Re: Over 2.5 GB ram problem

On Thu, Jul 11, 2013 at 9:03 AM, Gary Roach <gary719_list1@verizon.net> wrote:

On 07/10/2013 02:53 PM, Joel Rees wrote:

On Thu, Jul 11, 2013 at 1:47 AM, Gary Roach <gary719_list1@verizon.net> wrote:

[...]
Now I am really confused. I have two systems; one with an Intel P4, 3GHz, multi-thread and one with an Intel i5-750. Both are running Wheezy with a KDE desktop. The i5-750 system recognizes the 4GB of memory but the P4 system does not. They are both running "pae" kernels.

My impression is that the P4 is likely to be a 32 bit cpu. P4s may or may not have the hardware extensions that allow (bank-switch) access to memory beyond the 4G boundary (2G on some systems).

Is your i5-750 a 32 bit cpu or 64 bit? My memory is that there are some intel "core" series cpus that were 32 bit. But those would all be recent enough to include the "bigmem" or pae hardware extensions in the cpu.

But I think the i5 is likely to be 64 bit, anyway. (And the 32 bit pae kernel includes the "magic" that allows the 32 bit mode to access memory above the 32 bit boundary.)

The Kinfocenter shows that the P4 system is running a Linux 3.2.0-4-686-pae kernel and the Aptitude listing agrees. The i5-750 system Kinfocenter shows a Linux 3.8-1-686-pae kernel but the Aptitude listing shows a Linux 3.2.0-4-686-pae kernel installed. Further, in the the layout of the firmware-linux package in Aptitude, the P4 system list it in kernel / main and the i5-750 system list it in kernel / non-free.

What is going on here?

Gary R.

No idea about Kinfocenter, but the hardware on a motherboard can definitely include stuff that the manufacturer refuses to release developer specs without an NDA, if at all. Older motherboards tend to have better workarounds, found by reverse-engineering.

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Joel Rees

First, i believe that Kinfocenter is showing the version of the linux-image-686-pae dummy package and not the actual kernel version. One problem solved.

Dummy packages in your system are something you'll know more about than I will, of course.

Second, you are right. The i5-750 is a 64 bit processor. But the documentation for the pae enabled kernel specifically lists all of the Intel P series processors as compatible. Ergo, the P4 should be able to access well over 4GB of ram. So what is wrong?

Gary R.

There is "compatible" (in the sense of bank switching works), and there is "compatible" (in the sense of the kernel runs, anyway).

A little more information:

http://en.wikipedia.org/wiki/Physical_Address_Extension

note the "Support" section, where it mentions "cat /proc/cpuinfo" (but does not mention that you'll want to " | grep pae" to strip out the noise).

And it sort-of mentions another point, ergo, support is required in the OS, in the CPU, *and* in the motherboard.

For example, the cpu on my family box supports pae, but the motherboard only wires out enough address lines for 2G max. So, even if I installed, say, two matched ppairs of physically compatible 1G RAMs, all that any OS is going to see is 2G.

In other words, your P4 may have limits in the motherboard that overrule the cpu's capabilities.

(You can get hints in the advanced settings in the BIOS, but even then you can't be sure.)

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Joel Rees