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Build a minimal multi-tasking OS kernel for ARM from scratch


./configure --disable-werror --enable-debug \
    --target-list="arm-softmmu" \
    --extra-cflags=-DSTM32_UART_NO_BAUD_DELAY \
    --extra-cflags=-DSTM32_UART_ENABLE_OVERRUN \


  • 00-Semihosting
    • Minimal semihosted ARM Cortex-M "Hello World"
  • 00-HelloWorld
    • Enable STM32 USART to print trivial greetings
  • 01-HelloWorld
    • Almost identical to the previous one but improved startup routines
  • 02-ContextSwitch-1
    • Basic switch between user and kernel mode
  • 03-ContextSwitch-2
    • system call is introduced as an effective method to return to kernel
  • 04-Multitasking
    • Two user tasks are interatively switching
  • 05-TimerInterrupt
    • Enable SysTick for future scheduler implementation
  • 06-Preemptive
    • Basic preemptive scheduling
  • 07-Threads
    • Implement user-level threads
  • 08-CMSIS
    • Illustrate hardware abstraction layer (HAL) by introducing CMSIS
    • Both emulator (based on stm32-p103) and real device (STM32F429i discovery) are supported.

Building and Verification

  • Changes the current working directory to the specified one and then
make qemu

Guide to Hacking 08-CMSIS

08-CMSIS implements preemptive round-robin scheduling with user-level threads for STM32F429i-Discovery (real device) and STM32-P103 (qemu).

Get the dependencies:

git submodule init
git submodule update

Install additional utilities:

Quick Start / Support Devices:

  • STM32F429i-Discovery(physical devices)
    • Details in Chinese by NCKU
    • STM32F429i-Discovery uses USART1(Tx=PA9,Rx=PA10,baud rate=115200) as default serial port here.
      • You would need a terminal emulator, such as screen
        • Installation on Ubuntu / Debian based systems: sudo apt-get install screen
        • Then, attach the device file where a serial to USB converter is attached: screen /dev/ttyUSB0 115200 8n1
        • Once you want to quit screen, press: Ctrl-a then k

Available commands:


  • make all
    • Build all target's bin,elf,objdump files in the "release" directory.
    • NOTE: make doe NOT equal to make all here because Makefile uses eval for targets.
  • make clean
    • Remove the entire "release" directory.


  • make p103 or make target PLAT=p103
    • Build "p103.bin"
  • make qemu
    • Build "p103.bin" and run QEMU automatically.
  • make qemu_GDBstub
    • Build "p103.bin" and run QEMU with GDB stub and wait for remote GDB automatically.
  • make qemu_GDBconnect
    • Open remote GDB to connect to the QEMU GDB stub with port:1234 (the default port).

STM32F429i-Discovery(physical device)

  • make f429disco or make target PLAT=f429disco
    • Build "f429disco.bin"
  • make st-flash
    • Build "f429disco.bin" and flash the binary into STM32F429 with st-link.
  • make st-erase
    • Sometimes, STM32F429i-Discovery will not be able to flash new binary file, then you will need to erase flash memory with st-link.
    • Erase the entire flash on STM32F429.
  • make gdb_ST-UTIL
    • Using GDB with ST-LINK on STM32F429.
    • Remember to open another terminal,and type "st-util" to open "STLINK GDB Server"

Directory Tree:

  • core
    • Hardware independent source and header files.
  • platform
    • Hardware dependent source and header files.
  • cmsis
    • With cmsis,porting would be much easier!
  • release
    • This directory will be created after "Target" in Makefile is called.
    • Containing the elf,bin,objdump files in corresponding directory.
    • make clean will remove the entire directory,do not put personal files inside it!

Porting Guide:

You should know what CMSIS is and why it saves us a lot of efforts.

cmsis is a submodule from TibaChang/cmsis, maintained by Jia-Rung Chang.

The full project can be divided into two layer:

  • hardware-dependent part (HAL)
    • "platform" directory
  • hardware-indepentent part
    • "core" directory

Steps to launch the kernel on real devices:


Select a target name for your device, such as f429disco.

In this guide, we assume its name is example_device with vendor name "f429disco".

Create example_device directory in "platform" and f429disco directory in "cmsis" directory.

Create "include" and "src" directory in platform/example_device/


Introducing your CMSIS for your target, where it should be in the mbed repo.

For example, the CMSIS for STM32F429i-discovery could be found here.

We only need ".h" files, do not copy any ".c" files.

Put the header files into cmsis/f429discoexample_device.

cmsis is a submodule in this project, maintianed by Tiba Chang.

NOTE: You may encounter some error messages during building binary for your target. You need to solve it mannually. Usually, some files may be missing caused by some specific "define". You could just comment out that definition to resolve this problem.


This is the most difficult part.

You have to implement the files in platform/example_device/include/ and platform/example_device/src/.

According to different device vendor(such as STMicroelectronics, NXP, etc), the implementation is very different.

Please look into the current example:f429disco,and you will figure it out!

The function interface must be as same as the function interface in "platform/STM32F429/inc/" due to this is HAL for the entire project.


Add your target rules into Makefile.

Please look the example f429disco in the Makefile.

Most of the rules are reusable,so all you need is copy-n-paste, modifying the variable/target name and knowing what gcc arguments suit your target!

    • You should NOT modify this file!  - All of the rules are encapsulated into macro, used in Makefile.
  • Makefile:
    • If your device vendor name does not exist, create new variable and assign a name to it!
      • E.g.STM32 := STM32
    • Add your device name
      • E.g.STM32F429_DEVICE := f429disco
    • Check your device CPU type(Cortex-M3/4)
      • In target_using_CM4_list
    • Will you use CMSIS?
      • If NOT,add to target_NOT_using_CMSIS_list
    • Use the predefined macro to produce the corresponding directory and device specific variable(device name,vendor name)
      • E.g. $(eval $(call eval_all_variable,$(STM32F429_DEVICE),$(STM32)))
      • The vendor name is used in the cmsis directory name. They must be associated with each other.
    • Use the predefined macro to produce the corresponding GCC commands
      • E.g.: $(eval $(call eval_build_command,$(STM32F429_DEVICE)))
      • This will derive lots of variables that you don't see in the Makefile.
    • Add your device name to all
      • In the specific form:$($(YOUR_DEVICE_NAME)_TARGET) , this variable will be automatic derived by
      • E.g.: $($(STM32F429_DEVICE)_TARGET)



Now, you can try the "Available commands" in this README.


mini-arm-os is freely redistributable under the two-clause BSD License. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.



Build a minimal multi-tasking OS kernel for ARM from scratch







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