RTLinux is a hard Real-Time OS (RTOS) which uses an original approach to develop complex hard real-time applications in a fairly easy way: Executing a hard RTOS (RTLinux itself) jointly with Linux in the same box. This approach enables splitting up a hard real-time application according to its criticality, the part with deadline requirements is run on the RTOS while the part with no ``special'' time requirements is executed on Linux.

However, this approach presents two drawbacks. Firstly, Linux has a big footprint, especially in the 2.6 series, which is unsuitable for system with low resources. On the other hand, it is possible for a Linux application to directly disable interrupts, disrupting the real-time properties of RTLinux.

Several years ago, people from our department developed Stand-Alone RTLinux (SA-RTL), a RTLinux-compatible standalone kernel which was able to execute RTLinux applications in a bare machine. However, this approach was not completely optimum since was built cutting and pasting code from RTLinux, being rather difficult to maintain. Besides, it did not provide binary compatibility with RTLinux applications.

In this paper we present Embedded RTLinux (ERTL), our second approach of what a Stand-Alone RTLinux system should be. This second kernel, unlike SA-RTL, just replaces Linux with a minimal set of drivers, allowing to execute the original RTLinux jointly with a RTLinux application on a bare machine.