A dual-voltage CPU uses a split-rail design to allow lower voltages to be used in the processor core while the external Input/Output (I/O) voltages remain 3.3 volts for backwards compatibility. A single-voltage CPU uses a single power voltage throughout the chip, supplying both I/O power and internal power.As of 2002 Microprocessor#Market statistics, most CPUs are single-voltage CPUs.All CPUs before the Pentium MMX are single-voltage CPUs.