Keil RTX version 5 (RTX5) is a real-time operating system (RTOS) for Arm Cortex-M and Cortex-A processor-based devices that implements the CMSIS-RTOS2 API as its native interface.

The following sections provide further details:

Revision History lists the changes made in each version of CMSIS-RTX.
System Requirements lists hardware, software, and resource requirements, as well as supported toolchains.
Theory of Operation provides general information about the operation of CMSIS-RTOS RTX v5.
Create an RTX5 Project explains how to setup an RTX v5 project in Keil MDK.
Building the RTX5 Library explains how to build your own CMSIS-RTOS RTX v5 library.
Configure RTX v5 describes configuration parameters of CMSIS-RTOS RTX v5.
MISRA C:2012 Compliance describes the violations to the MISRA standard.
Tutorial is an introduction into the usage of Keil RTX5 based on real-life examples.

Access to CMSIS-RTX

CMSIS-RTX kernel is actively maintained in CMSIS-RTX GitHub repository and is also provided as a standalone CMSIS-RTX pack in the CMSIS-Pack format.

‍Note

CMSIS-RTX replaces and extends RTX5 implementation previously provided as part of ARM::CMSIS pack.

See Migrating projects from CMSIS v5 to CMSIS v6 for a guidance on updating existing projects to CMSIS-RTX.

The table below explains the content of the ARM::CMSIS-RTX Pack.

File/Directory	Content
📂 Config	RTX configuration files RTX_Config.h and RTX_Config.c
📂 Documentation	A local copy of this documentation
📂 Examples	Example projects (MDK uVision and CMSIS-Toolbox)
📂 Include	Public header files of RTX software component
📂 Library	Library project files and pre-built libraries
📂 Source	Private header and source files of RTX software component
📂 Template	User code templates for creating application projects with CMSIS-RTX
📄 ARM.CMSIS-RTX.pdsc	Pack description file in CMSIS-Pack format
📄 LICENSE	License Agreement (Apache 2.0)
📄 RTX5.scvd	SCVD file for RTOS-aware debugging with Component Viewer and Event Recorder

See CMSIS Documentation for an overview of CMSIS software components, tools and specifications.

Security Considerations

RTX5 has not been designed with strong security considerations in mind. The implementation has a focus on high code quality assured by applying coding rules according to MISRA-C:2012 and checking compliance using CodeQL. The analysis results are published on GitHub Code scanning page.

Users of RTX5 are strongly advised to run security analysis for their applications if any kind of exposure is given. The main security assumption made for RTX5 is no untrusted code is ever executed on a device. This assumption is expected to apply to the vast majority of use cases of an embedded RTOS like RTX5.

For use cases (such as firmware update) where the firmware image (or parts of it) can be modified/updated by an untrusted user, the application developer must implement appropriate security measures to prevent execution of untrusted code.

For use cases where executing untrusted code is required (like customizable plugins), the application developer must carfully analyze the execution sandbox boundaries. Given that untrusted code is executed within an unprivileged RTX5 thread, it must be assured that no RTOS operation can be used to run untrusted code as part of a service call nor interrupt routine, see Safety features (Source variant only).

License

CMSIS-RTX is provided free of charge by Arm under the Apache 2.0 License.