ForgeRTOS

ForgeRTOS is a small preemptive real-time operating system kernel for ARM Cortex-M4, developed from scratch in C and ARM Thumb assembly. The project is built around the STM32F446RE and focuses on understanding the low-level mechanisms behind an RTOS, including task contexts, exception handling, scheduling, synchronization, inter-process communication, timing, tracing, and kernel diagnostics.

Rather than relying on an existing RTOS or CMSIS-RTOS layer, ForgeRTOS implements its own kernel primitives and Cortex-M4 port while keeping architecture-specific code separated from the portable kernel logic.

Development Roadmap

• Bootstrap the bare-metal Cortex-M4 target with custom startup code, linker script, CMake build system, and GPIO test.
• Document and inspect the Cortex-M4 execution model, vector table, MSP, PSP, CONTROL, IPSR, and VTOR.
• Implement the MSP-to-PSP transition and inspect Cortex-M exception stacking with SVC.
• Configure the Cortex-M4 SysTick timer and establish a 1 ms hardware time base.
• Add the kernel tick layer and wrap-safe time API.
• Implement nested BASEPRI-based critical sections and kernel interrupt-priority rules.
• Add Cortex-M fault diagnostics for HardFault, MemManage, BusFault, and UsageFault exceptions.
• Define task control blocks, task states, priorities, stack ownership, and a fixed-capacity task registry.
• Initialize synthetic Cortex-M4 task contexts with software-saved registers and hardware exception frames.
• Add the first cooperative scheduler and start the highest-priority READY task through SVC exception return.
• Implement cooperative task yielding and full task-to-task context switching.
• Move context switching from SVC to PendSV.
• Preserve and restore R4-R11 through the PendSV context-switch path.
• Validate repeated task context switching and register preservation under stress.
• Enable preemptive scheduling using SysTick and PendSV.
• Implement fixed-priority scheduling.
• Add round-robin scheduling for tasks with equal priority.
• Add an idle task and defined CPU idle behavior.
• Implement task sleeping and the BLOCKED state.
• Add timeout and wake-up management for blocked tasks.
• Implement binary semaphores.
• Implement counting semaphores.
• Implement mutex synchronization.
• Add a reproducible priority-inversion test.
• Implement mutex priority inheritance.
• Add kernel message queues.
• Implement blocking queue send and receive operations.
• Add task event flags.
• Add timeout support to kernel synchronization and IPC objects.
• Add task stack watermark and overflow diagnostics.
• Add kernel assertions and runtime invariants.
• Implement kernel event tracing.
• Add a UART-based kernel monitor interface.
• Add RTOS stress tests covering tasks, semaphores, mutexes, queues, and timing.
• Prepare the final demo, benchmarks, architecture documentation, and the first ForgeRTOS release.