Editing Multiuser DOS (section)

==={{anchor|CCPM286|CDOS286|FlexOS}}Concurrent DOS 286 and FlexOS 286===
In parallel to the Concurrent DOS 68K effort,<ref name="Knox_1986_CDOS68K"/> Digital Research also previewed Concurrent DOS 286<ref name="BYTE_1985_CDOS286"/> in cooperation with [[Intel]] in January 1985. This was based on [[MP/M-286]] and Concurrent CP/M-286, on which Digital Research had worked since 1982.<ref name="Swaine_1982_MPM286"/><ref name="Bidmead_1985_CCPM286"/>

Concurrent DOS 286 was a complete rewrite in the [[C (programming language)|C language]] based on a new system architecture with dynamically loadable device drivers instead of a static BIOS or XIOS.<ref name="BYTE_1985_CDOS286"/> One of its main architects was Francis "Frank" R. Holsworth<!-- born 19??-09-04 https://amaus.net/static/S100/software/DRI/Digital%20Dialogue/03x04%20198408%20Digital%20Dialogue.pdf -->.<ref name="Wein_2002"/><ref name="Holsworth_2006"/> The operating system would function strictly in [[Intel 80286|80286]] native mode, allowing [[protected mode]] [[multi-user]], multitasking operation while running [[8086 emulation mode|8086 emulation]].<ref name="Infoworld_1985_Super"/><ref name="BYTE_1985_CDOS286"/> While this worked on the B-1 [[stepping level|step]] of prototype chip samples, Digital Research, with evaluation copies of their operating system already shipping in April, discovered problems with the emulation on the production level C-1 step of the processor in May, which would not allow Concurrent DOS 286 to run [[Intel 8086|8086]] software in protected mode. The release of Concurrent DOS 286 had been scheduled for late May, but was delayed until Intel could develop a new version of the chip.<ref name="Infoworld_1985_Super"/> In August, after extensive testing E-1 step samples of the 80286, Digital Research said that Intel had corrected all documented 286 [[errata]], but that there were still undocumented chip performance problems with the prerelease version of Concurrent DOS 286 running on the E-1 step. Intel said that the approach Digital Research wished to take in emulating 8086 software in protected mode differed from the original specifications; nevertheless they incorporated into the E-2 step minor changes in the [[microcode]] that allowed Digital Research to run emulation mode much faster (see [[286 LOADALL|LOADALL]]).<ref name="Infoworld_1985_80286"/><ref name="DRI_1986_CDOS68K-2"/> These same limitations affected [[FlexOS 286]] version 1.x, a reengineered derivation of Concurrent DOS 286,<ref name="DRI_1986_FlexOS286"/><ref name="CBR_1987_FlexOS-286"/> which was developed by Digital Research's new Flexible Automation Business Unit in Monterey, California, since 1986.

Later versions added compatibility with PC DOS 2.x and 3.x.

Known versions include:

* Concurrent DOS 286 1.0 (1985)<ref name="DRI_1986_CDOS68K-2"/>
* Concurrent DOS 286 1.1 (1986-01-07)<ref name="DRI_1986_CDOS68K-2"/>
* Concurrent DOS 286 1.2 (1986)<ref name="Meta_1986_HighC12"/>
* [[FlexOS 286 1.3]] (November 1986)<ref name="DRI_1986_FlexOS286"/>
* [[FlexOS 286 1.31]] (May 1987)