[PATCH v2 02/14] [x86] Add basic support for .code16

[Joerg Sonnenberger]

On Sun, Jan 05, 2014 at 09:57:21PM +0000, David Woodhouse wrote:
> On Sun, 2014-01-05 at 22:51 +0100, Joerg Sonnenberger wrote:
> > At the moment, In64BitMode is a toogle between two valid modes.
> > Without copying things around in random places (like to the
> > fragments), that's somewhat sane. But the more places are dealing with
> > feature bits, the more important it becomes to ensure the correctness.
> > Does that make sense?
> 
> Yeah, but the correctness aspect is simply and succinctly stated as
> "Mode16Bit and Mode64Bit are mutually exclusive" — that is, the value
> set {1,1} is invalid. 
> 
> I don't see the merit in inverting them both just to declare that {0,0}
> is invalid instead. Can you show a "likely" coding error or paradigm
> which would make that actually useful?

If the feature bits are not copied in one place, a bad mode is choosen.
Not using {0,0} as invalid encoding prevents exactly that.

Joerg