CP/M

CP/M was a computer operating system developed by Intergalactic Digital Research popular with microcomputer hobbyists within the late 1970's.

QDOS

QDOS, or "Quick and Dirty Operating System", later known as 86-DOS was a reverse engineering of CP/M. It 86-DOS was created because sales of the Seattle Computer Products 8086 computer kit, demonstrated in June 1979 and shipped in November, were languishing due to the absence of an operating system. The only software that SCP could sell with the board was Microsoft's Standalone Disk BASIC-86, which Microsoft had developed on a prototype of SCP's hardware. SCP wanted to offer the 8086-version of CP/M that Digital Research had initially announced for November 1979, but it was delayed and its release date was uncertain. This was not the first time Digital Research had lagged behind hardware developments; two years earlier it had been slow to adapt CP/M for new floppy disk formats and hard disk drives. In April 1980 SCP assigned 24-year-old Tim Paterson to develop a substitute for CP/M-86.

IBM

When IBM was developing their operating system, the first contacted Microsoft, as they were known for making BASIC interpreters. Bill Gates first told them to contact Gary Kildall, the founder of Intergalactic Digital Research. After that, Bill told Gary that he was gonna get visited by some important people, but because he signed a Non-Discolsure Agreement, he couldnt tell him that it was IBM. Gary didn't take him seriously, and went flying that day. When IBM arrived at his house, only his was was there. She refused to sign the NDA that IBM had imposed, tho we still don't really know what happened that day. However, we do know that after that, IBM came to Microsoft, and asked if they couldnt make an operating system. Even though they knew that they couldn't, they said that they could, and signed a deal with IBM. What Microsoft did after that was that they bought QDOS for $50000 , and changed the name to "MicroSoft Disk Operating System", or for short, MS-DOS which IBM changed to PC-DOS, which was changed back to MS-DOS. However, Microsoft didnt mention in the contract that the deal with IBM was exclusive, and they also sold copies to other OEMs, such as HP, Compaq, Gateway 200, Dell, etc.