Linux Find Out CPU Support 36-bit Physical Addressing Or Not

by on November 23, 2008 · 3 comments· LAST UPDATED November 23, 2008

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Q. Physical Address Extension (PAE) refers to a feature of x86 and x86-64 processors that allows more than 4G bytes of physical memory to be used in 32-bit systems. The x86 architecture presently uses only 36 bits out of 52 bits possible. On x86-64 processors, PAE is obligatory in native long mode; currently 40 bits are used out of 52 bits possible. How do I find out if my Linux kernel supports 36-bit or more physical addressing?

A. You can easily find this information by visiting /proc/cpuinfo file. cat /proc/cpuinfo and look for the physical address size. You can also use grep command extract exact information:
cat /proc/cpuinfo
OR
grep physical /proc/cpuinfo
Sample output:

physical id	: 0
address sizes	: 36 bits physical, 48 bits virtual
physical id	: 3
address sizes	: 36 bits physical, 48 bits virtual
physical id	: 0
address sizes	: 36 bits physical, 48 bits virtual
physical id	: 3
address sizes	: 36 bits physical, 48 bits virtual
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{ 3 comments… read them below or add one }

1 Javad November 23, 2008 at 7:40 pm

Not worked on Debian etch with Xeon processors.

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2 DataMatrix November 24, 2008 at 7:42 am

This doesn’t work on ubuntu 8.10 either.
~$ grep physical /proc/cpuinfo
physical id : 0
physical id : 0

Address sizes isn’t displayed.
Some info: The generic kernel doesn’t support more than 3GB RAM. the “server” kernel does support it.

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3 nixCraft November 26, 2008 at 8:10 pm

Can you give more information about current kernel and CPU? This is more applicable to 64 bit XEON CPUs.

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