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Old 08-26-2008, 08:36 PM
James Wilkinson
Default PAE and 4 GB of memory or less

Roger Heflin wrote:
> If they don't remap above 4GB they will maximize the memory
> usable for 32-bit windows (depending on the exact hw, they may have to
> remap
> larger amounts than are actually covered and that would lower the ram
> usable under 32-bit windows, so no remap is better for 32-bit windows in
> this case), but a proper remap would allow anything using PAE (I thought
> the server versions
> of Windows did use PAE) or 64bit to have more ram that without the remap.

Actually, I understand that all 32 bit editions of Windows XP support
PAE, but still support no more than 4 GB of memory¹.

For a similar reason, you may want to use the PAE kernel on 32-bit
Fedora even if you don’t have 4 GB of memory.

That reason is that recent processors (since 2003 or 2004) can be told
which bits of memory come from a proper program, and won’t even try
running anything else¹. This makes certain security bugs impossible to
exploit and others much more difficult.

This only works in PAE or 64 bit modes. Fedora can do something similar
with earlier processors or non-PAE kernels, but it’s not as fine-grained
and doesn’t fully protect all programs.

See http://fedoranews.org/updates/FEDORA-2004-137.shtml (NX feature) for
more details.

Hope this helps,


¹ It’s not the PAE that’s the problem – it’s the third-party drivers.
They believe that a *lot* of drivers won’t work properly if they’re
told to access memory above 4 GB. It’s possible to get around this
with performance penalties, which Microsoft aren’t willing to pay.

² And they won’t run data from inside proper programs, either.

E-mail: james@ | Legacy (adj): an uncomplimentary computer-industry
aprilcottage.co.uk | epithet that means 'it works'.
| -- Anthony DeBoer

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