Needless to say, we couldn't sit this one out, so last week we ordered up an Athlon XP-M 2500+, and have spent the last 24 hours or so putting it through its paces. Read on to find out more about this overclocking wonder, and see how our particular chip fared.
XP-M: It's not just for notebooks anymore
The first question in many people's minds is: What's up with putting a mobile chip in a desktop system? Well, there are a couple of important factors that make the XP-M such a desirable chip from an overclocking standpoint. The first is the default voltage of the chip. Desktop Athlon XP 2500+ chips run at a frequency of 1.8GHz using a default voltage of 1.65V. That's fine for desktop systems that don't have to worry about power consumption or battery life, but it's a different story for mobile systems.
Because voltage relates directly to power consumption, one way to cut down on the amount of power used in a mobile application would be to lower the core voltage of the processor. Of course, it's not quite that simple. Some 2500+ chips may continue to run properly at 1.8GHz if their core voltage is set below 1.65V, but others may become unstable or crash outright at anything below 1.65V. Every chip is different in this regard, and you won't know until you try.
AMD does in fact use this method for the Athlon XP-M. Basically, they test their chips and determine which 2500+ chips can maintain their 1.8GHz clock speed properly with only 1.45V of core voltage instead of 1.65V. Chips that pass this test can then be set to a default voltage of 1.45V and sold as Athlon XP-Ms. Essentially, the XP-M 2500+ is the cream of the crop within its speed grade. Not only can it do 1.8GHz, it can do it with one hand tied behind its back, so to speak.
Typically, overclocking is a crapshoot because you never know how fast a particular chip is going to go. There are many tales of people taking Pentium 4 2.4Cs and overclocking them to 3.2GHz or higher with ease, but we have an example in our labs that refuses to go any higher than 2.6GHz. Overclocking a processor is like a box of chocolates....
In the case of the Athlon XP-M, however, AMD has basically created a part consisting of cherry-picked processors. After all, if a 1.8GHz chip is good enough to stay at 1.8GHz even when undervolted by 0.2V, how much higher than 1.8GHz might it go at default voltage? This is one big reason for the XP-Ms popularity with overclockers; grab an XP-M, and you know you're getting something good. The only question is: how good?
The second important mark in the XP-M's favor is what's missing: a multiplier lock. AMD mobile systems use the PowerNow! power management feature to conserve battery life. Unlike a desktop system, where bus speed times processor multiplier equals processor speed, things in a PowerNow! system are much more fluid. The processor has a minimum and maximum speed, and the system is constantly adjusting CPU speed based on processor load. If your system isn't working very hard (you've gone to lunch, or you're working on a Word document) the processor will limp along at 800MHz. Fire up a game, and it may jump up to 1500MHz and stay there until you quit. The speed is constantly updated relative to system load, and can be set to any number of intermediate speeds between the minimum and maximum.
One of the ways that the system works this magic is by manipulating the multiplier on the processor. Thus, in order for PowerNow! to work, the processor's multiplier must be unlocked. Since PowerNow! is an important part of AMD's mobile strategy, all Athlon XP-M chips are factory unlocked.
Add these two factors together, and you wind up with a guaranteed high-quality, unlocked Athlon XP core for only $25 more (at the time of this writing) than a "regular" Athlon XP 2500+. Whatta bargain!
Putting theory into practice
So now that we've looked at why the XP-M should be a good overclocker, let's see if our hypothesis holds up to the test. Our subject is an Athlon XP-M 2500+, manufactured in week 49 of 2003, with a stepping code of IQZFA.
We started out on an Asus A7N8X motherboard, and quickly got the CPU up to 2.4GHz@1.8V on a 200MHz front-side bus with little drama. Any attempts to push harder, however, were met with failure, possibly because the A7N8X refused to crank the voltage any higher than 1.825V.
At that point, we switched over to Abit's new nForce 2 Ultra 400 board, the AN7 (review forthcoming). The AN7 had no qualms about pushing the voltage up to and past our own comfort level. In the end, though, it didn't make much difference with this chip. We managed to get 2.5GHz@1.96V (yes, 1.96, that's one of the settings on the AN7), but it wasn't stable enough to complete our benchmark suite, so we decided to back down to 2.4GHz at 1.8V and run our benchmarks there.
Abit's uGuru utils kept us aware of our temps, shown here at 2.4GHz and 1.8V Although we didn't hit the 2.7GHz+ levels that some others have apparently reached, let's not lose sight of what we did accomplish. This is a 33% overclock resulting in a 200MHz advantage over an Athlon XP 3200+ (currently going for $195), from a chip that cost $98.
We decided against experimenting with bus overclocking, because our intent was to test the processor's capabilities, and bus overclocking would've thrown too many other variables into the equation, such as non-standard PCI and AGP clock speeds. We said it before, but it bears repeating: One of the big advantages of the XP-M is its unlocked multiplier, which allows for substantial performance gains without having to resort to bus overclocking.