how does the kernel go about mapping memory for amd64?

[Gary Jennejohn]

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?

---
Gary Jennejohn