Re: HIGHMEM

[Matt Domsch]

On Fri, Feb 13, 2004 at 01:20:36PM +0100, Nagy Tibor wrote:
> We have two Dell Poweredge servers, an older one (PowerEdge 6300) and a
> newer one (PowerEdge 6400). Both servers have 4GB RAM, but the Linux
> kernel uses about 500MB less memory in the newer machine.

This is a FAQ on the Linux-PowerEdge mailing list, and is captured here.
http://lists.us.dell.com/fom-serve/cache/68.html

I have 4GB (or more) RAM in my system. How come Linux sees less than
that?

BIOS must reserve some address space below 4GB for PCI devices such as
RAID controllers, SCSI controllers, NICs, etc. RAID controllers in
particular may request and be given 256MB each. This is address space
that would normally be occupied by RAM, but instead is used by PCI
devices.

RAM addresses start at 0 and grow up. PCI device addresses start at
~4GB and grow down. As long as there is no overlap, the OS will see
all available RAM and make use of it. If there is overlap, the PCI
devices win, and that RAM is not made available to the OS.

This is working as designed per PCI, BIOS, and system chipset
specifications. 


You may wish to subscribe to the Linux-PowerEdge@xxxxxxxx mailing list
at http://lists.us.dell.com/.

Thanks,
Matt

-- 
Matt Domsch
Sr. Software Engineer, Lead Engineer
Dell Linux Solutions linux.dell.com & www.dell.com/linux
Linux on Dell mailing lists @ http://lists.us.dell.com
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