Strictly defined, overclocking is the act of running a component (CPU, graphics card, etc.) at a higher clock speed than designated by the manufacturer.
How can a processor run faster than it's supposed to?
Chips are typically rated by their manufacturers with a bit of "wiggle room" to account for extreme operating conditions, degredation with time, etc. Additionally, sometimes a company is so good at manufacturing chips that even the worst chips they make are still able to run much faster than the lowest rating the manufacturer sells. If the manufacturer were to sell all its chips with higher clock speeds, it would cause a shortage of the low-end chips and a glut of higher-end chips, driving down prices. To maximize profits at various points along the supply and demand curves, the manufacturer sells chips capable of running at a higher clock speeds with lower MHz ratings. This business practice is economically sound, but it does leave room for overclockers to do their thing.
In short, the parts are often capable of being pushed well past what the label on the box says.
Probably. All indications are that AMD's fabrication process is producing excellent yields, and the quality seems to be improving over time. Most Athlons seem to have a little bit of flexibility in terms of clock speed, though each chip is different. With any luck, your Athlon will probably overclock fairly well.
For instance, the results of an informal survey posted on Usenet revealed that some time around week 42 of production, AMD started selling Athlon 650's as Athlon 500's. The core itself is stamped with a 650, and the cache chips are 3.3 or 3.1 ns chips capable of running at 650 MHz. In other words, AMD was apparently doing so well that the worst cores they were producing were capable of 650 MHz.
For a much more comprehensive survey of how rated speeds, weeks of production and core/cache speeds relate, take a look at this site.
I heard something about .25 or .18 micron. What's that?
These numbers refer to the kind of manufacturing process used to make the chips. The smaller the process, the cooler the chips run and the less power they consume. As a result, chips manufactured with a smaller process will typically run at a higher clock speed. Generally, if you can get a hold of an Athlon made on AMD's .18-micron process, it's likely to tolerate higher clock speeds than a .25-micron chip with the same MHz rating.
However, with the Athlon, cache timing is typically the biggest barrier to higher clock speeds; while the .18 micron processor cores may have a higher theoretical speed, the timing of the L2 cache may keep an Athlon from realizing its full potential. If, however, you are using an overclocking method that allows you to alter the L2 cache divisor, you may be able to overcome this barrier, as well.
Regardless, you should probably try to find a .18 micron chip, as it will typically reach a higher stable speed than a .25 micron chip. Most Athlons being sold now are built on AMD's .18-micron process.
How can I tell the difference between a .25 micron and a .18 micron Athlon?
A Tech Report reader wrote AMD with this question and was told that the .25 micron and .18 micron chips had slightly different part numbers. The part number is printed on the top of the CPU cartridge, so if you're buying locally you can check it before purchase. Examples of the two part numbers are as follows:
AMD-K7550MTR51B C ( a550 in 0.25µ )
AMD-K7550MTR51B A ( a550 in 0.18µ )
How can the Athlon be overclocked?
The Athlon can be overclocked either by manipulating the bus speed of the system and/or by manipulating the multiplier of the processor. See below for more detail on how these methods work.
What's all this about bus speeds and multipliers?
The various components in a modern computer operate at different speeds. In most current Athlon systems, the processor and the RAM interface with the system bus at a speed of 100 MHz. The processor itself operates at some multiple of the 100 MHz bus. For example, a 500 MHz Athlon operates at a 5X multiplier on a 100MHz bus. In other words, the processor runs at five times the speed at which it interfaces with the rest of the system.
Because the processor's speed is based on a 100 MHz bus, raising the speed of the bus raises the speed of the processor, as well. For example, changing the system bus speed to 105 MHz would make the aforementioned Athlon 500 operate at 5 times 105 MHz, or 525 MHz.
Can I overclock the system bus?
Not all motherboards are capable of altering bus speed, but many are. With the right motherboard, raising the system bus speed is a relatively easy way to run the Athlon above its rated clock speed. If it works, it's as simple as changing a setting via a jumper or a BIOS menu.
Note that there is the potential for problems when manipulating the system bus speed, especially when running well outside of spec. Other components, such as PCI cards, use the bus speed as a reference for their own speed. If you overclock the system bus, you may be overclocking the PCI interface, as well as the hard drive controller built into the motherboard. Some cards and/or drives are more tolerant of higher bus speeds than others. The bottom line is this: if you bump up the speed and start to notice problems, something isn't happy. You may be able to figure out which component is unwell and—if possible—replace it with a more tolerant one. Otherwise, you'll have to kick the bus speed back down.
