VSTa
VSTa (Valencia's Simple Tasker) was an operating system with a microkernel architecture, with all device drivers and file systems residing in userspace mode. It is mostly POSIX compliant, except when POSIX compatibility got in the way of extensibility and modularity. It was conceptually inspired by QNX and Plan 9. Written by Andy Valencia, and released under the GPL license. Currently the licensing for VSTa is Copyleft.
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