Why Build a Custom RTOS in 2026?
Why build a custom RTOS when there are already tons of battle-tested ones like FreeRTOS or Zephyr available?
It’s a fair question, but there's much more to it than just reinventing the wheel. Building a kernel from scratch forces you to understand low-level hardware interactions, assembly context switching, and memory layout. It fundamentally transforms you into a better embedded firmware engineer, sharpens your low-level debugging skills (hello, hard fault handlers!), and gives you complete freedom to architect a system tailored to your exact specifications.
How to Get Started
If you've decided to embark on building your own RTOS, here are the three critical decisions you need to make first:
1. Target Instruction Set Architecture (ISA)
You need to choose a target architecture—common choices include ARM Cortex-M, RISC-V, MIPS, or x86.
I chose the ARM Cortex-M4 architecture. It provides dedicated features tailored for OS design—such as the NVIC (Nested Vectored Interrupt Controller), SysTick Timer, and the PendSV interrupt for safe context switching—along with an incredible community and ecosystem for developers.
2. Hardware vs. Simulation
Select a development board featuring your target architecture (e.g., STM32). Alternatively, you can use QEMU to simulate the hardware environment before flashing physical silicon. In fact, many professional RTOS teams rely heavily on QEMU for automated testing and rapid prototyping.
3. Toolchain & Build Setup
If you aren't using a Hardware Abstraction Layer (HAL) and want to write bare-metal code, you'll need a cross-compiler toolchain. Because I'm writing it in C and assembly for ARM, I am using the arm-none-eabi-gcc toolchain alongside GNU Make/CMake.
What’s Next?
In the next post, we’ll dive into startup scripts, linker scripts, and setting up the vector table.
(Note: Throughout this series, I’ll be moving forward with the ARM Cortex-M4 setup, but the core operating system concepts will apply across most architectures.)
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