there is no software required to control the CPU. For example, DOS will run on both a Pentium 60 and Athlon XP2000 perfectly fine, when it was designed for the 386.
The reason is that all x86 CPUs share the same instruction set- that is what makes them x86 CPUs. All x86s respond to instructions in the same way. For all intents and purposes, that P60 is identical to that XP2000 in all ways except speed and cache size.
The 'driver' is actually the mobo chipset- programs will make calls and the chipset decides what to do with them, where to send them, etc. All of it is done in hardware. So, if you want to tweak the way the system handles instructions, you want to tweak the chipset.
There is a great app for doing this- WPCredit
. It's hard to use, but can completely transform a system.