Implementing Memory Protection Unit Boundaries

Configure the MPU to isolate critical tasks from non-critical processes, preventing memory corruption from propagating across the entire system.

In this lesson, you will learn how to configure your system’s Memory Protection Unit (MPU) to act as a physical sandbox, preventing non-critical code from overwriting sensitive task memory.

The Logic of Isolation

In a safety-critical system, you cannot assume that all software components are equally trustworthy. If a low-priority logging task suffers a buffer overflow, it shouldn't be allowed to corrupt the stack of a high-priority motor control task. The MPU is a hardware feature that segments your address space into specific regions, assigning each a permission set (Read, Write, Execute).

By defining these boundaries, the hardware monitors every bus transaction. If a task attempts to access an address outside its assigned "sandbox"—such as writing to a kernel register or another task's stack—the MPU triggers a MemManage fault immediately, halting the illegal operation before the corruption can propagate.

Practical Configuration

Think of your memory map as a partitioned floor plan. You define these partitions using start addresses and size masks. For instance, you might configure an MPU region to protect a shared memory buffer from unauthorized modification:

// Example: Restricting a task to Read-Only access for a configuration table
MPU_Region_Config config = {
    .BaseAddress = 0x20001000,
    .Size        = MPU_SIZE_256B,
    .AccessPermission = MPU_AP_READ_ONLY,
    .ExecuteNever    = true,
    .Enable          = true
};

MPU_ConfigureRegion(&config);

By enforcing MPU_AP_READ_ONLY on critical configuration data, any coding error resulting in an accidental pointer write will cause an immediate hardware exception. While this might trigger a system reset or a graceful degradation routine (as we discussed in previous lessons), it is far safer than allowing the system to continue operation in a corrupted, unpredictable state.

This hardware-level enforcement complements your static analysis and defensive pointer arithmetic by providing a runtime "safety net" that is immune to software bugs. Even if a MISRA C violation slips through, the MPU ensures the failure remains localized rather than systemic.

Key Takeaway: The MPU provides hardware-enforced compartmentalization, ensuring that memory corruption remains isolated to the specific task where the fault originated, thereby preserving the integrity of the overall system.