Die-locked CPUs

Starting from week-39 of 2003, Athlon CPUs have certain bridges locked via the core, so modifying these bridges does not affect operation at all. The approximate date ranges are:
The die locks affect the L3 (desktop multiplier) and L2 (L2 cache size) bridges only. All other bridges remain modifiable.

In order to regain multiplier control, the CPU needs to be turned into a Mobile CPU through the L5 bridge. The multiplier can then be changed through software, though no current BIOSes support setting the software multiplier on boot.

See also:
Set to Mobile CPU
Mobile CPUs cannot do SMP