80286 Protected Mode Test

Liam Proven lproven at gmail.com
Mon Mar 15 08:16:59 CDT 2021

On Sun, 14 Mar 2021 at 19:37, Guy Sotomayor via cctalk
<cctalk at classiccmp.org> wrote:
> At the time I was fairly familiar with the LOADALL instruction.  I had
> modified PC/AT Xenix to use the LOADALL instruction to allow for running
> Xenix programs and multiple DOS programs simultaneously.

Incidentally, I believe that OS/2 1 was not the only 286 OS affected by this.

The development versions of DR's Concurrent DOS 286 could multitask
DOS apps in protect mode on pre-release 286s, but Intel "fixed" the
feature that permitted this in the first shipping version of the
80286, to DR's dismay and horror.


AIUI the feature was later restored, but customer uncertainty,
together with suddenly-questionable compatibility with all the 286s
out there, killed CDOS 286.

I suspect this is instrumental in why DR took FlexOS and X/GEM down
the RTOS route instead... in which form it survived and a distant
descendant, formerly IBM 4690 OS, is still sold by Toshiba.

Liam Proven – Profile: https://about.me/liamproven
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