kernel/system can't see all 4G memory

Lamar Owen lowen at pari.edu
Thu Nov 3 13:38:53 UTC 2011


On Nov 3, 2011, at 3:46 AM, George R Goffe wrote:
> I upgraded my lenovo 60t to 4G memory and don't seem to be able to  
> see all 4G of the upgrade.
>
> The hardware spec for this machine says it supports 4G memory.
>
> Any hints/clues/tips would be GREATLY appreciated.

It is a hardware limitation in the 945M chipset; the chipset  
physically only has 32 address lines, and the PCI/PCI-e memory map has  
to go somewhere.  That 'somewhere' is typically the top 1GB of address  
space, leaving 3GB (give or take) available for RAM.  If you don't  
have that 33rd address line, you can't go above the 32-bit barrier PAE  
or 64 bit regardless.

Same thing is true in my Dell Precision M65, although the Dell BIOS  
squeezes a little more out of it, 3.5GB or so is what I get with 4GB  
of RAM installed, and the 64-bit kernel, Fedora 14.

The 965 chipset fixes this by adding physical address lines to the  
chipset and adding remapping support in hardware.

In the Dell case, the Precision M65 is virtually the same machine as a  
Latitude D820; both have the 945; the Latitude D830 and equivalent  
Precision model have the 965, and can address all 4GB of RAM and can  
map the RAM that is displaced by the PCI/PCI-e address space above the  
4GB (32-bit) line. (If your BIOS has 'PCI Memory Hole Remapping'  
support you can try that; see http://en.wikipedia.org/wiki/PCI_hole  
for more information.  But this won't help go above the 32-bit  
addressing barrier if the chipset physically only has 32-bits of  
addressing capacity, regardless of processor installed.

Again, neither PAE nor 64-bit OS will help since the 945 chipset  
physically does not have a 33rd (and above) address line, so the PCI/ 
PCI-e memory map has to go somewhere in the 4GB (32-bit) address  
space, taking away from the 4GB of installed RAM.

Please also see http://www.dansdata.com/askdan00015.htm

and

http://www.linuxquestions.org/questions/linux-hardware-18/intel-945-chipset-and-4gb-of-memory-578912/

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