F9 Microkernel Brings L4 Architecture to ARM Cortex-M Devices

F9 targets ARM Cortex-M hardware

The F9 microkernel provides a hard real-time operating system environment for ARM Cortex-M processors based on the L4 architecture. It brings high-end microkernel principles like address spaces and Inter-Process Communication (IPC) to resource-constrained embedded devices.

Developers designed F9 to handle tasks that require strict determinism and hardware-level security. It specifically targets the Cortex-M3, M4, and M4F architectures, utilizing the hardware features found in modern microcontrollers. The kernel remains small by offloading complex services to user space while maintaining core control over hardware resources.

The system follows the L4 family design philosophy, drawing inspiration from seL4 and L4Ka::Pistachio. It balances the minimal footprint required for embedded chips with the advanced isolation features usually reserved for larger systems. This makes it suitable for industrial applications where failure is not an option.

Predictable scheduling for embedded chips

F9 uses a Priority Bitmap Scheduler to ensure constant-time thread selection. This O(1) dispatcher supports 32 distinct priority levels, allowing the system to find the next task to run without searching through long lists. The scheduler remains efficient regardless of how many threads the developer creates.

The kernel implements Preemption-Threshold Scheduling (PTS) to reduce unnecessary context switches. This feature, popularized by ThreadX, allows a thread to disable preemption for specific priority ranges during critical sections. It minimizes overhead while guaranteeing that high-priority interrupts still receive immediate attention.

To prevent performance bottlenecks, F9 includes several specialized scheduling protocols:

• Priority Inheritance Protocol: This automatically boosts the priority of lower-tier threads holding shared resources to prevent priority inversion.
• Round-Robin Scheduling: The kernel ensures fair execution time for multiple threads operating at the same priority level.
• Tickless Operation: The system only wakes the processor for scheduled events or hardware interrupts, significantly extending battery life in energy-sensitive deployments.
• Lazy Context Switching: For Cortex-M4F chips, the kernel only saves floating-point registers when a different thread actually uses the FPU.

Memory protection through hardware isolation

F9 relies on the ARM Memory Protection Unit (MPU) to enforce security boundaries between different parts of the system. It partitions memory into 8 hardware-protected regions, preventing a bug in one driver from crashing the entire kernel. This hardware-level isolation is a core requirement for secure embedded applications.

The kernel manages memory using Flexible Pages, which are power-of-2 aligned regions mapped directly to the MPU. These pages form the basis of isolated Address Spaces. Developers can manipulate these spaces using Grant, Map, and Flush operations to share memory between processes safely.

F9 also utilizes Physical Memory Pools to manage specific hardware attributes. This allows the system to distinguish between different types of RAM or flash memory on a single chip. By categorizing memory into pools, the kernel ensures that sensitive data stays within designated high-security or high-speed areas.

Fast communication via synchronous IPC

The kernel uses a Synchronous IPC model for all communication between isolated components. This L4-style message passing uses blocking semantics to ensure that data transfer is both predictable and fast. It eliminates the need for complex buffering schemes that can lead to memory exhaustion.

For small data transfers, F9 supports Short IPC, which passes data exclusively through CPU registers. It uses registers MR0 through MR7 to send payloads without touching main memory. This method provides the lowest possible latency for simple signals or status updates between threads.

Larger payloads move through User-level Thread Control Blocks (UTCBs). These blocks are always mapped in memory to provide fast access to system call arguments and complex messages. The kernel copies data directly between the UTCBs of the sender and receiver, maintaining a high throughput for data-heavy operations.

Standard APIs and developer tools

F9 exposes a native system call interface derived from the L4 family. This API allows for fine-grained control over the system's core primitives. Developers use these calls to manage the lifecycle of threads and the distribution of memory pages.

The native system calls include:

• L4_Ipc: Handles all message passing and synchronization between threads.
• L4_ThreadControl: Manages the creation, deletion, and configuration of execution units.
• L4_SpaceControl: Defines the boundaries and permissions of memory address spaces.
• L4_Schedule: Adjusts thread priorities and time slices dynamically.
• L4_ExchangeRegisters: Allows for direct manipulation of thread states for debugging or context management.

For better portability, F9 includes a POSIX compatibility layer implemented entirely in user space. This layer supports PSE51 and PSE52 profiles, allowing developers to run standard real-time code without kernel modifications. While most threading and clock functions are fully operational, the POSIX timer functions currently have limited support.

Advanced debugging and instrumentation

The kernel includes KDB, an in-kernel debugger that provides deep visibility into the system state. Developers can inspect thread statuses, memory maps, and active timers directly from a serial console. This tool is essential for diagnosing complex timing issues in real-time environments.

F9 also features KProbes, a dynamic instrumentation tool inspired by the Linux kernel. KProbes allows developers to break into any kernel routine and collect debugging information without recompiling the code. This enables real-time profiling of uptime, stack usage, and memory fragmentation while the system is running.

To ensure reliability, the project includes an automated test suite integrated with QEMU. This allows for regression testing and continuous integration without requiring physical hardware for every build. The QEMU environment emulates the Netduino Plus 2 board to provide a consistent platform for software verification.

Hardware support and build system

F9 officially supports several popular development boards based on the STMicroelectronics STM32F4 series. These boards are widely used in the maker community and industrial prototyping. The kernel utilizes the Nested Vectored Interrupt Controller (NVIC) for low-latency interrupt handling across all supported devices.

The current hardware compatibility list includes:

• STM32F4DISCOVERY: Featuring the STM32F407VG microcontroller.
• STM32F429I-DISC1: A high-performance board with the STM32F429ZI chip.
• NUCLEO-F429ZI: A standard development platform for industrial testing.
• Netduino Plus 2: Supported primarily through QEMU for automated testing.

The build system uses Kconfiglib, following the same configuration style as the Linux kernel. Developers run make config to open a menu-driven interface for enabling or disabling kernel features. This system manages complex dependencies, such as enabling FPU support or the KDB debugger, ensuring that the resulting binary fits within the target device's flash memory.

F9 is released under the two-clause BSD License, allowing for both open-source and commercial use. This permissive licensing makes it an attractive option for companies building proprietary hardware that requires a verified, high-performance microkernel foundation.