Does it all appear in the e820 map? If not, then the chipset is
probably masking it; check to see if there's any likely-looking BIOS
options to make it appear.
I assume you're compiling for either 32-bit PAE or 64-bit. If you're
compiling 32-bit without PAE (HIGHMEM64G) then the remaining Gbyte may
simply be above 4G and out of the kernel's reach.
J
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