FlexOS was a powerful and innovative operating system from Digital Research, the early 1990's.
Here is a Byte Magazine review of it from back then: https://raw.githubusercontent.com/bootrino/vintage_software/master/1991_01_BYTE_FLEXOSS_MUSCLE.pdf
Here's the Wikipedia article: https://en.wikipedia.org/wiki/FlexOS
Maybe someone at WindRiver knows where the source code of FlexOS is? The wikipedia article above says:
>>>>When Novell decided to abandon further development of the various Digital Research operating systems such as Multiuser DOS (a successor to Concurrent DOS) and Novell DOS (a successor to DR DOS), they sold FlexOS off to the Santa Clara, California-based Integrated Systems, Inc. (ISI) for US$3 million in July 1994.[22] The deal comprised a direct payment of half this sum as well as shares representing 2% of the company. The company already had pSOS+, another modular real-time multitasking operating system for embedded systems, but they continued to maintain FlexOS as well.[22] FlexOS version 2.33 was current as of May 1998 and with FlexOS 2.34 to be released soon after with added support for faster CPUs, 64 MB of memory, EIDE and ATAPI CDROM drives.
>>>>Integrated Systems was bought by their competitor Wind River Systems in February 2000.