Mailinglist Archive: opensuse (3605 mails)

["Bryan J. Smith" <b.j.smith@xxxxxxxx>]

On Mon, 2006-06-12 at 13:32 +0200, Per Jessen wrote:
> Would a Linux application even be able to use PAE by itself?
> Does the kernel provide that option/interface?

Yes.  It's really a _complex_ discussion.  And my answer below is
_still_ a _mega_-simplification.

On x86, the kernel has various code to do this, along with GCC
compile-time/directives.  Applications that want to address more than
1-3GiB (depending on the model) of user-space memory have to be built
differently.  It's _never_ a matter of getting access to _all_ 4GiB of
memory, even if the pointers are capable of (or being rationalized to)
address upto 32-bit/4GiB.  This is in _addition_ to how the
kernel-processor actually do PAE (36-bit) -- which is a _serious_
performance hit.

On x86-64, the kernel does even a crapload more.  It provides the new
"flat" 64-bit address model to the "Long Mode" 48-bit/256TiB pointer
model, as well as uses a 7-layer PAE mode (52-bit).  So _all_ programs
can use it -- flat 32-bit to 1/2/3GiB user, >4GiB x86 object code, as
well as new 48-bit "Long Mode" applications.  There is also a major
reduction in performance hit for even legacy apps built against the old
PAE (36-bit) model.


-- 
Bryan J. Smith        Professional, technical annoyance
mailto:b.j.smith@xxxxxxxx  http://thebs413.blogspot.com
-------------------------------------------------------
Illegal Immigration = "Representation Without Taxation"



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