PAE kernel and 4GB of memory

[Roger Heflin]

Howard Wilkinson wrote:
> I am looking for a definitive answer to the question of where the PAE 
> kernels become useful. I have seen various articles that mention needing 
> PAE kernels if you have more then 4GB of physical memory in a 32-bit 
> processor environment. I have also seen statements that say you need 
> them if you have 4GB or more of memory. Now which is right? Also, even 
> if you need a PAE kernel because the last few bytes are not addressable 
> when you have exactly 4GB is this useful or is the trade off of larger 
> page tables and pages going to eat any benefit of being able to address 
> these few bytes and if so when does the PAE kernel become useful?
> 
> Howard.
> 

It depends on the bios, you would have to try with and without the PAE kernel 
and see if the amount of usable ram changes.

Some bioses won't remap any memory below 4GB (that is covered by something else) 
to over 4GB, if your bios does not remap anything above 4GB when you only have 
4GB (or less) then PAE won't buy you anything.    And since often moving the 
covered memory from below 4GB, sometimes means moving some non-covered memory 
and therefore lowering the memory usable for an OS that does not support PAE-ie 
that other OS, often the bios *WON'T* move the covered memory at all because it 
would lower the usable memory below 4GB for that other OS.

This is almost always true on the Desktop class MB's, and it is sometimes true 
on the higher end stuff also.

                               Roger