Concurrent DOS, great stuff!
One of the first multiuser systems I really learned as a spotty teenager back in the late '80s. Working with my Dad on getting shared DataEase databases working at his workplace. We had C/DOS on the "master" PC system and a couple Wyse terminals hanging off the serial ports, 3 systems independently using a shared, albeit cutdown, PC based RDBMS system. I loved it so much as a kid that I ended up with a career working in database systems.