80286 Protected Mode Test

Rob Jarratt robert.jarratt at ntlworld.com
Sun Mar 7 01:46:11 CST 2021

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Chuck Guzis via
> cctalk
> Sent: 07 March 2021 00:08
> To: Sean Conner via cctalk <cctalk at classiccmp.org>
> Subject: Re: 80286 Protected Mode Test
> 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.

I wondered if that might be how it works after reading that you can only switch to real mode with a reset. I will follow this line of inquiry. Thanks for the suggestion!

> --Chuck

More information about the cctech mailing list