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

Guy Helmer ghelmer at palisadesys.com
Tue Aug 16 21:46:27 GMT 2005

Scott Long wrote:

> Intel Xeon systems typically do the remapping trick, so when you boot 
> FreeBSD i386+PAE or amd64 on them, they might show 4.5GB of RAM when 
> there reall is only 4GB (this is a limitation of how we compute RAM and
> is purely cosmetic, but should be fixed).  Many AMD Opteron systems do 
> not do the remapping and result in you loosing effective RAM.  The 
> difference between Intel and AMD is because AMD puts the memory 
> controller into the CPU instead of in the PCI host bridge, so it's 
> much harder to have the two work together to do the remapping.  I 
> believe that there are some Opteron systems that can do this, though.

Ahh, that helps explain something.  I got excited yesterday when I 
noticed a Supermicro H8DAR-8 that I thought had 8MB of RAM reported 10GB 
of physical memory instead.


Guy Helmer, Ph.D.
Principal System Architect
Palisade Systems, Inc.

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