HDOS

{{short description|Computer operating system}}

{{About||the Microsoft operating system|Hebrew MS-DOS|the fast food company|Hot Dog on a Stick}}

{{Infobox OS

| name = HDOS

| logo =

| screenshot =

| caption =

| developer = Heath Company / Gordon Letwin, Richard Musgrave

| source_model = Open source

| kernel_type =

| supported_platforms = Heathkit H8, Heathkit H89, Zenith Z-89

| ui =

| family =

| released = {{Start date and age|1978}}

| latest_release_version = 3.02

| latest_release_date =

| latest_test_version =

| latest_test_date =

| marketing_target =

| programmed_in =

| prog_language = BASIC, Intel 8080 assembly language, COBOL, FORTRAN

| language = English

| updatemodel =

| package_manager =

| working_state = Discontinued

| license = Public domain

| website =

}}

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.

Similar to how Heath/Zenith published complete schematics and part lists for its computers, the company sold to users the source code for HDOS.{{cite news | url=https://books.google.com/books?id=az4EAAAAMBAJ&pg=PT56 | title=Untitled | work=InfoWorld | date=1980-09-01 | accessdate=16 February 2015 | pages=17}}{{cite news | url=https://books.google.com/books?id=EDAEAAAAMBAJ&pg=PA24 | title=Personal touch: Zenith inherits do-it-yourself Heath fans | work=InfoWorld | date=1982-09-13 | accessdate=29 June 2014 | author=Needle, David | pages=24–25}} Item references (Heathkit part number) are HOS-1-SL part number 595–2466.

Heath/Zenith also offered the widely used CP/M as an alternative operating system. Although differing in design and internals, CP/M and HDOS are technical comparable and sold for the same price;{{cite journal | last=Wilkinson | first=Bill | date=October 19, 1981 | url=https://books.google.com/books?id=Gj0EAAAAMBAJ&pg=PA16 | title=Software interchangeability problems in the 6502 marketplace | journal=InfoWorld | publisher=IDG Publications | volume=3 | issue=22 | page=16 | via=Google Books | access-date=April 19, 2023 | archive-date=April 20, 2023 | archive-url=https://web.archive.org/web/20230420013852/https://books.google.com/books?id=Gj0EAAAAMBAJ&pg=PA16 | url-status=live }} from 1981 CP/M became the company's preferred operating system.{{Cite magazine |last=Freiberger |first=Paul |date=1982-09-13 |title=Zenith challenges IBM's share of micro market |url=https://books.google.com/books?id=EDAEAAAAMBAJ&lpg=PA35&pg=PA35#v=onepage&q&f=true |access-date=2025-03-19 |magazine=InfoWorld |page=35}}