This project’s last update was 2017, but it seems to have got a good distance before going quiet. For CPC, MSX, PCW Joyce and Enterprise machines:
SYmbiosis Multitasking Based Operating System
Preemptive multitasking - 1024 Kb dynamic memory - 2 terabyte filesystem - 100% flexible windows GUI - network capable
available for your Amstrad CPC, MSX, Amstrad PCW and Enterprise 64/128
According to these facts, it’s a microkernel design.
It is an impressive OS, but unfortunately, closed source. At one point I was looking for something new to put on an FPGA with my A-Z80 cpu in order to test it (and to see something different from the standard batch of retro designs) and could not manage to get the sources for this. In fact, all discussion groups, web pages etc. were appearing dormant, silent and dead to any new activity. Perhaps that changed recently?
Well, it seems the project is alive! And there’s an in-browser MSX emulator too (has a Game of Life as well as much else, but I think I might have crashed it.)