The MERT (Multi-Environment Real-Time) operating system was one of the earliest to be constructed using an organizational concept that later became known as a "micro-kernel".

MERT was created in the 1970s at Bell Labs, and was a spinoff of Unix. It ran on Digital Equipment Corporation PDP-11 computers (models 11/45 and 11/70).

Although it was not exactly a micro-kernel as they are today, it was a definitely not a "monolithic kernel"; it was a major step down the road to micro-kernels. MERT was an operating system which was divided up into several semi-independent components, all of which ran on a lower-level "kernel" (as they described it).

The kernel provided only the lowest-level basic mechanisms (memory management, process scheduling , etc); the other components needed for an operating system (e.g. a file system) were constructed as processes which ran on top of the kernel. Inter-process communication was done with messages, event flags, and shared memory and shared files.

MERT was also intended to be used in real-time applications, and had a number of features to meet this goal. Process scheduling had real-time mechanisms, as did the file system.

References

• D. L. Bayer, H. Lycklama, MERT - a multi-environment real-time operating system, (Fifth ACM Symposium on Operating Systems Principles, Austin, Texas, 1975)
• H. Lycklama, D. L. Bayer, The MERT Operating System (The Bell System Technical Journal, July-August 1978, Vol. 57, No. 6, Part 2)