Does cortex M series have MMU?

The short answer is yes, the Cortex-M series of ARM processor cores do have an MMU (Memory Management Unit). The MMU allows Cortex-M cores to support memory protection, virtual memory, and other advanced memory management features.

Overview of the Cortex-M Series

The Cortex-M series is ARM’s family of 32-bit RISC processor cores designed for embedded and IoT applications. The Cortex-M cores are optimized for low power consumption and low cost, making them popular for use in microcontrollers, sensors, wearables, and other resource-constrained devices.

There have been multiple generations of Cortex-M cores over the years. The most widely used ones are:

• Cortex-M0/M0+ – Baseline MCU cores for ultra low power applications
• Cortex-M3 – Adds DSP extensions and improved performance
• Cortex-M4 – Adds single precision floating point unit
• Cortex-M7 – Adds double precision floating point unit
• Cortex-M23/M33 – Adds security extensions

All Cortex-M cores since the M0 have included an MMU, with some differences in capabilities between the generations. The MMU provides critical memory management features for Cortex-M devices.

MMU in Cortex-M

The key functions provided by the Cortex-M MMU are:

• Memory protection – The MMU allows assigning memory regions with different permissions like read-only, no access, privileged access only etc. This prevents corruption of code and data.
• Memory virtualization – The MMU enables mapping virtual addresses used by software to physical addresses in memory. This provides isolation between software modules.
• Memory attributes – The MMU allows configuring attributes like cacheability, bufferability for memory regions. This improves performance.

The MMU in Cortex-M cores is a relatively simple fixed-mapping unit optimized for low overhead compared to MMUs in high-end application processors. But it provides adequate functionality for most embedded use cases.

MMU Architecture in Cortex-M

The MMU in Cortex-M cores typically consists of:

• A Translation Lookaside Buffer (TLB) cache that caches recent translations from virtual to physical addresses.
• Memory Protection Unit (MPU) which holds the defined memory regions and their permission attributes.
• Logic that walks page tables in memory if a translation is not found in the TLB.
• Logic to update the TLB, manage faults, and control MMU behavior.

The MMU is integrated into the processor pipeline and memory interconnect to perform address translations and permissions checks seamlessly. The translation table format is standardized by ARM across Cortex-M generations for software compatibility.

MMU Usage in Cortex-M

Using the MMU in a Cortex-M based microcontroller involves the following steps:

1. The memory map is defined with regions for code, data, peripherals, etc.
2. Access permissions like read-write, execute are set up for each region using the MPU.
3. Optional memory attributes like cacheability and bufferability are configured.
4. A translation table is set up mapping virtual addresses to physical addresses.
5. The MMU is enabled and mappings are loaded into the TLB.

During operation, the processor generates virtual addresses which are translated by the MMU to physical addresses. Any permission violations or invalid mappings will raise exceptions that can be handled by software.

The translation table format can be simple single-level page tables for smaller address spaces, or multi-level page tables for larger 32-bit address spaces in M cores like Cortex-M7.

Benefits of the Cortex-M MMU

Here are some of the benefits provided by having an MMU in Cortex-M cores:

• Memory protection – Prevents corruption of code by errant writes. Makes systems more robust.
• Memory virtualization – Allows software isolation and partitioning into processes or tasks.
• Data sharing – Virtual memory regions can be shared between software modules.
• Address space control – Full 32-bit address space can be utilized even with smaller physical memory.
• Performance – Cacheability/bufferability attributes optimize memory access speeds.
• Real-time support – Improves determinism by partitioning memory resources.

These capabilities allow developers to build more advanced and robust embedded applications using Cortex-M cores. The MMU unlocks features traditionally only found in higher-end application processors.

Cortex-M Generations and MMU Capabilities

Different generations of the Cortex-M cores have incrementally improved MMU capabilities:

Cortex-M0/M0+

• No MMU. Only simplified memory protection through the Protected Memory System Architecture (PMSAv7).

Cortex-M3

• Basic MMU with a TLB and configurable memory regions.
• Supports memory protection and attributes.
• Limited address space virtualization.

Cortex-M4

• Enhanced MMU with larger TLB and more memory regions.
• Improved background table walk logic.
• Larger virtual address spaces supported.

Cortex-M7

• MMU with dual TLBs – instruction and data.
• Full 4GB virtual address space via multi-level page tables.
• Memory tagging and lockdown capabilities.

Cortex-M23/M33

• Secure MMU with TrustZone extensions.
• Virtualization support through Armv8-M architecture.
• Hardware memory tag checking for enhanced security.

So in summary, MMU capabilities have grown across Cortex-M generations, but all cores since the M3 have basic MMU functionality.

Use Cases Benefiting from the MMU

Here are some examples of Cortex-M use cases that can benefit from utilizing the MMU:

• Real-time control systems – The MMU provides memory partitioning to isolate tasks and meet real-time requirements.
• IoT edge devices – The MMU enables secure separation between application code and device firmware.
• Neural network accelerators – Concurrent data sharing between processors is enabled using shared virtual memory.
• Automotive systems – Hardware enforced memory protection improves safety and security.
• RTOS platforms – The MMU allows efficient implementations of demanding RTOS features.

Any Cortex-M application requiring stronger memory protection, hardware isolation, virtual addressing, or advanced memory use cases can benefit from utilizing the MMU.

Conclusion

In summary, all generations of Cortex-M processor cores, apart from the very first Cortex-M0/M0+, include a Memory Management Unit to support memory protection, virtual memory, and advanced attributes required in modern embedded applications. The capabilities of the MMU have improved with each core generation. Utilizing the Cortex-M MMU unlocks a whole range of features such as robustness, security, real-time performance, isolation, and efficient memory utilization that benefit many advanced embedded systems.