The Duron multiplier is set by two factors, FID and BP_FID (FID = Frequency ID). The signal from FID is sent from the CPU to the mainboard's northbridge. Then the northbridge sends a signal back to the CPU which actually sets the multiplier that the CPU is going to use. The trick is to modify the signal that is sent by the CPU to the northbridge to change the multiplier. The BP_FID pins send a signal directly to the CPU, which has to match the FID signal or the system won't boot.
Because BP_FID wasn't documentated in AMD's datasheets it was really hard to find the solution for the described problem. With quite a bit of help from Hiroji, who first found out about the undocumented BP_FID pins, and an informative Xoom site, who put us on the trail of the L1 bridges (among other things), a battleplan was made for attacking the problem.
The Duron's multiplier was locked by laser-cutting 4 golden bridges (marked L1). By doing this the connection with the BP_FID pins is cut off, resulting in a signal inchangeable from the socket. Some retail Durons and Thunderbirds are locked this way, including ours. Most at this time still seem to be coming with the L1 bridges intact (we have had esxactly one other report of a locked processor to date). This picture (borrowed from uncle Tom) shows an unlocked Duron: the L1 bridges are still intact.
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Further there are differences between mainboards. Some boards, like the Asus A7V, provide a possibility to change the mulitplier. The Gigabyte GA-7ZM that we used didn't have such a possibility.
Adding this together makes you end up with these four possible situations.
Duron not locked + mainboard with multiplier tweaking onboard
Duron not locked + mainboard without multiplier tweaking
Duron locked + mainboard with multiplier tweaking
Duron locked + mainboard without multiplier tweaking
The most simple situation is the first one (duh!). In that case you only need to set some dipswitches and you're done. With the third option you will "only" need to alter the Duron itself, a modification that eventually proved to be quite simple. If you happen to already own a Socket A mainboard without multiplier tweaking options, you have some more work to do. You'll have to build a multiplier tweaking system yourself.
Our task was the hardest: we had to unlock the Duron and try to modify our mainboard in a way that we could actually control the multiplier. The exact way we did this is explained in the next pages.