AMD's K6-2+ in Notebooks - Evaluating Performance with PowerNow

How it Works

PowerNow! works by manipulating the Effective Bus Frequency Divisor (EBF) which is initially set to the same value as the BF pins that control the clock multiplier of the CPU. For example, with a K6-2+ running at 550MHz (100MHz FSB x 5.5), the BF pins indicate a 5.5X clock multiplier and thus the EBF Divisor is set to a value of 5.5X.

In order to manipulate the processor frequency, PowerNow! changes the state of the EBF instead of changing the actual BF input pins in order to dynamically adjust the clock speed of the processor without any intervention on the part of the user and even without the user's knowledge of what's going on. As far as the user is concerned, the notebook is just as responsive as before since the CPU is only running at its higher clock frequency if there's a need for that kind of power.

Since PowerNow! is essentially changing the clock frequency by manipulating the clock ratio, this means that the only available operating frequencies occur in 50MHz increments (this is because the K6-2+ features 0.5x multiplier increments). More specifically, the K6-2+ supports the following clock multiplier ratios: 2.0x, 3.0x, 3.5x, 4.0x, 4.5x, 5.0x, 5.5x and 6.0x. Obviously, if you only have a 500MHz K6-2+, PowerNow! will never set the EBF state to anything greater than 5.0X.

The voltage being supplied to the processor is also adjusted, as we mentioned above, according to clock speed and the specific demands of the CPU at that given instant. The K6-2+ uses its VID outputs to instruct the motherboard as to exactly what voltage it needs at any given time. So if it's running at full speed, the K6-2+ would tell the motherboard to supply it with 2.0v however if it's running at a reduced clock speed (the lowest being 200MHz because the K6-2+ doesn't support any lower multipliers) the CPU will tell the motherboard that it only needs 1.4v.

If you compare this to Intel's Speedstep technology, you can truly see how PowerNow! is a completely different beast. Speedstep does not dynamically adjust the clock speed of the CPU or the voltage supplied to it, instead, it basically allows the processor to run at the maximum clock speed when using the AC adapter and a lower frequency/core voltage when running off of battery power alone.

PowerNow! takes this idea to the next level by allowing dynamic manipulation of operating frequency and voltage supplied to the CPU depending on the needs of the user at any given time. Even while simply playing a DVD movie, the K6-2+ because of its PowerNow! technology may run at full speed during certain scenes that require more complex decoding and drop down to half speed during other scenes that don't require as much CPU power. And while all of this is happening, the DVD playback is never interrupted and the user notices no visible difference in performance.

Intel does have another technology that isn't as prominently advertised as Speedstep which is their Quick Start technology. Intel's Quick Start is similar to PowerNow! in that it makes use of the "hurry up and get idle" principle. As taken from our coverage on the topic from this year's Fall IDF:

"Intel has figured out that it is best to use full CPU power for a split second to finish a task and then put the CPU to idle as this conserves battery life the best. Although one may suspect that when running complex operations the CPU would not have time to go idle, this is not the case. To illustrate this point, Intel used an example of DVD playback. Very stressful on the system as a whole, Intel's quick start technology allows the CPU to "hurry up" and perform the DVD decoding operations and then go idle until the frame is displayed to screen and the next scene needs to be calculated. This saves battery life because, although the system may require 3 watts or so to "hurry up", the power consumption goes down near .25 watts when idle. By averaging these two numbers, one can quickly see how quick start can extend battery life."

We will hopefully be looking at Intel's Speedstep and Quick Start in the future.