how does the kernel go about mapping memory for amd64?

Jeremy Chadwick koitsu at
Mon Oct 13 12:40:23 UTC 2008

On Mon, Oct 13, 2008 at 02:15:56PM +0200, Gary Jennejohn wrote:
> I have a new mother board, a Gigabyte GA-MA78GPM-DS2H, which shows
> really weird behavior under FreeBSD when I have 4GB of memory and a
> PCI card plugged into it.
> With an Adaptec 29160 plugged in FreeBSD sees only 3GB and there is _no_
> memory mapped above 4GB.  This seems like a bug in the BIOS.
> With no PCI card plugged in FreeBSD sees memory mapped above 4GB and
> almost 4GB are available.
> Note that this board has graphics in the chipset (AMD 780G) which uses UMA
> (128 MB) for the video buffer.
> This behavior leads me to believe that FreeBSD is only using the memory
> mapping hints provided by the BIOS and is not sizing and mapping memory
> itself.
> Is that correct?

You might also try freebsd-hackers for this kind of thing, as it's
pretty low-level.

Does Linux and/or Windows exhibit the same problem?  If so, I would open
up a case with Gigabyte.  This should be very easily reproducible,
although I would not be surprised if they ask for you to send them your
Adaptec controller.

| Jeremy Chadwick                                jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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