Section 4.2.4 described the read/write access to regions between 640k - 1MB There is a table on page 106. I think that is what you need.
Have a look to the src/cpu/amd/model_gx1 section
I'm not sure, but I think the BC_XMAP_2 and BC_XMAP_3 are untouched. Means all is set to zero.
In that case the region c0000 - fffff is disabled.
BC_XMAP_1 has a 0x60 entry in the cpu_setup table. How can I enable the other Memory regions.
as the same way, I'm not sure what will happen ;)
chris