86-DOS

86-DOS Instruction Manual (patersontech.com) 86-DOS provides the tools needed to develop programs for the 8086, as well as a hardware-independent environment in which to run these programs. It is a very modular system. At its core is the disk file manager and I/O device handler, and everything else is considered a “user program”. This allows the system to be easily trailored to any custom requirements.

CP/M

86 DOS Version 0.1 C Serial # 11 (ORIGINAL DISK) (archive.org) This is the precursor to MS-DOS and is likely the oldest known version to survive. (I had previously uploaded Version 0.34, which was at the time thought to hold that honor.)

86-DOS (Wikipedia) 86-DOS (known internally as QDOS, for Quick and Dirty Operating System) is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit.