HDOS
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HDOS is an early microcomputer operating system, originally written for the Heath H-8 computer system and later also available for the Heath H89 and Zenith Z89 computers. The author was Gordon Letwin, who later was an early employee of Microsoft and lead architect of OS/2.
HDOS originally came with a very limited set of system software tools, including an assembler, but commercial and freeware programs eventually became available for it.
HDOS 2.0, is notable because it was one of the first microcomputer OSs to use loadable device drivers to achieve a degree of device independence and extensibility.