A microkernel is a minimal computer operating system kernel providing only basic operating system services (system calls), while other services (commonly provided by kernels) are provided by user-space programs called servers. Commonly, microkernels provide services such as address space management, thread management, and inter-process communication, but not networking or display for example.
Later extensions of this concept lead to new architectures such as nanokernels, exokernels and hardware abstraction layers.
Microkernel :: Operating Systems
Submicrokernel :: Operating Systems

E.R.I.K.A. - Microkernel RTOSs and set of integrated tools to develop embedded realtime applications, made to support all architectures used in power train controllers in automotive industry. Kernels have 2 main layers: Kernel Layer, Hardware Abstraction Layer, HAL. [Open Source, GPL]
ERIKA Enterprise - A commercial realtime system with minimal footprint that supports advanced scheduling mechanisms for stack size reduction on multiprocessor-on-a-chip systems.
Real-Time Mach NTT - Another version of Real-Time Mach, by NTT Laboratories. Contains RT-Mach kernel, FreeBSD 2.2.8 server, real-time shell, RT-Java, Hurd loader. Site has fine 'Getting Started' section: readme, release notes, installation, hardware, compatibility, troubles QA, contacts.
rtmk - Realtime microkernel: preemptive, fully re-entrant, multithreaded, SMP, continuations, full locking primitives; shares many ideas with Mach, developed by CMU in late 1980's, early 1990's, stopped in 1994. [Open Source, GPL]
Sphere SP - A modular RTOS suitable for embedded systems, scalable from tiny stand-alone microkernel up to full client/server OS, supports different processors.
Meta Description: [ Miray is the manufacturer of the embeddable RTOSes Sphere and µnOS and the software tools HDClone, HDShredder, PCISniffer, DiskSpy, DiskCheck and NetSniffer. ]
Spring Project - Realtime microkernel and integrated environment, designed and implemented to support/provide predictability, guarantees (on-line dynamic, atomic), end-to-end scheduling, resource reservations; supports call/task admission model; and multiprocessors.
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