80286 Protected Mode Test

jim stephens jwsmail at jwsss.com
Sat Mar 6 22:23:05 CST 2021

On 3/6/2021 3:10 PM, Sean Conner via cctalk wrote:
>   Once the 80286 is in protected mode, there is no way to get out of
> protected mode except via the RESET signal.
The 286 can exit protected mode with the LOADALL instruction. 
Microsoft's extended memory driver pissed off the world (Intel) when 
someone noticed it was addressing +1mb memory with no hint of the 
reset.  The rest of the world used a driver and memory buffer algorithm 
which optimized accessing > 1mb because of the horrible latency involved 
in what you are describing.

Another client and some friends I had were caught with their s**t 
stinking because they'd used it in their bios (Micro5 systems).

I suspect there's some sort of failure involved like that, but not sure 
how it could live to print the message in the case of going into 
protected mode but not getting back out.

As far as a defective 80286 I can't imagine it passing a lot of the bios 
test code at all if it was internally damaged.

Any POST card (port 80) handy?  Maybe some hints there?

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