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From: Janne Johansson <jj AT it DOT kth DOT se>
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Subject: Re: pgcc does better, reboot, then does terrible!(SOLVED) 
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Date: Wed, 23 Jun 1999 09:57:19 +0200
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> As to -mpentium.  I'm not sure how -m, -mcpu, -march are different.
> 
> For example, the kernel compiles with -m486 -DCPU=686 (or something like
> that), with your CPU set to 686.  Why not use -mpentium at least??

The -m<something> tells the compiler to act differently or use instructions 
and optimizations for the architecture that you specified, while the 
-Dsomething_else is a #define that affects the source, and makes the source 
act in different ways.

Just because the -D happens to define a constant that has the name "686" 
doesn't mean anything, it could as easily be -Dmight_have_large_L2_cache,
-Dhas_variable_mmu_table_size, -Dimpress_users or whatever.

See the difference?

-- 
"Backwards compatible" means: "if it isn't backwards, it's not compatible."

Http://www.it.kth.se/~jj