UNIX99, a Unix-like OS for the TI-99/4A

Related - there's also a 6th Edition Unix port to "big iron" TI990 minicomputers by Dave Pitts, which are based on either the TMS9900 or discrete TTL implementations of it:

https://cozx.com/dpitts/ti990.html

You can find a lot of additional tools there, e.g. an assembler, a GCC port, a TI990 simulator and lots of disk images of and applications for TI's DX10/DNOS OS for the 990 series.