HYDRA was an early capability-based, object-oriented, microkernel designed to support a wide range of possible operating systems to run on top of it. HYDRA was created as part of the C.mmp project at Carnegie-Mellon University in 1971.
HYDRA was designed to be modular and secure, and intended to be flexible enough for easy experimentation. The system was implemented in the programming language BLISS.
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Hydra (operating system) Wikipedia(Text) CC BY-SA