First things first, I'm going to show you how to incorporate embOS into your project. In my case, I'm going to use our already famous project template for the for the Nucleo-G0B1RE board as starting point. If you're using a different STM32 board, you can follow the guide described in here to organize your project in a similar fashion and follow from there this guide.

Download the latest version of SEGGER embOS Classic for Cortex-M and GCC from the official SEGGER website and unzip the zip file.

You should find the following folder structure and documents

embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0
├── embOSView
├── Start
├── License.txt
├── Release_embOS.html
├── Release_embOS_Classic_CortexM_GCC.html
├── SYSVIEW_embOS.txt
├── UM01001_embOS.pdf
└── UM01039_embOS_Classic_CortexM_GCC.pdf

Locate the following directories

embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/Inc
embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/Lib

Copy and paste the Inc and Lib folders into the directory you created in the root of your project. Also create the Src sub-directory. You should have the following order:

template-g0
├── app
├── cmsisg0
├── halg0
├── embOS
│   ├── Inc
│   ├── Lib
│   └── Src
├── linker.ld
├── makefile
├── misra.json
...

Inside the Inc folder, just delete the files BSP.h and BSP_UART.h because we will not use these files. Inside the Lib folder, you'll find different files, we’ll only include the libraries our microcontroller needs. To find out which files we'll use, follow the table below.

To choose the correct files for our microcontroller we have to know the libraries are named as:

libosT<Architecture><VFP><Endianness><LibMode><Errata><TrustZone><PACBTI>.a

As you can see, the libraries I’m gonna use are those with the nomenclature libosT6Lxx.a because the architecture value 6 is for Cortex-M0/M0+/M1 (Cortex-M0+ in my case) and the byte order is little endian. Remember, you have to use the files according with your microcontroller arquitecture.

template-g0
├── app
├── cmsisg0
├── halg0
├── embOS
│   ├── Inc
│   ├── Lib
│   │   ├── libosT6LD.a
│   │   ├── libosT6LDP.a
│   │   ├── libosT6LDT.a
│   │   ├── libosT6LR.a
│   │   ├── libosT6LS.a
│   │   ├── libosT6LSP.a
│   │   └── libosT6LXR.a
│   └── Src
...

We need some extra files we have to copy from the BoardSupport directory, or more specifically from the corresponding board you are using in my case the Nucleo-STM32G0B1RE. From here copy the source files embOS use to work correctly. From the Setup folder copy the files below and paste these files into the Src folder you created in your project folder.

embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/BoardSupport/ST/STM32G0B1_STM32G0B1RE_Nucleo/Setup/RTOSInit_STM32G0xx.c
embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/BoardSupport/ST/STM32G0B1_STM32G0B1RE_Nucleo/Setup/OS_Error.c
embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/BoardSupport/ST/STM32G0B1_STM32G0B1RE_Nucleo/Setup/JLINKMEM_Process.c
embOS_Classic_CortexM_GCC_Obj_SFL_V5.20.0.0/Start/BoardSupport/ST/STM32G0B1_STM32G0B1RE_Nucleo/Setup/OS_ThreadSafe.c

We need to modify a few things in the file RTOSInit_STM32G0xx.c starting by comment the #include "SEGGER_SYSVIEW.h" we are not going to use it at the moment. And then in function OS_InitHW remove several lines to leave the function only with the lines below; the HAL library already init the Systick timer.

void OS_InitHW(void) {
   OS_INT_IncDI();
  // Inform embOS about the timer settings
  OS_SYSTIMER_CONFIG SysTimerConfig = {OS_TIMER_FREQ, OS_INT_FREQ, OS_TIMER_DOWNCOUNTING, _OS_GetHWTimerCycles, _OS_GetHWTimer_IntPending};
  OS_TIME_ConfigSysTimer(&SysTimerConfig);
  // Configure and initialize SEGGER SystemView
#if (OS_SUPPORT_TRACE_API != 0)
  SEGGER_SYSVIEW_Conf();
#endif
  // Initialize communication for embOSView
#if (OS_VIEW_IFSELECT == OS_VIEW_IF_JLINK)
  JLINKMEM_SetpfOnRx(OS_COM_OnRx);
  JLINKMEM_SetpfOnTx(OS_COM_OnTx);
  JLINKMEM_SetpfGetNextChar(OS_COM_GetNextChar);
#elif (OS_VIEW_IFSELECT == OS_VIEW_IF_UART)
  BSP_UART_Init(OS_UART, OS_BAUDRATE, BSP_UART_DATA_BITS_8, BSP_UART_PARITY_NONE, BSP_UART_STOP_BITS_1);
  BSP_UART_SetReadCallback(OS_UART, _OS_OnRX);
  BSP_UART_SetWriteCallback(OS_UART, _OS_OnTX);
#endif
  OS_INT_DecRI();
}

