[SeaBIOS] SeaBIOS PAE experiment

[Rudolf Marek]

Hi Kevin,

> * The reason why PAE is interesting (instead of standard i386 paging)
>   is that it allows for 64bit mappings and because one can set it up
>   with just a single level page directory of 2MB pages.  The single
>   level page directory makes maintaining it much easier.

There is a performance penalty if you do it for first 2MB of phys memory on some
P6/XeonP4 CPUs.  Others could trigger undefined behaviour.

The reason for this are the special regions in the 640KB-1MB covered by various
MTRRs.  See 11.11.9 Large Page Size Considerations in the Intel manual for the
details.

Thanks
Rudolf