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There are several "industry standard" voltages in use in processors today. I put the phrase in quotes because it seems that the number of different voltages being used continues to increase, and the new market presence of AMD and Cyrix makes this even more confusing than when it is was just Intel we had to worry about. The table below shows the current standard voltages with their names and the typical range of voltages that is considered acceptable to run a processor that uses that nominal voltage level:
Note a couple of things about this table. First, the names are rather ridiculous; "Standard" isn't really standard at all, and "Voltage Reduced" is actually a higher voltage than "Standard"! (This is because the term refers to being "reduced" from the original +5V used on older processors). The newest low voltages don't appear to have names any more (which is a good thing, I think). Second, note that both VR and VRE have two slightly different definitions, as they were revised by Intel. The differences are pretty subtle and generally aren't anything to worry about. Most motherboards will have enough voltage settings to support a wide range of processors. Many will have additional voltages besides the ones in the table.
At the lower end of voltages (below 3V) the industry is still evolving, with 2.8V or 2.9V being the standard for core voltage for chips such as the Intel Pentium with MMX, the Cyrix 6x86L and the AMD K6 (the K6-233 is a bit of an oddball at 3.2V core). Socket 7 and later motherboards supply several voltages (such as 2.5V, 2.7V, 2.8V and 2.9V) for compatibility with future devices. The voltage regulator built into the motherboard does the work of converting the power supply voltage into the different levels required by the processors the motherboard supports. Pentium II systems take this a step further; the technology is designed to allow Intel to change the internal voltage required by the CPU without changes being necessary to the motherboard.
Next: Power Management