Dell 690 only seeing 3 GB RAM (was: slow last 128MB...)

[Michael ODonnell]

>> we can now see all 4Gb.  I'm betting it was disabling the MTRR
>> that did it, not the PAE stuff...
>
> I have a feeling it was the PAE stuff.  Without PAE, the CPU can
> only see the 4Gb.  and the BIOS and PCI devices (especially ones
> that are only 32-bit aware) are all in the memory between 3-4Gb.
> With PAE (Physical Address extension) the Kernel can use memory
> above the 4Gb address range to map the previously unused 1Gb
> of memory.

Yeah, I get that, now, but I had just finished explaining to
my boss that (given the vintage of the kernel in question)
the code paths for disabling the MTRR were probably fewer and
better exercised than the PAE ones.  Maybe I should have said
I was "hoping" rather than "betting", primarily to keep the
risk-management hounds at bay...