MiThOS (Micro-kernel Threads Operating System) is an experimental operating system for embedded systems. The system kernel is a first implementation of the POSIX Minimal Real-Time System Profile. It is based on prior work of a library implementation of Pthreads (POSIX threads). The system is fully preemptive. It supports multi-threading within a single process environment with shared kernel and user space, i.e. real-time tasks are mapped onto POSIX threads. It exhibits remarkable timing predictability intended for hard real-time requirements. This is achieved by a careful design of only few device drivers. The system has been implemented and tested on the SPARC VME architecture. The system includes a fast context switching algorithm for the SPARC which outperforms the context switch under SunOS and matches the performance under Solaris. It supports selective enabling and disabling of hardware components (MMU, caches, etc.) since its sources are available. Furthermore, an implementation-defined extension of POSIX threads for deadline scheduling is presented. Overall, the system exhibits slightly faster performance than SunOS 4.x and is considerably more predictable in its timing behavior. Applications of the kernel range from evaluating the overhead of new language features in Ada 95 and its runtime system, verifying static timing predictions on a bare machine, to providing the operating system for small embedded system that require a high timing predictability.