4 GB RAM showing up as 3, BIOS memory hole and all that

Mikhail P. miha at ghuug.org
Fri Aug 19 22:17:21 GMT 2005

On Tuesday 16 August 2005 16:06, Scott Long wrote:
> Intel Xeon systems typically do the remapping trick, so when you boot
> FreeBSD i386+PAE or amd64 on them, they might show 4.5GB of RAM when
> there reall is only 4GB (this is a limitation of how we compute RAM and
> is purely cosmetic, but should be fixed).  Many AMD Opteron systems do
> not do the remapping and result in you loosing effective RAM.  The
> difference between Intel and AMD is because AMD puts the memory
> controller into the CPU instead of in the PCI host bridge, so it's much
> harder to have the two work together to do the remapping.  I believe
> that there are some Opteron systems that can do this, though.
> A junior doc writer task would be for someone to collect all of the
> email responses that I give on this topic (I seem to get at least one
> query a month) and turn it into an FAQ for the FreeBSD doc set.
> Scott

Here's a dual opteron system we've got recently. Currently it runs 
FreeBSD-4.11 GENERIC kernel (just fresh installed).
It has 4GB of actual RAM, and OS sees it:

CPU: AMD Opteron(tm) Processor 246 (1992.77-MHz 686-class CPU)
  Origin = "AuthenticAMD"  Id = 0xf5a  Stepping = 10
  AMD Features=0xe0500000<<b20>,AMIE,<b29>,DSP,3DNow!>
real memory  = 4227792896 (4128704K bytes)
avail memory = 4117520384 (4021016K bytes)

I'm not sure what BIOS settings it uses (system is overseas), but it is built 
on identical Celestia barebone:



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