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Xinu stands for Xinu Is Not Unix -- although it shares concepts and even names with Unix, the internal design differs completely. Xinu is a small, elegant operating system that supports dynamic process creation, dynamic memory allocation, network communication, local and remote file systems, a shell, and device-independent I/O functions. The small size makes Xinu suitable for embedded environments.

Xinu for STM32 is a port of the ARM version of Xinu. The STM32 family of 32-bit microcontrollers is a popular SoC based on the Arm Cortex-M processor. Besides porting the original version, Xinu for STM32 is extended with the following specific Cortex-M* features:

• Shadowed Stack Pointer: Two stack pointers are available. The MSP is used for the OS Kernel and interrupt handlers. The PSP is used by application tasks.
• PendSV exception for scheduling: If Xinu decides that context switching is needed, it sets the pending status of the PendSV, and delays the context-switching request until all other IRQ handlers have completed their processing before carrying out the PendSV exception

• UART
• GPIO
• SPI

1. STM32F103* - https://stm32-base.org/boards/STM32F103RCT6-STM32-Mini-V2.0
2. Qemu for STM32 - https://github.com/beckus/qemu_stm32

Xinu for STM32 uses simple Makefiles. The source code should be compiled with a cross compiler (i.e. GCC cross compiler for ARM Cortex-A/R/M processors). The following packages are required:

• gcc-arm-none-eabi
• flex
• flex-dev
• bison
• gawk
• make
• libc-dev
• picocom
• stm32-flash) (* qemu)

For a debian-like distribution: apt-get install gcc-arm-none-eabi flex flex-dev bison gawk make libc-dev

Make sure you have installed the above required packages.

cd compile
make

This creates a very light container based on Alpine Linux:

docker build -t . xinu-container
docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp xinu-container sh -c 'cd compile && make'

If required you can use the container interactively: docker run -it --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp xinu-container

Note: you will need a USB to UART device to set up communication between your PC and STM32 board. Any common chip will suffice. Pins are (RX1 - PA10, TX1 - PA9)

STM32Flash (https://sourceforge.net/projects/stm32flash) can be used for flashing the compiled code to the device's flash ROM.

stm32flash -w compile/xinu.bin -v /dev/ttyUSB0

Code layout is simple. You can just write directly at address 0x0.

Xinu boots and shows a prompt. Output is over UART. Baudrate is at 115200. You can use a tool like picocom to communicate with your PC: picocom -b 115200 /dev/ttyUSB0

Use make qemu to run Xinu for STM32 in Qemu. Please note that this version of qemu should be manually build (see https://github.com/beckus/qemu_stm32) Note: if you have installed qemu-system-arm, you will need to remove that package to avoid any conficts'

More general information about Xinu can be found at https://xinu.cs.purdue.edu/