Mailinglist Archive: opensuse (3513 mails)

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Re: [opensuse] Why does 10.3 x86_64 only see 3gig of memory?
  • From: "David C. Rankin" <drankinatty@xxxxxxxxxxxxxxxxxx>
  • Date: Thu, 08 Jan 2009 22:14:53 -0600
  • Message-id: <4966CF3D.4080405@xxxxxxxxxxxxxxxxxx>
Patrick Shanahan wrote:
Socket Designation: Socket 939

I have this socket and had to change the bios to not reserve above
3.2g for system processes (if memory serves).

anyway, it is a bios issue.


Sun put out a great paper that explains the problem: "Memory Hole in
Large
Memory X86 Based Systems", XES Product Development Team, May 19, 2004.

http://techfiles.de/dmelanchthon/files/memory_hole.pdf

To summarize, in the X86 architecture, there are 2 types of devices
that are
mapped into processor address space. The first type is physical memory. There
is a 1:1 correlation between memory size and processor address space. A system
with 512MB of physical memory will require 512MB of processor address space for
it to be accessed. The second, and perhaps less obvious, type is device memory
or device address space.

As long as you have less than 4 gig there is no problem because the
physical
memory and device address space all fit within the processor address space.
However, when you fill the processors address space up with physical memory,
then you have an overlap between the physical memory and device address space
causing a conflict.

The only way to resolve this conflict (and retain compatibility) is to
allow
the device address space to be mapped over (overlap) the memory causing a
“hole” in this address range. The memory for this space can’t be used and can’t
be remapped and will not be reported as available memory. This is why you see
less available memory.

This is also why a 64-bit layout isn't a complete solution. Many
'devices'
have to live within 32-bit address space regardless of whether you have 64-bit
address space available. So in the case of a 64-bit system with 4 gig of ram,
you will still lose some of your memory due to the device address space that
still requires a 32-bit address range.

Clear as mud right! I need some aspirin..


--
David C. Rankin, J.D.,P.E.
Rankin Law Firm, PLLC
510 Ochiltree Street
Nacogdoches, Texas 75961
Telephone: (936) 715-9333
Facsimile: (936) 715-9339
www.rankinlawfirm.com
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