eCos (the Embedded Configurable Operating System) is a free open source real-time operating system (RTOS) designed for use in embedded systems. A key feature of eCos is its source-level configuration system which provides the infrastructure necessary to define configurable operating system features and to specify the dependencies between them. Developers make use of this configurability to customise eCos for specific application requirements, increasing performance and reducing memory requirements. The configurability of eCos at the hardware abstraction level has enabled the porting of eCos to many processor architectures and development boards. eCos is developed as an open source community project under the guidance of a team of independent maintainers. Many sub-systems have been contributed by the community of developers including board support, device drivers, I/O infrastructure, networking, file systems and compatibility layers. Copyright is held by the Free Software Foundation. The core components of eCos are licensed under the eCos License which is derived from the GNU General Public License and incorporates an important exception clause which allows application code to remain proprietary.