The you rename the SysTick_Handler()function in the same file, to HAL_SYSTICK_Callback() to avoid problems with the fucntion in the ints.c file.

void HAL_SYSTICK_Callback(void);
void HAL_SYSTICK_Callback(void) {
#if (OS_SUPPORT_TRACE_API != 0)
  if (SEGGER_SYSVIEW_DWT_IS_ENABLED() == 0u) {
    SEGGER_SYSVIEW_TickCnt++;
  }
#endif

In file ints.c we need to comment (or remove) the functions SVC_Handlerand PendSV_Handler since this are used by embOS and add the call to HAL_SYSTICK_IRQHandler that in turn will call HAL_SYSTICK_Callbac, this is how for the moment we serve the main interrupt our operative system needs

void SysTick_Handler( void ){
    /* service HAL library tick */    
    HAL_IncTick( );
    /* service embOS rick*/    
    HAL_SYSTICK_IRQHandler( );  
}

In the make file we need to add the files to compile and also the directories with our new files are located, sources and headers

# Files to compile
SRCS += OS_Error.c JLINKMEM_Process.c RTOSInit_STM32G0xx.c OS_ThreadSafe.c
...

# directories with source files to compiler
SRC_PATHS += embOS/Src
...

# directories with header files
INC_PATHS += embOS/Inc
...

You also need to add the precompiled rtos library, as you may notice we are using an Friendly license version and the embOS source code is not shipped, and there is another detail we can only use some of the library versions with debug options, nothing with release options. You need to add the corresponding library and its corresponding define symbol

# library file
LIBF = osT6LD
...

# Global symbols (#defines)
SYMBOLS = -DSTM32G0B1xx -DUSE_HAL_DRIVER -DOS_LIBMODE_D
...

embOS expects specific symbols in order to retrieve information about stack and heap in your project: __heap_end__, __heap_start__, __stack_end__ and __stack_start__ These symbols need to be added in the ._user_heap_stack section in the linker file as shown below:

._user_heap_stack :
  {
    . = ALIGN(8);
    PROVIDE ( end = . );
    PROVIDE ( _end = . );
    __heap_start__ = .;
    . = . + _Min_Heap_Size;
    . = ALIGN(8);
    __heap_end__ = .;
    __stack_start__ = .;
    . = . + _Min_Stack_Size;
    . = ALIGN(8);
    __stack_end__ = .;
  } >RAM

To test the operative system is working correctly let create a simple task with a running blinky led, using the code down below

#include "bsp.h"
#include "RTOS.h"

static OS_STACKPTR int Stack[128];      // Task stacks
static OS_TASK  Task;                   // Task control blocks

void Task_Demo( void );  //* Task routine

int main( void )
{
    OS_Init();      // Initialize embOS (must be first)
    HAL_Init();
    OS_InitHW();   // Initialize Hardware for embOS
    
    // create a Task with priority of 100 and time slice of 1ms 
    OS_TASK_Create(&Task1, "HP Task", 100, Task_Demo, Stack, sizeof(pStack), 1u);
      
    OS_Start();     // Start multitasking

    return 0u;
}

void Task_Demo( void ){

    GPIO_InitTypeDef GPIO_InitStruct;
    __HAL_RCC_GPIOA_CLK_ENABLE( );

    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;
    GPIO_InitStruct.Pull  = GPIO_NOPULL;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_HIGH;
    GPIO_InitStruct.Pin   = GPIO_PIN_5;
    HAL_GPIO_Init( GPIOA, &GPIO_InitStruct );

    for( ;; ){
        HAL_GPIO_TogglePin( GPIOA, GPIO_PIN_5 );
        HAL_Delay(1000);
    }
}