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

Scott Long scottl at samsco.org
Tue Aug 16 05:04:38 GMT 2005

Martin Cracauer wrote:
> Steve Kargl wrote on Mon, Aug 15, 2005 at 10:22:50AM -0700: 
>>On Mon, Aug 15, 2005 at 12:56:57PM -0400, Martin Cracauer wrote:
>>>I have a Dual Opteron board with 4x 1 GB RAM.  The memory count in the
>>>BIOS goes up to 640 KB + 3072 MB, and FreeBSD says it sees 3 GB.
>>>Any way to get my GB back?
>>This should be a FAQ.  No, you can't get it back.
>>David O'brien has explained this more than once in 
>>the freebsd-amd64 list.  Unfortunately, I don't
>>have a handy URL for you to read.  I suggest scanning
>>the list for posts by David in the Feb to May timeframe. 
> Sorry, I just read every message David posted to amd64@ since January
> 2004 and I have only seen a reference that it has been explained too
> often :-) He also said that you only lose the space from 3.5-4 GB, not
> the full GB, and that the board can re-map that physical memory to > 4
> GB.
> http://groups-beta.google.com/group/fa.freebsd.stable/browse_frm/thread/9dc99b2be7f23d4/2d6de607055b020d?lnk=st&q=freebsd-amd64+david+%224+gb%22&rnum=7&hl=en#2d6de607055b020d
> Any further hints what to look for? I am sure I did a decent scan of
> the archive (both local and on google groups).
> Martin

You are definitely going to loose the address space from 3.75GB to 4GB
to normal PCI and APIC overhead.  If you have PCI Express then you'll
likely also loose the space from 3.5 to 3.75GB.  Any other space lost
beyond that would be possible but usual.  And no, most AMD systems do
*NOT* remap the lost space.

If you want a detailed analysis then please send a verbose boot message
along with a pointer to the specs on your motherboard.  That will give
enough to information to say if FreeBSD is at fault or if your
motherboard is simply sub-standard.


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