Editing Virtual DOS machine (section)

=={{anchor|8086 emulation}}Concurrent DOS 8086 emulation mode==
In January 1985 [[Digital Research]] together with Intel previewed [[Concurrent DOS 286 1.0]],<ref name="BYTE_1985_CDOS286"/> a version of [[Concurrent DOS]] capable of running real mode DOS programs in the [[80286]]'s protected mode.<ref name="BYTE_1985_CDOS286"/> The method devised on B-1 stepping processor chips, however, in May 1985 stopped working on the C-1 and subsequent processor steppings shortly before Digital Research was about to release the product. Although with the E-1 stepping Intel started to address the issues in August 1985, so that Digital Research's "8086 emulation mode" worked again utilizing the undocumented [[286 LOADALL|LOADALL]] processor instruction,<ref name="DRI_1986_CDOS68K-2"/><ref name="Deitel_1992"/> it was too slow to be practical. [[Microcode]] changes for the E-2 stepping improved the speed again.<ref name="Infoworld_1985_Super"/><ref name="Infoworld_1985_80286"/> This early implementation can be seen as a predecessor to actual virtual DOS machines.

Eventually, Concurrent DOS 286 was reworked from a potential desktop operating system to become [[FlexOS 286]] for industrial use in 1986.<ref name="DRI_1986_FlexOS286"/><ref name="CBR_1987_FlexOS-286"/> It was also licensed by [[IBM]] for their [[4680 OS]] in 1986.<ref name="InfoWorld_1986_Concurrent"/><ref name="DRI_1986_IBM"/>

When Intel's 80386 with its virtual 8086 mode became available (as samples since October 1985 and in quantities since June 1986), Digital Research switched to use this to run real mode DOS programs in virtual DOS machines in protected mode under [[Concurrent DOS 386 1.0]] (February 1987)<ref name="Weiss_1987"/> and [[FlexOS 386 1.0]] (June 1987).<ref name="CBR_1987_FlexOS"/> However, the architecture of these multiuser multitasking 
protected mode operating systems was not DOS-based by themselves.

[[Concurrent DOS 386]] was later developed to become [[Multiuser DOS]] (since 1991) and [[REAL/32]] (since 1995). [[FlexOS 386]] later became [[4690 OS]] in 1993.