Intel Developer Forum Fall 2002 - Preview

The Future of CPUs - Dual Threshold Voltage?

There has been a dramatic change in the way Intel goes about designing microprocessors over the past decade; power never used to be the primary concern when architecting a CPU, it was more of an afterthought. Over the past several years Intel has done some things to improve power efficiency but performance was still the primary goal. Today, Intel doesn't begin architecting a processor before they define a power limit for the CPU. Based on the "power budget" they will go on to design the CPU to meet other limitations and goals.

The most popular methods of reducing power consumption and heat production are by improving manufacturing processes and making cores smaller; an unfortunate downside of this is that with smaller cores, your power density obviously increases and you end up having to deal with a much bigger problem - removing a lot of heat from a very small core.

In order to deal with this problem of increasing power density, there are a lot of things that must be done on the inside of the core to help. It turns out that during normal CPU operation there are a number of transistors that are operating much quicker than they need to be. Transistors that are critical to the high performance execution of tasks obviously must operate as fast as possible, but there are a number of other transistors in the CPU that are either unused or relatively idle during normal operation - yet they still operate at the same frequency and voltage as the rest of the CPU. This is where the idea of Dual Threshold Voltage (Dual Vt) comes in to play.

You're very familiar with the single voltage that your CPU works off of, often referred to as its "core voltage" or "Vcore." This is the voltage that all of the millions of transistors in the CPU operate at but, as we just mentioned, this isn't the most efficient method of operation. Intel is currently researching into the idea of feeding future CPU's two different voltages, a high and a low voltage. These Dual Vt CPUs would then be architected to run non-critical transistors at the lower voltage while the rest would run at the higher voltage. The benefit of this is that transistors that don't need to be switching as fast end up running slower and thus reduce the overall power consumption of the CPU.

Slowing down the non-critical transistors won't actually reduce performance of the CPU any because of the fact that identical transistors don't all switch at the same speed. There's a distribution of speeds which identical transistors will switch at; the "high" voltage will effectively keep transistors running as fast as they can while the "low" voltage will ensure that transistors will operate at the slower end of their distribution curve.