86-DOS

86-DOS, known internally and in official documents as QDOS, Q-DOS, Seattle DOS, and SCP-DOS, is an operating system developed by Seattle Computer Products that was later licensed and then bought by Microsoft and used as the basis for IBM PC-DOS and MS-DOS.

Development History
Development of QDOS began in April 1980 out of the growing need for an 8086 operating system, as CP/M-86 had been delayed since December 1979. It was written on a Cromemco Z80 computer running Cromemco's CDOS, in a text editing software called MicroPro WordMaster. It was then translated using a Z80 to 8086 translator called TRANS-86 (which was later ported to 86-DOS and renamed to simply TRANS) and tested on the Seattle Computer Products 16-bit Computer System. QDOS was ready to ship in July, with Tim Paterson, the creator, spending about half of his time on its development. Though the operating system wasn't completely finished, Paterson figured a quick release was more important than adding all the features.

Though some parts of QDOS were written from scratch or by referencing the CP/M-80 manual, others were created in Z80 assembly and then translated using the TRANS command. Paterson claims that QDOS was created with CP/M translation compatibility being the goal - however, these extreme similarities between the OSes caused controversy, even going as far as a defamation case. It has also been rumored that QDOS' development was aided by not only the CP/M manuals, but the source code as well (or even Digital Research's original OEM translation tools). However, these rumors are both unsourced and unlikely.

Paterson later revealed he had mostly worked on hardware before QDOS, and that the idea came during his work on Seattle Computer products' 8086 board. He decided on aspects of the OS because of his experience with North Star and Cromemco's variants of CP/M, UCSD p-System, and Unix. He also took inspiration for the filesystem from an unfinished 8-bit operating system known as Microsoft Interrupt Driven Asynchronous System (MIDAS or M-DOS, called MDOS before 1980), written by Marc McDonald, which he learned about during the 1979 National Computer Conference.