      
      __EPS_CODE_START__
      0x8000

Trusted vs Non-Trusted OsApplication

Property	Trusted OsApplication	Non-Trusted OsApplication
Execution mode	Privileged (full register + peripheral access)	Unprivileged (MPU enforces region boundaries)
Memory access	All SRAM and peripheral registers	Only regions in OsApplicationMemoryRegions list
MPU reconfiguration	None on context switch (privileged = no restrictions)	MPU reprogrammed on every context switch to this OsApp
Typical users	BSW modules (CanIf, COM, NvM, DCM)	Safety SWCs, ASIL-D runnables
OsProtectionHook trigger	No (privileged access never faults MPU)	Yes — any access outside configured regions triggers hook

💡 BSW is Trusted for Performance

BSW modules run as trusted (privileged) because they access MCAL registers, require unrestricted DMA setup, and are already safety-qualified by the supplier. Making BSW trusted avoids the context-switch overhead of reprogramming MPU regions for every BSW main function call. Safety properties of BSW are assured by supplier qualification, not MPU enforcement.

Stack Overflow Guard Region

A dedicated MPU region of minimum 32 bytes is placed immediately below each task's stack. A stack overflow writes into this guard region, triggering an MPU fault before corrupting adjacent data.

XMLOs_StackGuard.arxml



  EPS_StackGuard
  
    
    OS-APPLICATION-MEMORY-REGION-ACCESS-TYPE-NONE
  
  
    __TASK_SAFETYCTRL_STACK_GUARD_START__
  
  0x20

COsProtectionHook_Handler.c

FUNC(ProtectionReturnType, OS_APPL_CODE) OsProtectionHook(
    StatusType FatalError)
{
    switch (FatalError) {
        case E_OS_PROTECTION_MEMORY:
            /* MPU fault: log offending task and address */
            DEM_LOG_PROTECTION_FAULT(GetTaskID_Current(), GetFaultAddress());
            /* Restart only the affected OsApplication */
            return PRO_TERMINATEAPPL_RESTART;
        case E_OS_PROTECTION_TIME:
            /* Timing budget exceeded */
            return PRO_TERMINATETASKISR;
        default:
            return PRO_SHUTDOWN;
    }
}

MPU Region Budget & Context-Switch Overhead

Cortex-M4/M7 supports 8 or 16 MPU regions total. AUTOSAR OS uses some regions for OS-internal data (typically 2–3), leaving 5–13 for OsApplication regions.

Cortex-M Variant	Total MPU Regions	OS Internal Regions	Available for OsApps
Cortex-M4	8	3 (OS code, OS data, shared SRAM)	5
Cortex-M7	16	3	13
Cortex-R5 (dual core)	16 per core	3 per core	13 per core

⚠️ Region Pooling Overhead

When there are more OsApplications than available MPU regions, the AUTOSAR OS must use a region pooling strategy: it loads only the regions for the currently active OsApplication and flushes the others. Each context switch requires multiple MPU region register writes (~5–15 ns per register on Cortex-M7 at 400 MHz). With 10+ OsApplications on a Cortex-M4 (only 5 user regions), context-switch overhead can reach 1–2 µs — significant on 1 ms tasks. Design the OsApplication count to fit within the hardware region budget.

Summary

MPU-based memory partitioning is the hardware enforcement mechanism that makes ASIL decomposition by OsApplication valid for ISO 26262. The combination of non-trusted OsApplication + correctly sized memory regions + stack guard page provides spatial FFI. Context-switch overhead from MPU reprogramming must be measured and included in task WCET budgets, especially on Cortex-M4 with only 8 MPU regions.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.