Kernel only detection 2 GB of memory on AMD

Vincent Stemen vince.dragonfly at
Sat Mar 20 12:47:00 PDT 2010

On Wed, Mar 03, 2010 at 07:20:28PM +0000, Thomas E. Spanjaard wrote:
> Vincent Stemen wrote:
> > On Tue, Mar 02, 2010 at 10:57:58PM -0800, Matthew Dillon wrote:
> >>     I think the only way the kernel will be able to see all the memory
> >>     on that box is if you run a 64-bit DFly kernel.
> > 
> > So it is normal to only be able to use 2GB of memory in 32-bit mode??  
> It is if the BIOS only maps 2Gibyte worth of RAM below the 4Gibyte mark.
> Most BIOSes allow you to disable memory remapping entirely though, in
> which case you can use as much memory as is left when subtracting all
> other physical address space consumers (most importantly memio).

BTW, I've been meaning to follow up on this and say thanks for the
information.  You were right.  I didn't realize that some of these
server boards mapped the memory so that only 2 or 3 GB is visible below
the 4GB mark.

We tested FreeBSD, just to confirm, and it had the same problem.
I could not find any option to disable memory remapping in the BIOS
though.  Apparently, these boards are pretty much meant to only be run
with 64bit kernels.

It does seem to work fine with a 64bit Dragonfly kernel :-).


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