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

David O'Brien obrien at freebsd.org
Sun Sep 4 01:37:11 PDT 2005

On Mon, Aug 15, 2005 at 12:56:57PM -0400, Martin Cracauer wrote:
> 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.
> Any way to get my GB back?

Depends on the BIOS.  Some BIOS's have knobs for the "memory hole" and
simply control how large the hole is.  Others have nobs for the "memory
hole" that AMD internal often calls the better named "memory hoisting" -
which is what you are asking for.  Revision E CPU's have better HW support
for memory hoisting, but some BIOS's control of it are still buggy.
-- David  (obrien at FreeBSD.org)

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