80286 Protected Mode Test

[Chuck Guzis]

On 3/6/21 3:10 PM, Sean Conner via cctalk wrote:

>   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.

I'll add that, at least in the PC AT world, the switch to real mode is
accomplished by writing a value into a reserved cell in CMOS
(configuration memory--I wish they'd lose that 4-letter
appellation--what, in a modern PC *isn't* CMOS?).  Upon executing the
reset code, the BIOS checks for the "reason for shutdown".   If it was a
switch to real mode, then all of the various hardware tests are
bypassed, the register file is restored and execution continues.

What this means that if your CMOS (ugh!) memory isn't functioning, the
switch to real mode won't work.

--Chuck