For example, a lot of systems used VxWorks that would run paravirtualized Windows 9x for user interface - you normally booted into windows, and during boot a .VXD file would get loaded that was actually a VxWorks bootstrap that preempted Windows kernel components and continued running them as "userspace" task in a way that allowed other realtime tasks to work.
This architecture was used for example in KUKA robots KRC1 controllers.
Similar setups are used by "PC PLC" products for example from Siemens.
It was one of the first, things t was the fastest, the other was Intel RTX, but it only ran on their hardware,and a very few others. QNX booted on anything PC, they were out soon after the Compaq 386. Their demo was late 1980s WindRiver did not get rolling until the early 90s, HP was doing Forth.
