[OpenBIOS] Why protected mode?

(Kevin P Lawton) bochs at world.std.com
Thu Jul 15 13:31:48 CEST 1999

> I also wanted to add that all this nonsense was necessary because int he
> design of the 80286 there was a method of ENTERing protected mode, but no
> clean way to get back.  I'm curious to know whether the double/triple fault
> was used as a way back because that's how Intel had *designed* it, or if the
> lack of method to come back to Real mode was an oversight.

I'd say it was by mistake.  Resetting upon tripple fault is
about the only intelligent thing you can do, giving your
system the chance to reboot.  That I'll given them.

But it took some hacking to get the rest of this to work.
Even the BIOS has to be aware of this aweful technique.
There's a boot flag in CMOS (reg 0xf) letting the BIOS
know what the cause of the shutdown was.  That way
an intentional switch back to real mode could be detected,
and the init routines are bypassed.  It jumps to a
prescribed address stored in 0x467.

Looks like Intel screwed up, and didn't give the 286
enough power to virtualize real mode stuff.
Can you hear that Intel commercial tune playing in
your head?  Bum bump bum bummmmmmmmmmmm

Also IMHO, by far their greatest mistake in history
was when they went to a new protected mode environment
in the 286, they carried forward the segmentation
architecture.  If you think about it, they could
have had a strict v8086 environment in the 286 which
would virtualize the 8086 completely giving compatability
there, then go to a whole new instruction set and
architecture in protected mode.

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