*PAE* Kernel Still Does Not Detect All RAM

[Mordac]

See, the thing is, I could see all the memory I had when I only had 2GB installed. That's what confuses me. If I could only see 1GB back then, I would've thought it natural...

[jpollard]

I think you are confusing physical address space with physical memory.

The address space is used already. Whether you have 1GB or 4GB. The only change is when the assigned physical address space is filled (4GB) you can't use all 4 - some is taken for video, some is taken for the I/O. The same memory map applies.

Look in /proc/cpuinfo and show us what is in the values are for address sizes.

[Mordac]

I'm not sure I fully understand that. *scratches head*

So, what you're saying is, that if I don't have the full 4GB, i.e., when I had 2GB of physical memory installed, I would be able to see all 2GB when running, say, 'free -m', but now that I have 4GB physically installed, I cannot see all that, but the kernel is still using it?

Looking at /proc/cpuinfo tells me this:

address sizes : 32 bits physical, 32 bits virtual

For each core. I think that's what you were looking for, but if not, just let me know and I'll post the whole thing.

Cheers!

[jpollard]

Yup. Thats the line.

The maximum physical address allowed on your processor is 32 bits (physical). That is 4G. BUT, some addresses must be reserved for I/O, and that subtracts from the physical memory you can address. There is a minimum of 1MB for the hole, and then the memory for the video (reachable by the CPU, but mapped out because the video is using it). The remainder gets divided into system and user space. I belive the current mapping reserved 1G for the kernel to use for mapping I/O buffers and access to the user mode process. That leaves a maximum of something in the range of 2.5-2.9G for user processes.

As a comparison, my system (x86-64) has:

address sizes	: 38 bits physical, 48 bits virtual

The physical range I can reach are 264G. My motherboard can't go anywhere near that (only 12G). This means I have address bits to spare, and can completely map the physical memory I have, and not worry about loosing any for the I/O hole, or even that consumed by the kernel.

Note the 48 bit virtual address. That is the largest process I can have (of course, most of that would have to be swapped out).