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RTX Real-Time Operating System

The Keil RTX is a royalty-free, deterministic Real-Time Operating System designed for ARM and Cortex-M devices. It allows you to create programs that simultaneously perform multiple functions and helps to create applications which are better structured and more easily maintained.



  • Royalty-free, deterministic RTOS with source code
  • Flexible Scheduling: round-robin, pre-emptive, and collaborative
  • High-Speed real-time operation with low interrupt latency
  • Small footprint for resource constrained systems
  • Unlimited number of tasks each with 254 priority levels
  • Unlimited number of mailboxes, semaphores, mutex, and timers
  • Support for multithreading and thread-safe operation
  • Kernel aware debug support in MDK-ARM
  • Dialog-based setup using µVision Configuration Wizard



While it is certainly possible to create real-time programs without an RTOS (by executing one or more functions in a Super-loop), there are numerous scheduling, maintenance, and timing issues that an RTOS like the Keil RTX solves for you. For a more detailed comparison between RTOS and Super-loop, take a look at the advantages of using an RTOS.

Feature Products
MDK-Professional MDK-Standard MDK-CortexM MDK-Lite

RTX Real-Time Operating System (Including Source Code)



CMSIS-RTOS RTX is a royalty-freedeterministic real-time operating system implementing the CMSIS-RTOS API, a generic RTOS interface for Cortex-M processor-based devices. Keil RTX version 4 implements CMSIS-RTOS v1, whereas Keil RTX 5 implements the latest CMSIS-RTOS v2 API.

Modern microcontroller applications frequently have to serve several concurrent activities. RTX manages the switching between the activities. Each activity gets a separate thread which executes a specific task and to simplify the program structure. CMSIS-RTOS RTX is scalable and additional threads can be added easily at a later time. Threads have a priority allowing faster execution of time-critical parts of a user application.

CMSIS-RTOS RTX offers services needed in many real-time applications, such as periodical activation of timer functions, memory management, and message exchange between threads with time limits.


What's new in CMSIS-RTOS v2 and Keil RTX v5?


The new CMSIS-RTOS API v2 addresses new requirements such as dynamic object creation, support for ARMv8-M, and full support of C++ run-time environments. This webinar recording compares the CMSIS-RTOS API versions and shows how to utilize the new features in Keil RTX5, the first real-time operating system that implements the new API as its native RTOS interface for ARM Cortex-M processor-based devices.



  • Included in MDK Version 5 - No additional cost to use a commercial level RTOS.
    CMSIS-RTOS RTX (including source code) is shipped with Keil MDK Version 5. This full-featured, commercial grade RTOS is fully configurable and has no restrictions. It is shipped under the permissive Apache 2.0 license and is fully developed on GitHub.
  • Easy to Use - Reduced learning curve, faster product development.
    CMSIS-RTOS RTX is well supported within the µVision IDE/Debugger, which features RTX aware tools enabling you to quickly debug your RTX applications.
  • Royalty-Free - No on-going costs.
    There are no run-time royalty payments or other hidden charges. Ship your RTX based products without further fees or recurring costs.
  • Flexible Scheduling - Choose the best scheduling for your application.
    CMSIS-RTOS RTX offers various kernel scheduling options, allowing you to use the best suited one in your application.
  • Deterministic Behavior - Known behavior for time critical tasks.
    CMSIS-RTOS RTX delivers fully deterministic behavior meaning that events and interrupts are handled within a predefined time (deadline). Your application can rely on consistent and known process timings.
  • Designed for Embedded Systems - Uses the minimum amount of system memory.
    CMSIS-RTOS RTX is specifically written for applications running on ARM Cortex-M based MCUs. It runs quickly and requires only minimal MCU resources with a memory footprint as small as 5KB (ROM).
  • Source Code - Useful for certification.
    The source code is included in all MDK editions. Use it if your product requires certification.