Pentium microprocessors with MMX actually require two voltage regulators -- one for the internal (core) voltage, and one for the I/O drivers at 3.3 volts.

Some voltage regulators, particularly those packaged as a voltage regulator module (VRM), are voltage ID (VID) programmable, which means that the microprocessor can program the voltage regulator to provide the correct voltage during power-up.

Processor Power and Voltage 
Describes external and internal voltage levels, standard voltage levels and motherboard voltage support, power management, and power and voltage for specific processors.