>From the fine manual http://gcc.gnu.org/onlinedocs/gcc-3.0.1/gcc_3.html#SEC35
-mcpu=cpu-type Assume the defaults for the machine type cpu-type when scheduling instructions. The choices for cpu-type are `i386', `i486', `i586', `i686', `pentium', `pentiumpro', `k6', and `athlon' While picking a specific cpu-type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the `-march=cpu-type' option being used. `i586' is equivalent to `pentium' and `i686' is equivalent to `pentiumpro'. `k6' is the AMD chip as opposed to the Intel ones.