Some hard drives, especially IDE drives in Ultra DMA modes, may corrupt data when run at bus speeds too far out of spec. Beware of this risk, and make backups!
I thought Athlon machines had a 200 MHz bus. Is that not the case?
Although the current Athlon architecture is touted as having a 200 MHz bus, in practice the bus runs at 100MHz. The Athlon's bus, which is based on the EV6 bus protocol from the DEC Alpha architecture, transfers data on both the rising and falling edges of the clock, much like the AGP bus does. Because of this practice, the Althon's bus can attain a data transfer rate equivalent to a 200 MHz bus while running at 100 MHz.
Soon, Athlon systems will be available with a 133/266 MHz bus using this same "double data rate" capability.
What about VIA KX133 chipset-based motherboards? Don't they have a 133MHz bus?
Motherboards based on VIA's KX133 chipset are capable of running a 133MHz memory interface. They do so by running the memory interface at 4/3 the speed of the 100 MHz system bus. KX133 motherboards can also run the memory interface at 100MHz (the same speed as the system bus) if need be, since non-PC133 RAM often won't run properly at 133 MHz.
What's the story on the multiplier lock?
The Athlon is multiplier locked; that is, it has a multiplier that is set during manufacture and is not intended to be changed. Intel's Pentium II/III and Celeron processors are multiplier locked, as well. As mentioned above, it's the multiplier that determines the speed of a particular Athlon. For instance, a 5X multipler makes an Athlon 500, while a 7.5X multiplier is found on Athlon 750's. If you can change the multiplier, you can change the speed of the CPU. In the case of current Athlons, the multiplier can be set at anywhere from 5X to 10.5X, for a speed range of 500 MHz to 1050 MHz.
OK, let's just crank this mother up to 1050MHz and be done with it! Right?
Not so fast, hotshot. There are limits. Each processor is different; some Athlons can overclock to 800 MHz or more, and some can't make it past 600. The usual procedure is to start out by bumping the multiplier up one notch (.5X) at a time and testing for stability. Keep going until it becomes unstable, then back it down a notch and you're done. As noted above, AMD's fab process seems to be doing very well. Having said that, it's highly doubtful that any of the current chips will make it to 1050 MHz. Try it, and you may to cause permanent damage to your CPU.
So it's just a matter of how fast the processor core will go, right?
Not exactly. There are two pieces to the Athlon overclocking puzzle; the processor core and the L2 cache. Current Athlons have cache chips which are mounted on the same PCB as the processor core but are not part of the core itself. Because the cache chips aren't built on the same manufacturing process as the processor core, they won't run nearly as fast as the processor core itself. Thus, the cache chips are set to run at a fraction of the speed of the processor core.
For 500 MHz to 700 MHz Athlons, the cache runs at 1/2 the speed of the processor, or between 250 MHz and 350 MHz. Cache that runs at over 350 MHz is difficult to produce and relatively expensive, so for speeds of 750 MHz to 850 MHz, the cache runs at 2/5 the speed of the processor, or between 300 MHz and 340 MHz. For 900 to 1000 MHz Athlons, the cache runs at 1/3 the speed of the processor, or between 300 MHz and 333 MHz.
This fact is important for two reasons. First, the relaxing the cache timing degrades performance, which creates "flat spots" in the Athlon line. For example, a 700 MHz Athlon and a 750 MHz Athlon are very close to one another from a performance standpoint, because the core clock speed goes up, but the cache speed goes down. The same is true of an 850 MHz Athlon and a 900 MHz Athlon.
Second, this arrangement typically means that the cache will become a limiting factor before the processor core itself will. For example, let's say you purchase an Athlon 550 manufactured on the .18 micron process. This is the same process used to manufacture the 1000 MHz Athlon, so it's entirely possible your processor core will overclock to 800 or 900 MHz, or even higher. However, the cache on your 550 will typically run out of steam somewhere between 700 and 750 MHz, effectively limiting the processor to 700 MHz.
Why not just change the cache divisor?
Ah, if only it were that easy. As you'll find out later on in the FAQ, the cache divisor can't be changed as easily as the multiplier or core voltage; while the latter two parameters require only a plug-on card to be manipulated, the cache divider requires soldering on the Athlon itself.
Thus, if you don't want to subject your new CPU to a soldering iron, the available Athlons get sorted into three groups: 500-700 MHz, 750-850 MHz, and 900-1000 MHz. While it's possible that your cache can be pushed past these boundaries (our Athlon 800 engineering sample was rock solid at 900 MHz) the odds aren't terribly good.
