> AFAIK, the Mach microkernel was version 3.0 onwards, and used in projects such as IBM Workplace OS, MKLinux, and GNU HURD. GNU Hurd was based on CMU Mach 3 f

> AFAIK, the Mach microkernel was version 3.0 onwards, and used in projects such as IBM Workplace OS, MKLinux, and GNU HURD. GNU Hurd was based on CMU Mach 3 for a while in lieu of Mach 4, which was an oft cited reasons for delays with the Hurd development due to missing critical features anticipated to arrive in Mach 4. Then CMU dropped Mach, the University of Utah picked it up and developed for some time, and finally the active development fizzled out altogether, which added the further delays with GNU Hurd. GNU Mach was branched off Mach 4 at some point and has been (was?) developed as a separate endeavour. In reality, the CPU's at the time were not powerful enough to operate message passing, microkernel based operating systems so, whilst the idea itself was great, it was not a viable one. It could have been a different story today, but with the large scale operating system reasearch and experimentation all but having ceased in the last couple of decades, it does not seem likely there will be a large scale microkernel (or something else) success story any time soon, which is kinda sad.