FreeBSD-6.3 only detects 3GB of RAM

[Erik Osterholm]

On Thu, Mar 27, 2008 at 02:26:05PM +0000, lhecking at users.sourceforge.net wrote:
> fred writes:
> > Hello all,
> > 
> > I am trying to fix an issue with my dual xeon ibm server, it only detects
> > 3GB or RAM but I have 4GB:
> [...]
> 
>  I have seen this problem under Linux on IBM Intellistations (6225).
>  IBM pointed me to a tech document (the number of which I do not
>  remember) that says when the machine detects more than 3GB, it uses
>  some of that memory internally for ... stuff. Still, I find stealing
>  1GB quite unacceptable.

It's not really stealing it.  It has to do with how the computer
hardware handles memory mapped IO.  A certain amount of memory is
allocated per device which needs MMIO; in a typical computer, this
could be as much as 1GB.  It's mapped from the highest portion of the
address space that the computer can handle--in a 32-bit environment,
that means that it's mapped from 4GB on down.

I haven't looked to see if it's arcitecturally the same on 64-bit
machines, but if so, we'll see the same problem once we start putting
more than a few terabytes of RAM in computers.  Luckily, I think that
day is quite a ways off, and who knows what changes will be made in
computing by then.

An MSDN blog actually has a decent description of the issue, including
some points I didn't mention here:
http://blogs.msdn.com/hiltonl/archive/2007/04/13/tbhe-3gb-not-4gb-ram-problem.aspx

Erik