Add FreeRTOS wrapper.

Author: dmitrykos

README.md

@@ -51,6 +51,7 @@ It is an [open-source project](https://github.com/SuperTinyKernel-RTOS), navigat
 | Safety-critical systems ready         | No dynamic heap memory allocation                                                                                                                                                                                            |
 | C++ and C API                         | Can be used easily in C++ and C projects                                                                                                                                                                                     |
 | CMSIS-RTOS2 compatible                | Full CMSIS-RTOS2 wrapper (`cmsis_os2_stk.cpp`) maps the standard ARM CMSIS-RTOS2 C API onto STK, enabling drop-in compatibility with STM32CubeMX, MCUXpresso, and other CMSIS-aware middleware                               |
+| FreeRTOS compatible                   | Full FreeRTOS wrapper (`freertos_stk.cpp`) maps the standard FreeRTOS C API onto STK, enabling drop-in migration of existing FreeRTOS codebases with minimal or no application changes                                       |
 | Easy porting                          | Requires very small to none BSP surface                                                                                                                                                                                      |
 | Traceable                             | Scheduling is fully traceable with a SEGGER SystemView                                                                                                                                                                       |
 | Development mode (x86)                | Run the same threaded application on Windows                                                                                                                                                                                 |
@@ -257,12 +258,91 @@ For a seamless integration with C projects STK provides a dedicated, fully-featu
 
 ---
 
-## CMSIS-RTOS2 Wrapper (`interop/cmsis/rtos2`)
+## Migrating from CMSIS-RTOS2 or FreeRTOS
 
-STK provides a complete **CMSIS-RTOS2** compatibility layer (`cmsis_os2_stk.cpp`) that maps standard ARM [CMSIS-RTOS2 C API](https://arm-software.github.io/CMSIS_6/latest/RTOS2/group__CMSIS__RTOS.html) (`cmsis_os2.h` **v2.3.0**) onto the STK C++ kernel. This allows you to use STK as a drop-in RTOS backend in any project that targets the CMSIS-RTOS2 interface, including code generated by STM32CubeMX, MCUXpresso, or any other CMSIS-aware IDE or middleware stack. See [interop/cmsis/rtos2](https://github.com/SuperTinyKernel-RTOS/stk/tree/main/interop/cmsis/rtos2) for more details and example.
+STK ships two ready-made compatibility wrappers that let you swap out your existing RTOS backend and replace it with STK without rewriting application code. Both wrappers live under `interop/` and share the same design principles: thin translation of the source API onto STK primitives, ISR-safe where the original API requires it, and zero heap usage when the caller supplies static memory.
 
-CMSIS-RTOS2 API works as documented in the CMSIS-RTOS2 specification.
+### CMSIS-RTOS2 Wrapper (`interop/cmsis/rtos2`)
 
+STK provides a complete **CMSIS-RTOS2** compatibility layer (`cmsis_os2_stk.cpp`) that maps the standard ARM [CMSIS-RTOS2 C API](https://arm-software.github.io/CMSIS_6/latest/RTOS2/group__CMSIS__RTOS.html) (`cmsis_os2.h` **v2.3.0**) onto the STK C++ kernel. This allows you to use STK as a drop-in RTOS backend in any project that targets the CMSIS-RTOS2 interface, including code generated by STM32CubeMX, MCUXpresso, or any other CMSIS-aware IDE or middleware stack.
+
+**Covered API groups:** Kernel Management, Thread Management, Thread Flags, Event Flags, Mutex, Semaphore, Timer, Message Queue, Memory Pool.
+
+**Quick integration:**
+
+```make
+# 1. Add wrapper source
+SRCS     += libs/stk/interop/cmsis/rtos2/src/cmsis_os2_stk.cpp
+
+# 2. Add include paths
+INCLUDES += -Ilibs/stk/include
+INCLUDES += -Ilibs/stk/interop/cmsis/rtos2/include
+```
+
+```c
+// 3. No application changes required — existing CMSIS-RTOS2 calls work as-is
+#include "cmsis_os2.h"
+
+int main(void)
+{
+    osKernelInitialize();
+    osThreadNew(app_thread, NULL, NULL);
+    osKernelStart();   // never returns
+}
+```
+
+See [interop/cmsis/rtos2](https://github.com/SuperTinyKernel-RTOS/stk/tree/main/interop/cmsis/rtos2) for the full API coverage table, design notes, and configuration macros.
+
+---
+
+### FreeRTOS Wrapper (`interop/freertos`)
+
+STK provides a complete **FreeRTOS** compatibility layer (`freertos_stk.cpp`) that maps the standard FreeRTOS C API onto the STK C++ kernel. Existing FreeRTOS projects can migrate to STK with minimal or no changes to application code, while immediately gaining STK's lower scheduling overhead, reduced jitter, and smaller RAM footprint (see [Benchmark](#benchmark) above).
+
+**Covered API groups:** Kernel Control, Task Management, Queue, Semaphore / Mutex, Software Timers, Event Groups, Task Notifications.
+
+**Quick integration:**
+
+```make
+# 1. Add wrapper source
+SRCS     += libs/stk/interop/freertos/src/freertos_stk.cpp
+
+# 2. Add include paths
+INCLUDES += -Ilibs/stk/include
+INCLUDES += -Ilibs/stk/interop/freertos/include
+```
+
+```c
+// 3. No application changes required — existing FreeRTOS calls work as-is
+#include "FreeRTOS.h"
+#include "task.h"
+
+int main(void)
+{
+    xTaskCreate(app_task, "app", 256, NULL, 2, NULL);
+    vTaskStartScheduler();   // never returns
+}
+```
+
+See [interop/freertos](https://github.com/SuperTinyKernel-RTOS/stk/tree/main/interop/freertos) for the full API coverage table, design notes, known limitations, and `FreeRTOSConfig.h` requirements.
+
+---
+
+### Wrapper Comparison
+
+|                     | CMSIS-RTOS2                                     | FreeRTOS                                          |
+|---------------------|-------------------------------------------------|---------------------------------------------------|
+| Source file         | `cmsis_os2_stk.cpp`                             | `freertos_stk.cpp`                                |
+| Header to include   | `cmsis_os2.h`                                   | `FreeRTOS.h`, `task.h`                            |
+| Priority model      | Linear map: `osPriority` 1–56 → STK levels 0–31 | Direct clamp: FreeRTOS 0–N → STK levels 0–31      |
+| Max tasks macro     | `CMSIS_STK_MAX_THREADS` (default: 16)           | `FREERTOS_STK_MAX_TASKS` (default: 16)            |
+| Default stack macro | `CMSIS_STK_DEFAULT_STACK_WORDS` (default: 256)  | `FREERTOS_STK_DEFAULT_STACK_WORDS` (default: 256) |
+| Static allocation   | `cb_mem` / `cb_size` attributes                 | `xTaskCreateStatic` / `xQueueCreateStatic`        |
+| Scheduler backend   | `SwitchStrategyFP32`                            | `SwitchStrategyFP32`                              |
+| Tickless support    | `STK_TICKLESS_IDLE=1`                           | `STK_TICKLESS_IDLE=1`                             |
+
+
+> **Note:** Wrapper is missing important API your project is using? Contact with inquiry: [contact@supertinykernel.org](mailto:contact@supertinykernel.org)
 ---
 
 ## Synchronization API (`stk/stk_sync.h`)