Originally Posted by syg00
If you are seeing 3 Gig, you have the appropriate option selected.
I don't think that is correct, but I'm not certain. I just did some more google searches on
But I still can't figure out which config options enable which actual features.
I do understand the underlying features. Without using PAE, you only have access to 4GB of physical address space
which gives you access to less than 4GB (typically 3.25GB) of ram.
The only 32 bit system I have handy with 4GB of ram is an old Centos system. Its config has CONFIG_HIGHMEM64G set and doesn't mention CONFIG_X86_PAE at all.
There is some feature that I think may be part of CONFIG_HIGHMEM64G that makes the kernel use a separate 4GB virtual address from the process 4GB virtual address space. (To support more than 16GB physical, the kernel needs a larger virtual address space). I hope I don't have that enabled on that old Centos system, but I'm not sure. I hope you can have PAE without that (I know on other old Centos systems I had PAE without splitting the address space).
The missing memory is probably a motherboard limitation.
That is possible and worth checking. See my post at
If that table of data from the BIOS includes the extra chunk outside 4GB of address space then the motherboard and BIOS are OK for a full 4GB of ram and your problem is lack of PAE in the kernel. If that table doesn't include the extra chunk, then the problem is in the motherboard or BIOS.