Is Athlon overclocking worth the trouble?
You have to look carefully to decide whether overclocking your processor is worth it. An example: Using prices current as of this writing, you could purchase an Athlon 550 for $159. Without manipulating the cache divisor, chances are good you could overclock it to 700 MHz pretty easily. But a "real" Athlon 700 is going for $243, and Golden Fingers devices (the easiest way to overclock the Athlon) cost at least $40 plus shipping. Thus, you're going to all this trouble and voiding your warranty in the process (not to mention the small chance that the chip won't hit 700) to save around $40. Doesn't really seem worth it.
On the other hand, the Athlon 750 goes for $337. Add in $50 or so for an overclocking device, and you're at $387. You can almost certainly overclock the 750 to 850, and a "real" Athlon 850 goes for $725 at the moment. You've just saved yourself $338. Now that's more like it. If you get lucky and the thing will push to 900, you're even further ahead of the game.
Of course, if you choose an overclocking method that allows you to manipulate the cache divisor, taking a chance on overclocking starts looking a lot more attractive. If the cache isn't a factor, maybe that 550 core will hit 1000 MHz; who knows? But if you're sticking with the stock cache divisor, just be sure to pay attention to the economics before you make the decision to buy a processor with the express intention of overclocking it.
However, if you're like us, you'll probably wind up overclocking. Raising the bus speed a little with the right motherboard doesn't cost any extra, and overclocking success is its own reward.
What exactly are the risks of overclocking?
Overclocking can reap some big dividends, but if you're not careful, it can also turn your nifty new CPU into a paperweight. As stated above, every processor has its limits. Exceed them at your own peril. Generally, even if you go past the point where the processor is stable, if you realize this and back the speed down, you should be fine. One way to hose your processor in a hurry, however, is to push the voltage too high.
Any time you overclock your system components, you void a warranty of some sort, and you risk burning up your CPU, motherboard, hard drive, video card, and the neighbor's cat, plus you might start a fire, burn down your house, and the heat could release an obscure microbe hidden deep in the bowels of your home that could bring plague and desolation to the entire human race.
However, you'll probably just experience a system hang and have to reboot.
Why manipulate the voltage supplied to the CPU?
Current Athlon processors (except those from 900 - 1000 MHz) run at a core voltage of 1.6 volts. When you're overclocking, however, you might run into a situation where a processor is almost stable at a given speed, but not quite. One thing you can try to increase stability is to slightly increase the core voltage of the processor. If you increase the voltage too much, however, there is a danger of causing permanent damage to the processor. Our recommendation is to increase the voltage no more than .2 volts, to 1.8 volts. Still, you do so at your own risk.
AMD's newer 900, 950 and 1000 MHz Athlons have a "stock" core voltage of 1.8 volts. Because these chips are manufactured using the same process as the lower speed chips, these newer Athlons are already near the upper end of the safe voltage range. For this reason, we would not recommend increasing the voltage of 900, 950 or 1000 MHz Athlons above "stock" levels.
OK, so how do I change the multiplier?
There are several ways. You may change the multiplier by manipulating resistors on the Athlon's PCB, by purchasing or performing a modification that adds DIP switches to your Athlon, or by purchasing or making an overclocking card (called a "Golden Fingers" card).
AMD Athlon overclocking FAQ:
Answers to frequently asked questions
What is the resistor method of Athlon overclocking?
The resistor manipulation method of overclocking the Athlon was first publicized by Tom Pabst. He published an article detailing how multiplier settings and voltage settings are determined on an Athlon via several sequences of resistors mounted on the PCB. By desoldering and soldering resistors on the PCB to change the sequences, one can use this information to modify the multiplier on the chip and run it at a higher speed. Additionally, other resistor sequences may be changed to bump the voltage up slightly as discussed above.
What are the advantages of the resistor method?
The main advantage is that it gives you complete control over every aspect of the processor that you might want to change, from the multiplier to voltage settings to cache divisor. Additionally, once you're done, if you've done a good job of things, the modification doesn't increase the fragility of the processor in any way; it's just as durable to physical shock as it always was.
What are the disadvantages of the resistor method?
There are several disadvantages to this method. The resistors are very small and require a skilled hand to solder and desolder without damaging the PCB and rendering the CPU useless. As many as twelve resistor locations must be manipulated to change the multiplier value of the processor. If you wish to modify the core voltage of the processor as well, as many as four more resistor locations must be changed.
