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External and Internal Voltage Levels
Early processors had a single voltage level that was used by the motherboard and the processor, typically 5 volts. As processors have increased in speed and size the desire to use lower voltage levels has led designers to look at using lower voltage levels. The first step was to reduce the voltage level to 3.3 volts. Newer processors reduce voltage levels even more by using what is called a dual voltage, or split rail design.
A split rail processor uses two different voltages. The external or I/O voltage is higher, typically 3.3V for compatibility with the other chips on the motherboard. The internal or core voltage is lower: usually 2.5 to 2.9 volts. This design allows these lower-voltage CPUs to be used without requiring wholesale changes to motherboards, chipsets etc. The voltage regulator on the motherboard is what must be changed to supply the correct voltages to the processor socket.