The first version of Microsoft DOS was written by Tim Paterson of Seattle Computer Products. He originally marketed it as SCP 86-DOS. In 1980 Bill Gates and Paul Allen bought exclusive rights to it in order to license it to IBM for their soon to be released Personal Computer. 

      After licensing DOS from Microsoft, IBM shipped it with their Personal Computer as PC DOS version 1.0 in 1981. A new version 1.10 was released shortly after to take advantage of the new double sided floppy disk drives released by IBM and to fix a few bugs in the first version. 

      With the introduction of the IBM XT in early 1983, a new version of DOS was shipped, the PC DOS 2.0. This version was a major improvement from the version 1.10. It provided support for fixed hard drives, hierarchical (tree) disk directories, and support for 360k floppy disk drives (previously only 320k drives were supported).