80286 Protected Mode Test

Rob Jarratt robert.jarratt at ntlworld.com
Sun Mar 7 06:04:33 CST 2021

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Fred Cisin via
> cctalk
> Sent: 06 March 2021 23:17
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: 80286 Protected Mode Test
> A stupid idea:
> Could the test require, and be failing, access to memory above 1M?

I think that is unlikely because the board comes with 1M onboard and I
believe the system is designed to work with just that memory. I can try it
though if I can find compatible 30-pin simms (I do have some somewhere).
Curious why you think it might require access to memory above 1M though? I
am currently working to the hypothesis that it needs the non-volatile memory
to work to remember that the reset was part of the POST. The NVR is provided
by the RTC element of the 82C206 chip. I may have to get the logic analyser
out to see if that is happening or not, but at the moment I still have an
intermittent problem getting the board to start at all, I think due to my
poor work on the repair wires for the damaged tracks.



> On Sat, 6 Mar 2021, Rob Jarratt via cctalk wrote:
> > I have a DECstation 220 (Olivetti M250E) which is failing POST on a
> > "simple test of the 80286 protected mode". It says in a service manual
> > I have that for this test the CPU is set in the protected mode, the
> > machine status word is checked to see whether it indicates the
> > protected mode and then exits protected mode. This test seems to be
> > failing. Is there any possible explanation for this other than a
> > failed 80286 CPU? Could there be any external reason? This board
> > suffered some battery leak damage. Clearly the
> > 80286 is working well enough to execute this diagnostic and send some
> > text to the screen, so it basically works.
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob

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