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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