And changing them once probably won't be enough. Overclocking is hardly an exact science, and, as we discussed before, in the past the preferred method has been to start with the smallest step up and gradually work up to the point where the chip becomes unstable. At this point, one might bump up the voltage to the chip slightly to see if that allowed the unstable speed to work properly. If it didn't, one would simply back the chip down to the last stable setting and be done with it.
This is all well and good when you're simply manipulating jumpers or, on more recent boards, changing BIOS settings. But when each change in multiplier or voltage means carefully soldering and desoldering a series of resistors on a $200+ processor, overclocking starts looking a lot less attractive.
What is the DIP switch method of Athlon overclocking?
The DIP switch method was conceived to eliminate some of the headaches of the resistor manipulation method. It was conceived more or less simultaneously by Mark L. Sorensen at Trinity Micro, and Mike Honn. To some extent, the DIP switch method is an evolution of the resistor manipulation method.
Rather than soldering the resistors into a hard-coded configuration, the resistors are removed from the PCB completely and instead wires are soldered to each side of the resistor junction. The wires lead to a series of DIP switches and resistors. Once the modification is performed, one can choose to place or not place a resistor at each resistor junction by flipping the DIP switches in specific patterns. This modification enables the end user to manipulate the multiplier, voltage and cache divisor at will with the flip of a few DIP switches.
What are the disadvantages of the DIP switch method?
There are disadvantages to this method, as well. Obviously, the warranty is right out the window here; the chip has a rat's nest of wires dangling off it. Another problem is it makes the CPU extremely vulnerable; accidentally brush against the CPU and break a wire off while plugging in that new GeForce card, and you're in a world of hurt.
Trinity Micro has altered the layout of their DIP switch method so the wires are protected inside the original Athlon casing. A small hole is cut out of the casing and the DIP switches are mounted there. This setup eliminates the chance of accidentally snagging a loose wire, but we have no way of knowing how fragile the processor remains to physical shock using this method.
How difficult is the DIP switch method?
From the looks of it, performing this modification would make the aforementioned resistor manipulation look easy. Soldering resistors isn't exactly child's play, but now you're soldering 32 wires to those resistor locations, and then soldering them onto the DIP switches. If you want to manipulate the cache divisor, you'll be doing even more soldering.
Fortunately, however there are individuals who will do it for you. The cost is currently around $100 when you supply the CPU. Shell out the cash and all you need to overclock is a ball point pen to flip the switches.
However, if the CPU doesn't want to overclock very well, you're basically out of luck. Once the modification is done, it's highly doubtful the CPU could be returned to its original state without it being fairly obvious that the modification had been performed. Thus, returning the chip for a hopefully more overclockable one isn't likely to happen. While this method is more flexible than the first one, it is still less than ideal.
What is a Golden Fingers card?
The publication of the first public information on Golden Fingers method can also be credited to Dr. Pabst, and it is almost certainly the best means of Athlon overclocking to date. The Golden Fingers card (credit JC at JC's PC News'n'Links for the name) enables you to overclock your Athlon without putting a soldering iron anywhere near it. Simply plug the card onto the Athlon, flip some switches and you're ready to go.
Where does a Golden Fingers card plug into the Athlon?
The Athlon is manufactured with an edge connector on the top corner of the PCB. The Golden Fingers device is a card that fits onto this connector and overrides the resistor settings on the PCB. As a result, the user can use DIP switches to manipulate both the multiplier of the CPU and the voltage. The card is unable to manipulate the L2 cache divisor, however.
Because the Athlon is shipped with a plastic casing that covers the PCB (and thus the edge connector) the casing must be removed before the overclocking card can be utilized. Removing the casing involves breaking it away from the pins that hold it on.
So how much is a Golden Fingers card going to run me?
The card is actually cheaper than the original DIP switch modification; we've seen the cards selling for US$40 to $69.
Where can I get a Golden Fingers Athlon overclocking card?
An extremely comprehensive list of Athlon overclocking cards may be found www.anative.com/goldfinger/gfbg.htm.
What disadvantages are there to using a Golden Fingers card?
Installing a Golden Fingers card is more or less reversible, save the fact the Athlon's plastic casing must be removed. Simply unplug the card, and the processor once again runs at the speed at which it was sold.
The only other deficiency of most cards is the inability to manipulate the cache divisor. Interestingly, the cache divisor can actually be manipulated in software; H. Oda has created a program that will do this from within Windows. The problem is that the machine needs to boot successfully in order for this program to be used; if the multiplier is set too high for the cache, the computer will typically crash well before the program can back the cache timing off. For more information on H. Oda's L2 cache divisor program, see our preview of the program, written before it became widely available.
