Phoenix-RTOS is an open-source, microkernel-based, real-time operating system for resource-constrained devices. OS written from scratch Based on several decades of development in the field of operating systems it was possible to create from scratch fully featured, effective microkernel with clean design and compact source code. OS for software-defined solutions The software-defined solution era is rising. Advanced functionalities restricted to specialized chips (ASICs) now can be implemented in software. Phoenix-RTOS is an ideal platform for software-defined IoT communication stacks and other applications with real-time processing requirements. OS with many faces Based on one microkernel many operating system version could be created. The smallest version can occupy tens of kilobytes and work on resource constrained devices. Adding emulation servers it is possible to scale it out and create UN*X application environment.