# [Initialize the operating system](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/initialize/)

## In this learning path

- [Introduction](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/)
- [Create and setup Keil MDK project](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/create/)
- [Initialize the operating system](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/initialize/)
- [Create RTOS threads](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/threads/)
- [Build and run the application](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/build_and_run/)
- [Using Event Recorder](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/eventrecorder/)
- [Next Steps](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/cmsis_rtx/_next-steps/)

<a href="https://www2.keil.com/mdk5/cmsis/rtx">Keil RTX5</a> is quite a feature rich real-time operating system (RTOS). CMSIS and the <a href="https://arm-software.github.io/CMSIS_5/RTOS2/html/index.html">CMSIS-RTOS2</a> API makes it very easy to work with.

When setting up the project Run-time environment, the appropriate system initialization code (`C Startup`) was added.

From there, the `RTX5` initialization code is always essentially the same, setting up the `SysTick` timer with the <a href="https://www.keil.com/pack/doc/CMSIS/Core/html/group__system__init__gr.html#gae0c36a9591fe6e9c45ecb21a794f0f0f">SystemCoreClockUpdate()</a> function, then initializing and starting the RTOS.

## Create main()
Right click on the `Source` folder under the `FVP` target, and `Add new item`.

Select `C file (.c)`, and create the `main.c` file with the contents below:

```c
#include "RTE_Components.h"
#include CMSIS_device_header
#include "cmsis_os2.h"

void app_main(void *);

int __attribute__((noreturn)) main(void) {
    SystemCoreClockUpdate();                    // initialize clocks etc

    osKernelInitialize();                       // initialize RTOS

    osThreadNew(app_main, NULL, NULL);          // Create application main thread

    if (osKernelGetState() == osKernelReady)    // If all OK...
        osKernelStart();                        // Start thread execution

    while (1);                                   // If you get to here, something has gone wrong!
}
```

Right-click on the project, and select `New` > `Source File` from the pop-up menu.

## Understanding the code
The function <a href="https://arm-software.github.io/CMSIS_6/latest/RTOS2/group__CMSIS__RTOS__KernelCtrl.html#ga9ae2cc00f0d89d7b6a307bba942b5221">osKernelStart()</a> should never return.

If your code gets to the infinite `while()` loop, something has gone wrong - most likely with the platform initialization code.

All threads use a prototype of the form:

```c
void thread(void *);
```

where the argument is passed as the second parameter of the <a href="https://arm-software.github.io/CMSIS_6/latest/RTOS2/group__CMSIS__RTOS__ThreadMgmt.html#ga48d68b8666d99d28fa646ee1d2182b8f">osThreadNew()</a> function. Use `NULL` if no argument to pass.

In the above, `app_main` is used as the main application thread, but this naming is arbitrary. From here, you shall spawn all other threads of the RTOS.