In theory, a motherboard manufacturer could probably put L2 cache divisor controls in the BIOS, but no manufacturer has yet done so.
Several Athlon overclocking cards do offer the ability to manipulate the cache divisor. We have no experience with the cache speed manipulation abilities of any of the cards, so we can't comment on their effectiveness. Since the edge connector on the Athlon doesn't allow for the manipulation of the cache divisor, soldering would typically be required.
There are a multitude of reasons to go with an Athlon over a Pentium III. First, there is the fact that, clock speeds being equal, the Athlon will beat the Pentium III in just about any test you can throw at it. See our review of the Athlon at Ars for a performance comparison.
Second, while changing the multiplier on an Athlon isn't much fun, it is at least possible. Information on how to change the multiplier on a bus-locked Intel processor isn't publicly available. With an Intel chip, you're stuck with manipulating bus speed--a less than ideal solution.
Finally, there is the fact that, subjectively speaking, Athlon chips seem more overclockable than the newest Intel chips. AMD designed the core of the Athlon with high clock frequencies in mind, and it appears they succeeded. A number of people have reported taking Athlon 500's up to over 800 MHz with nothing more than a good heat sink/fan combo. The biggest clock frequency increase we've heard of for an overclocked Athlon is a rumored case of someone using a water-based cooler to hit 912 MHz on an Athlon 500.
How important is cooling to Athlon overclocking?
On modern processors, good cooling is extremely important. AMD ships retail Athlons with beefy heat sink/fan combinations, and overclocking only heightens the need for good cooling. The more heat dissipated by the heat sink, the cooler the processor runs and the faster it can run without encountering problems. Of course, a good heat sink/fan combo doesn't help enough if your PC's case doesn't have good ventilation, as well.
So what should I look for in a heat sink?
AMD has a list of recommended Athlon coolers on their web site, and we've heard that a number of retail Athlons have shipped with Alpha coolers.
How do I test my overclocked processor's stability?
Generally, folks run highly CPU-intensive programs over an extended period of time in order to test the seaworthiness of an overclocked processor. A truly stable system should be able to run a really nasty program overnight without overheating, locking up, or crashing. Some common torture tests:
· Load up Quake 2 and let it run its looping game demo for hours on end. If you're trying to pinpoint the source of crash problems, Q2 can be run with a software renderer, so you can take your 3D video card out of the equation if needed.
· Or, more simply, run Prime95 or the SETI@Home client for a good, long time. These number crunching beasts will strain your PC's processor quite well, also.
Keep an eye on the system, especially early on, when running these sorts of tests. If an overclocked processor overheats and begins to get a little woozy, shut down the PC before any real damage is done.
What is the future of Athlon overclocking?
We're glad you asked. :) There are some new challenges on the horizon as the Athlon product line evolves.
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The changes coming down the pike for the Athlon raise some interesting questions about how Athlon overclocking will look in the near future. One potential problem is the fact that the current Athlon resistor configurations—adjustable via overclocking cards—is mapped out to 1050MHz. Will "Golden Fingers" cards be obsoleted by 1GHz+ Athlons? Among the possibilities:
The other big change on the way is the move to on-die L2 cache and, with it, AMD's Socket A. Obviously, socketed Athlons won't work with GF cards, so what do we do? This interesting post at JC's has speculation about a range of future Athlon developments, including this tasty morsel: Oh, and BTW, if anyone is wondering how to O/C the socket A cpus, here's how... certain pins on the cpu will designate voltage/multiplier. current way to o/c is to break/short the pin that corresponds to the setting you want. Next wave of o/c devices will sit between the cpu and the socket and short these pins automatically (it wouldn't be too cool to break those pins off of theat new 2ghz cpu would it?)"This kind of thing does seem possible. Dr. Evil's keen eye spotted the resistors on top of what appears to be a socketed Athlon in the lower picture here. (Credit JC's for this link, too.) Assuming these resistors control the CPU's multiplier setting, a device that sits between the CPU and the socket just might be able to adjust those settings. Anybody have any further insight into this one? Finally, there is the possibility—a likelihood, in my view—that we'll see "slotket" adapters to allow socketed Athlons to work with Slot A motherboards. Athlon slotkets could allow the kind of overclocking options GF cards provide now, kind of like Intel slotkets enable Celeron SMP. |