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Date:      Fri, 30 Nov 2012 18:58:06 +0400
From:      Ilya Kazakevich <kazakevichilya@gmail.com>
To:        Thomas Mueller <mueller23@insightbb.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: i386 vs amd64
Message-ID:  <CAHv=rM0vjUd6haTetUh=52R2QAnLyhvLW6S2oE_zzFKPOUHSSg@mail.gmail.com>
In-Reply-To: <30.96.29719.275C8B05@smtp01.insight.synacor.com>
References:  <30.96.29719.275C8B05@smtp01.insight.synacor.com>

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> How does the system know what is OS and what is 32-bit apps?
>
"OS" works in kernel space while application is not.

PAE affects paging system allowing software to address 2^36 bytes of memory.
You can access it in kernel space, but user space applications are limited
to 2^32 bytes of virtual memory (even less than 2^32 because of mappings).

http://en.wikipedia.org/wiki/Physical_Address_Extension

If you are interested in memory management in IA-32 (and IA-32e) here are
good links:
1) official guide:
http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html
2) nice (human-readable) book:
http://mindshare.com/shop/?c=b&section=0A6B17101710



> Where would GCC fit in this regard, or Clang for that matter?
>
If you write app for user-space (not kernel module) you should not care
about PAE.
You simply compile it as you would do it for system with out of PAE.

Ilya.



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