CP/M
From Sasara
CP/M was a microcomputer operating system by Gary Kildall and his Digital Research company, from which DOS was pilfered. It pioneered many of the common file name extensions we use today, along with drive letters and even the basic structure of programs.
Rubenerd was born too late to use it when it was contemporary, but he's making the time to explore it now! He currently uses it on his Apple II via an emulated AppliCard, and CP/M-86 on his emulated IBM 5160 XT.
