Differences betweem x86_64 and i686

[Mike A. Harris]

Mark Thomas wrote:
> A noobie asks
>  
>     I just installed FC6 on a Ferrari 3400 with no problems so far. My 
> question is about the differences between a x86_64 and a i686 code.

i686 code is intended to be executed on processors which are compatible
with the 32 bit Intel x86 platform, which includes all Intel 32bit x86
processors up to and including the Pentium 4, etc. as well as processors
from AMD and other vendors which make compatible 32bit chips.

x86_64 code is intended to be executed on processors which are
compatible with the AMD64 64bit platform, including the AMD Athlon 64,
AMD Opteron and related AMD chip families, and the Intel EM64T based
processors.

These 64bit processors are fully backward compatible with their 32bit
predecessors.  So if you have a 64bit AMD Athlon 64 or Intel EM64T
processor, it is up to you whether to install the 64bit x86_64 OS or
the 32bit i386/i686 OS.

 > The Ferrari has a i686 and FC expects an x86_64. It works, but am I
 > on a train wreck waiting to happen?

Fedora Core is available for i386, x86_64, and ppc architectures.  You
must download the ISO images for the correct architecture, as each ISO
is built for a specific CPU.  In other words, you can't download a
single ISO of Fedora Core and install it on any random CPU - each is
intended for a specific CPU so you must download the correct one.

If you have a 32bit-only AMD or Intel CPU, then you have no choice but
to install the i386 distribution.  If you have a 64bit AMD or Intel
EM64T CPU, then you can install either the 64bit x86_64 OS, or the
32bit i386 OS.

If you are unsure which one to install, then you probably should
install the i386 version, as the majority of software available
out there (open source or proprietary) is 32bit x86 code.  So if you
want to run things like Macromedia Flash, Adobe Acroread, or any
other proprietary software which is not available in 64bit form yet,
you'll want to use the 32bit distribution.

If however, you don't care about proprietary software availability,
and want to get the highest level of performance out of your system,
then you may want to use the 64 bit x86_64 distribution instead.

It is also possible to install both the 32bit OS and the 64bit OS
and dual boot between them, possibly sharing parts of the filesystem
such as /tmp, swap, /home, etc. if desired.

Another thing worth mentioning is that the 64bit x86_64 distribution
does come with runtime compatibility for 32bit x86 applications, so
it is not entirely a all or nothing decision.  Many if not most 32bit
apps should work under the 64bit OS properly so long as the compatible
32bit packages are installed.  If you do this however, keep in mind
that things like 32bit plugins for a particular application will not
necessarily work with a 64bit application, so if there is an app you
want to use which has 3rd party plugins which are not available as 64bit
plugins, you'll either have to use the 32bit app, or not use the
plugin.

Hope this sheds some light on the decision.

Take care,
TTYL