How FreeBSD manage more than 4GB on 32 bits architecture

Valeri Galtsev galtsev at
Thu Apr 9 16:58:15 UTC 2015

On Thu, April 9, 2015 11:19 am, Adrian Chadd wrote:
> Hi!
> This sounds... very exciting. I'd start by taking a look at how PAE
> works - you may be able to reuse all those pieces and just implement
> more than the 4 bits of physical address space. The interesting bits
> of PAE aren't just the extension of the physical addresses, but all
> the management of mapping things into virtual address space as
> required.

The only problem with all these exercises is: on 32 bit CPU box a single
process still can only have 4GB of address space (according to my
definitely not deep enough knowledge of kernel)...

Just my $0.02.


> I haven't looked at the MIPS side of things though. IIRC, on MIPS32 we
> only implement up to what the direct map KSEG supports, so a little
> PAE ish work would be required there to make things work right.
> So, I'd start by looking at how PAE on i386 works. I think there's a
> little bit of KVA exhaustion going on in -HEAD with i386 PAE that can
> be fixed with a little bit of attention. The rest of the basic PAE
> stuff should still work fine. Then if you need it for MIPS, you'll
> need to sketch out how you expect PAE to work on MIPS, and then make
> up a project for that. :)
> -adrian
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247

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