|Company / developer||Heath Company / Gordon Letwin, Richard Musgrave|
|Source model||Open source|
|Latest stable release||3.02|
|Available programming languages(s)||BASIC, Intel 8080 assembly language, COBOL, FORTRAN|
|Supported platforms||Heathkit H8, Heathkit H89, Zenith Z-89|
HDOS is an early microcomputer operating system, originally written for the Heathkit H8 computer system and later also available for the Heathkit H89 and Zenith Z-89 computers. The author was Heath Company employee Gordon Letwin, who later was an early employee of Microsoft and lead architect of OS/2.
HDOS originally came with a limited set of system software tools, including an assembler, but many commercial and large set of freeware programs from HUG (Heath User Group) became available for it eventually.
HDOS 2.0 is notable because it was one of the first microcomputer operating systems to use loadable device drivers to achieve a degree of device independence and extensibility. Device names followed the RSX-11-style convention of DKn: where the first two letters were the device driver file name and n was a number (DK0:, DK1:, and so on would all be handled by DK.SYS). Other similarities to RSX included the use of PIP for file transfer, and the use of EOT for file termination.
The full source paper listing is held at yesterpc.org, old computer museum. Item references (Heathkit part number) are HOS-1-SL part number 595-2466.
- HDOS 1.0 – written in 1978 by J. Gordon Letwin
- HDOS 1.5
- HDOS 1.6
- HDOS 2.0 – released in 1980, written by Gregg Chandler, released into the public domain in April 1988
- HDOS 3.0 – released into the public domain in August 1986
- HDOS 3.02 – enhanced version by Richard Musgrave
See also 
- Michael A. Pechuria, Comparing Two Microcomputer Operating Systems: CP/M and HDOS. Communications of the ACM, March 1983, vol. 26, no. 3.
- Society of Eight-Bit Heathkit Computerists A web site dedicated to preserving the Heathkit 8-bit computers, source listing in PDF form