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.
Dual-voltage CPUs were introduced for performance gain when increasing clock speeds and finer semiconductor fabrication processes resulted in excess heat generation and power supply concerns, especially regarding laptop computers. Using a voltage regulator, the external I/O voltage levels would be transformed to lower voltages in order to reduce power usage, resulting in less heat being produced with the ability to operate at higher frequencies.
VRT is a feature on older Intel P5 Pentium processors that are typically intended for use in a mobile environment. It refers to splitting the core voltage supply from the I/O voltage. A VRT processor has a 3.3 V I/O and 2.9 V core voltage, to save power compared to a typical Pentium processor which had both I/O and core voltage at 3.3V. All Pentium MMX and later processors adopted this so-called split rail power supply.
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