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Chipset Register For Changing The Multiplier

AthlonXP Underclocking for a Low-Power Fix
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To make all this happen, the motherboard BIOS must activate a few registers to provide the required functionality. If this is not the case, the system simply crashes at the very attempt.

Chipset Register Bit/Function
NVIDIA nForce2* F6, Bit 4
E7, Bit 4
Halt Disconnect
FID_Change Detect
VIA KT133A 55 Bit 2=1
VIA KT266, KT266A, KT333 95 Bit 2=1
VIA KT333CF, KT400, KT400A, KT600 D5 Bit 2=1
*only works on a few boards

Unfortunately, it is only possible to change the multiplier on a few motherboards. One of the fastest, the Asus A7N8X-E, crashed every time we tried.

Multiplier In The BIOS: Boot With What?

As the end-user is mostly also prevented from changing the CPU voltage on the fly (without a reboot), a well-functioning combination of multiplier and CPU voltage must already be selected in the BIOS. What makes this difficult is that most motherboards offer the multipliers x5.0 or x6.0 as their smallest value - none of the mobo-makers offers anything below that.

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