On 02/14/2013 01:27 PM, David Woodhouse wrote:
>
> So it *is* jumping to 0xfffffff0 but the memory at that location isn't
> what we expect? Do the PAM registers affect *that* too, or only the
> region from 0xc0000-0xfffff? Surely the contents at 4GiB-δ should be
> unchanged by *anything* we do with the PAM registers?
>
> Or maybe not... after also downloading the i440fx data sheet, I'm even
> more confused. There's some aliasing with... not the region at 1MiB-δ
> but the region at 16MiB-δ:
>
> (From §4.1 System Address Map):
>
> 2. High BIOS Area (FFE0_0000h−− FFFF_FFFFh)
> The top 2 Mbytes of the Extended Memory Region is reserved for System
> BIOS (High BIOS), extended BIOS for PCI devices, and the A20 alias of
> the system BIOS. The CPU begins execution from the High BIOS after
> reset. This region is mapped to the PCI so that the upper subset of
> this region is aliased to 16 Mbytes minus 256-Kbyte range.
>
That is presumably a 286 compatibility hack -- the 286 had 24 address
lines. I doubt anyone gives a hoot about it, and neither EDK2 nor
SeaBIOS should care.
-hpa
--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.
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