6 Mar
2021
6 Mar
'21
11:46 p.m.
-----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:
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!
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