CPU Speed Trap:

The abstract of the Intel patent reads as follows:

An over-clock deterrent mechanism of a chipset which comprises an over-clock detection circuit for detecting over-clocking of a system (processor) clock signal based on comparison of ratio of the system (processor) clock signal which is likely to be over-clocked and a fixed, stable reference clock signal which is highly unlikely to be over-clocked, and an over-clock prevention (thwarting) circuit for deterring such an over-clocking by either disabling operations of a computer system or significantly undermining key operations of a computer system.

Unlike the rest of the patent description, this is comparitively easy to understand. An Intel scientist invented a way to keep a microprocessor running at a specified clock speed by comparing it to a reference clock. Further, the patent details ways of acting upon an out-of-spec clock speed, all of which involve slowing the processor down in one way or another.

To explain the base technology that enables this CPU speed trap, we need to look no further than our television sets. Specifically, a television set tuned to TNN on Sunday afternoon. For those that haven't a clue what I'm talking about, check out TNN on Sunday sometime. For those that do know what I'm talking about, you probably still don't get what microprocessors have to do with NASCAR racing. Lucky for you, this curious metaphor is easily explained.

Imagine a racecar doing laps around a superspeedway of known length - say 1 mile. Then, imagine that I ask you to figure out how fast the racecar is going, but there are no radar guns around. How would you do it? The easiest way to determine the racecar's speed would be to count the number of times it goes around the track in one hour and since each lap is 1 mile, this would give us the answer in miles per hour. Pretty simple, isn't it?

Now imagine that we are on another track, of an unknown length. I again ask you to determine the racecar's speed. In this case, the best you could come up with would be a wild guess, because we no longer know the length of one lap. So, we decide that we'll settle for knowing if the racecar is going fast enough to beat an (until now unmentioned) opponent.

We have two cars, both running around the track. We're not running an actual race, so we're not looking for the first to cross the finish line. Rather, we're interested in one thing: whether or not our racecar is going fast enough to beat the other one. To figure this out, we go back to our solution to the first problem. By counting now many times each racecar goes around the track in a specified time period, we know if our racecar is going fast enough to beat the other one. If our racecar goes around the track more times during a fixed time period it's going faster than the opponent's; if it goes around the track fewer times it's going slower. If they go around the track EXACTLY the same number of times, then they're going the same speed and there will be a tie.

If you've followed along to this point, then you understand the technique that Intel is using to determine whether or not a processor is running at a clock speed other than that specified. In our racing metaphor, one lap is equivalent to a clock pulse. We can determine whether the device that generates the clock pulses (our racecar) is going faster than a known reference that is set at the processor's specified clock speed (the opponent's racecar) by counting the number of pulses from each over a fixed time period and comparing them. If we count more pulses for the system clock than the reference (in terms of our metaphor, our racecar is winning), then the system is overclocked. If we count fewer pulses (our racecar is losing), then we are "underclocked". If we count exactly the same number of pulses, there will be a "tie" and we know that both clocks are running at the same rate. Because the reference is actually a part of the processor, it theoretically cannot be tampered with and therefore the processor is running at its specified speed.