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

Martin Cracauer cracauer at cons.org
Mon Aug 15 16:56:59 GMT 2005

Somebody here must know this:

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.

I know/assume/heared this has to do with the I/O space placed in the
upper GB and that you can re-map the top GB to avoid the problem.

The BIOS has the options to have a memory hole which I set to 1 GB,
thinking it would put the 3G-4G memory at 4G-5G instead.  It seems I
am wrong about this, I am stuck with 3 GB.

Obviously I don't understand how that top GB for I/O works, how can
that work when the machine has less than 3 GB?

Any way to get my GB back?

I know the hardware is OK by cross-checking all modules individually.
And I can see that the memory bandwidth is that of dual-channel, so it
is not a case of one stick not being recognized.

- Armima Rioworks HDAMB, 2x Opteron 246, 4x 1 GB PC2100
- FreeBSD-7.0-current/AMD64, SMP kernel
- dmesg, pciconf etc on http://www.cons.org/cracauer/machines/wings/

Martin Cracauer <cracauer at cons.org>   http://www.cons.org/cracauer/
 No warranty.    This email is probably produced by one of my cats 
 stepping on the keys. No, I don't have an infinite number of cats.

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