Re: That horrible hack from hell called A20

["H. Peter Anvin" <hpa@transmeta.com>]

Linus Torvalds wrote:
> 
> So the "orb $2,%al ; andb $0xfe,%al" will potentially change both of
> these. And I'd feel a hell of a lot more safe, if we avoided using 0x92
> except when we find that we absolutely _have_ to.
> 
> How about making the keyboard controller timeouts shorter, and moving all
> the 0x92 games to after the keyboard controller games. That, I feel, would
> be the safest approach: try the really old approach first (that people are
> the least likely to use as GPIO - it's just too damn painful to go through
> the keyboard controller, and the keyboard controller A20 logic is just too
> well documented, so nobody would use it for anything else).
> 
> If the keyboard controller times out, or if A20 still doesn't seem to be
> enabled, only _then_ would we do the 0x92 testing.
> 
> Btw, do we actually know of any machine that really needs the "and $0xfe"?
> That register really makes me nervous.
> 

Good question.  The whole thing makes me nervous... in fact, perhaps we
should really consider using the BIOS INT 15h interrupt to enter
protected mode?

	-hpa

-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt
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