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Real-Time Kernels: Cs/OS2 and Cs/OS3

Cs/OS2 and Cs/OS3 are preemptive, highly portable, and scalable real-time kernels.

Designed for ease of use on a huge number of CPU architectures, these kernels are a key component of the Cesium real-time operating system.


Key Features


Portable. Offering unprecedented ease of use, Cs/OS kernels are delivered with complete source code and in-depth documentation. The Cs/OS kernels run on a huge number of processor architectures, with ports available for download.

Scalable. The Cs/OS kernels allow for unlimited tasks and kernel objects. The kernels' memory footprint can be scaled down to contain only the features required for your application, typically 6–24 KBytes of code space and 2-5 KByte of data space.

Reliable. The Cs/OS kernels include debugging features that reduce development time. The kernels provide extensive range checking, including checks for pointers passed in API calls, task-level services from ISRs, arguments within an allowable range, and valid specified options.

Efficient. Cesium RTOS kernels also include valuable runtime statistics, making the internals of your application observable. Identify performance bottlenecks, and optimize power usage, early in your development cycle.


The features of the Cs/OS kernels include:

  • Preemptive multitasking real-time kernel with optional round-robin scheduling
  • Delivered with complete, clean, consistent source code with in-depth documentation
  • Highly scalable: Unlimited number of tasks, priorities, and kernel objects
  • Resource-efficient: 6K to 24K bytes code space, 2K+ bytes data space)
  • Very low interrupt disable time
  • Extensive performance measurement metrics (configurable)
  • Certifiable for safety-critical applications



Memory Footprint

 Scalable Yes (compile and run-time configurable) 
 ROM (Min-Max) 6K-24K (bytes) 
 RAM 2K+(bytes) 

Task Management

Multitasking model Preemptive
Maximum number of tasks Unlimited
Scheduling Real-time scheduler
Round-robin scheduler
Run-time stack checking Configurable
Catch/delete tasks that return Yes
Support for thread local storage ARMCC, CrossCore Embedded Studio, GNU, IAR
Kernel time base Configurable (constant and dynamic tick rate)
Timeout on calls Yes (prevents deadlocks)
Interrupt Disable Time Near Zero

Kernel Services

Critical sections Yes
Event Flags Yes
Lock/Unlock scheduler Yes
Memory partitions Yes (prevents fragmentation)
Message queues Yes
Mutual exclusion semaphores Yes (full priority inheritance and nestable)
Semaphores Yes
Software timers Yes
Task registers Yes
Task suspend/resume Yes
Data Queues Yes


Feature Comparison

Which real-time kernel should you use? Cesium RTOS kernels meet different needs.




Preemptive Multitasking
Maximum number of tasks 255 Unlimited
Number of tasks at each priority level 1 Unlimited
Round robin scheduling -
Mutual exclusion semaphores ✔ (nestable)
Event flags
Message mailboxes - (not needed)
Message queues
Data Queues -
Fixed sized memory management
Signal a task without requiring a semaphore -
Option to post without scheduling -
Send messages to a task without requiring a message queue -
Software timers
Task suspend/resume ✔ (nestable)
Deadlock prevention
Code footprint 6 to 26 Kbytes 6 to 24 Kbytes
Data footprint 1 Kbyte + 2 Kbyte +
Run-time configurable -
Compile-time configurable
ASCII names for each kernel object
Pend on multiple objects -
Task registers
Built-in performance measurements Limited Extensive
User-definable hook functions
Timestamps on posts -
Built-in kernel awareness support
Optimizable scheduler in assembly language -
Catch a task that returns -
Dynamic tick rate -
Source code available
Number of services ~90 ~70
MISRA-C:1998 ✔ (except 10 rules) N/A
MISRA-C:2012 - ✔ (except 8 advisory and 8 required guidelines)
DO178B Level A and EUROCAE ED-12B
Medical FDA pre-market notification (510(k)) and pre-market approval (PMA)
SIL3/SIL4 IEC for transportation and nuclear systems


Supported CPU Architectures

The following table lists CPU architectures for which Cs/OS ports are readily available. New ports can be written easily or adapted to your specific needs either by yourself or us.


Manufacturer CPU
Altera (Intel) Nios II
SoC FPGA (Cortex-A)
Analog Devices Blackfin
ADSP-CM4xx (Cortex-M)
Atmel (Microchip) AVR, AVR32
SAM3, SAM4, SAM7, SAM9, SAMA5 (ARM Cortex-based)
Cypress Semiconductors (Infineon) PSoC 4, PSoC 5 (Cortex-M)
EnSilica eSi-RISC
Imagination Technologies M14K
Infineon XMC4000 (Cortex-M)
Microchip PIC24, PIC32
Microsemi SmartFusion2 (Cortex-M)
Microsoft Win32
NXP ColdFire
Kinetis (Cortex-M)
LPC (Cortex-M)
MPC5xxx, MPC8xxx
VFxxx (Cortex-A & Cortex-M)
Renesas H8S
RX100, RX200, RX600, RX700
RZ/A (Cortex-A), RZ/T1 (Cortex-R & Cortex-M)
R-IN32 (Cortex-M)
Silicon Labs Gecko (Cortex-M)
ST Microelectronics STM32F (Cortex-M)
STM32L (Cortex-M)
STM32H (Cortex-M)
STM32U (Cortex-M)
Texas Instruments C28x
MSP430 (Cortex-M), MSP432 (Cortex-M)
Hercules RM (Cortex-R), Hercules TMS570 (Cortex-R)
Xilinx MicroBlaze
Zynq-7000 (Cortex-A)
Zynq Ultrascale+ MPSoC (Cortex-A & Cortex-R)


You can also contact us for more information about supported platforms or port development. 


Commercial License

All Cesium RTOS software components are available under a commercial license.

Click here for more details


Technical Support

Cesium Support Plan

If you are currently without technical support for the Cs/OS software don’t go at it alone. Technical support services for any of the Cs/ software is readily available to all customers currently using or interested in the Cs/ family of RTOS components.

Learn how our Cs/OS product experts, with years of embedded software experience, can save you valuable engineering time and money.

Click here for more details


Consulting Services

Specialized Engineering Services

Do you need specialized assistance with your Cesium-based embedded project? Do you need real-time embedded software expertise for your mission critical application? For these or any other embedded software projects, help is available via our Consulting Services that are tailored to your unique needs. Please contact us with your requirements to learn how we can quickly deliver our expertise to your team.

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Cesium software includes comprehensive documentation, full source code, powerful debugging features, and support for a huge range of CPU architectures.


Cesium software offers unprecedented ease-of-use, a small memory footprint, remarkable energy efficiency, and all with a full suite of protocol stacks.


Engineers across the world building industrial controls, medical devices, and aerospace systems know they can rely on Cesium software for the foundation of exceptional products.