[coLinux-users] Re: [coLinux-devel] coLinux 0.6.2-pre2

[Nuno L. <lu...@nl...>]

peter green, escreveu :
> what exactly is PAE and how does it relate to the NX bit?
> 

PAE - Physical Address Extensions - is a way Intel made for 32 bits
processors to address more than 4 GB of memory. It was introduced in
the Intel Pentium Pro, so every CPU made in the last years also
implements it (not sure, but I think Pentium M doesn't have it).

This is done by extending the paging mechanism and, when enabled,
extends the page frame tables structures from 32 to 64 bits and
adds another table to the virtual memory scheme.

I don't want to go too deep on this. With PAE enabled, every process is
still limited to 4 GB of directly addressed memory, but it can now use
more by requesting pages from the OS (the same as overlays in DOS days).
Also, the total memory that can be addressable is usually limited by the
OS, not the PAE limit (52 bits, if I remember correctly).

NX needs that PAE be enabled because the 32 bits page structures didn't
had more space to hold more protection bits in it. With PAE enabled, 
there is now space for the NX bit and more.

Regards,
~Nuno Lucas