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Increasing minimum 'i386' processor

The i386 architecture was the first in Linux and in Debian, but we have
long since dropped support for the original i386-compatible processors
and now require a minimum of a 486-class processor.

I think it is time to increase the minimum requirement to 586-class, if
not for wheezy then immediately after.  (Later it should be increased
further, and eventually i386 should be reduced to a partial architecture
that may be installed on amd64 systems.)  This would allow the use of
optimisations and new instructions throughout userland that improve
performance for the vast majority of users.

The 486-class processors that would no longer be supported are:
1. All x86 processors with names including '486'
2. AMD Am5x86
3. Cyrix/IBM/ST 5x86, 6x86 and MediaGX
4. UMC U5D and U5S
5. AMD/NSC Geode GX1, Geode SC1100, Elan SC4xx and SC5xx
Also possibly:
6. DM&P/SiS Vortex86 and Vortex86SX.  These supposedly have all
   586-class features except an FPU, and we could probably keep FPU
   emulation for them.

So far as I know, all processors in groups 1-5 have been out of
production for several years.  Soekris still advertises boards using the
Geode SC1100 and Elan SC520, but they seem quite uncompetitive with
ARM-based systems and at least the SC1100-based products are being

Starting from version 2.6.24 or earlier (early 2008), Debian '486'
kernel packages had a bug that caused them to crash on boot on 486-class
processors, but this was not reported until early 2009 (#511703),
suggesting that there were few users with such systems.  Debian 7.0
'wheezy' should be released in late 2012 or early 2013 and in the
intervening 4 years the numbers of running systems with such a processor
will have declined still further.


Ben Hutchings
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