80286 Protected Mode Test

Sean Conner spc at conman.org
Sat Mar 6 17:10:07 CST 2021

It was thus said that the Great Rob Jarratt via cctalk once stated:
> 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.

  There might be damage to the keyboard controller that could cause the
issue.  Once the 80286 is in protected mode, there is no way to get out of
protected mode except via the RESET signal.  If I remember correctly, you
could program the keyboard controller to send a RESET signal to get out of
protected mode.  Also, the keyboard controller also managed the state of
address line A20, which is another important factor on PCs.


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