Overclocking AMD Chips

General information
Overclocking is the setting of CPU speed above its rated speed. The speed of a CPU is set by multiplying the system speed known as the front side bus (FSB), by a set value called the multiplier, to create the CPU speed. To overclock, either the multiplier or the bus speed can be raised. Raising the multiplier only increases the CPU speed, but raising the bus speed means that the other parts of the system will also be overclocked. The memory is the main concern, since the other buses (pci, agp, isa and usb) all have changeable dividers to keep them close to normal speed. Above certain values, depending on the motherboard, no more dividers are available so then the other buses may not take it. For instance, on my system, after a bus speed of 133, there is no more dividers, so I am limited by the pci bus to about 145 (I have a lot of pci cards). Read the Bios guide to find out what your current bus speed is and what your memory can stand. The settings for adjusting the bus speed and multiplier can either be found on the motherboard itself or in the bios on better motherboards - check your motherboard manual for details. If you do not have one, find out what motherboard you have and then look it up on the web as they normally have the manuals online. Raising these settings rarely causes problems unless taken too far for the chip, which may result in no booting or windows instability. This may lead to data corruption, but most of the time resetting the bios or dropping back the settings will result in normal operation again. Hardware problems after overclocking are very rare. The CPU may stop working if taken far too far for the setup due to heat, or after years of operation, which is usually long after most people upgrade. Another problem is data corruption due to the hard drive controllers not taking high bus speeds. Some computers cannot take a bus speed of 83 due to the big difference in the pci and agp bus speeds for instance.
The big bane of overclocking is heat. Raising the speed increases the heat given out by a CPU at each step. Increasing the voltage exponentially increases the heat as well. A powerful heatsink and fan is necessary before attempting overclocking. I will not suggest which to buy, look at the pages on my links page for information.
The other settings that some motherboards provide are voltage adjustment. This is the only setting you should be careful with - if you adjust it to far, you could blow your chip! You should be able to overclock without using these, Increasing them may result in the ability to overclock further, or better stability.

5 rules for overclocking

1) never overclock too far - take it in small steps, checking that the system is stable at all steps (run a 3d benchmark or game)
2) keep an eye on temperatures (use motherboard monitor). Above about 53C is too high - get a new heatsink and fan. If your case temp is above about 35C, invest in some case fans
3) be very careful when raising the voltage. most boards limit the voltage, and for a very good reason !
4) do not overclock too far. If the system does not seem to be stable, do not go higher. Try raising the voltage (if possible) to stabilise its current speed. I know i basically already said this, but it is the most important rule
5) all chips and systems are not equal. Just because your friend/brother/etc got the same chip to xxxxMHz doesn't mean yours will automatically make it

For more information on CPUs, check out my links page, since this guide is purely about overclocking.
AMD Specific information
AMD has only recently started locking its CPU multipliers. The K6-2s and K6-3s are all unlocked. The classic Athlon is locked, but unlockable by using cards that fit to the CPU - but you must remove the casing first. The new Thunderbirds can either be unlocked or locked, depending on the age (chips made after June 2000 AMD are locked) and speed of the chips. For instance, all chips are unlocked above a speed of 1.1Ghz. All new Durons are locked, early ones may be unlocked. The Thunderbirds and Durons can be unlocked by connecting the 'golden bridges' on the surface of the cpu with a conductive substance e.g. HB pencil. The new Athlon XP are all locked, and it is very hard to unlock them. Check online for guides. The Athlon mp as far as i know is always unlocked. AMD chips are not as overclockable as Intels as they do not produce all chips in the same way across a range. However, on a MHz for MHz comparison, the new Thunderbirds are more powerful than the PIII and P4, the classic Athlons comparable to the PIII and the K6-2/K6-3 series more powerful than the Pentium or comparable to Pentium II.
The older chips - The K6-2 and K6-3
The K6-2 and K6-3 run so close to their maximum speed that you are lucky to get 50MHz out of one. Because of this, raising the multiplier more than 0.5 is usually not the best idea since that means at least 66MHz. When overclocking a 66MHz bus speed chip, you should be able to make 75MHz, but not much more. A motherboard with many bus speed adjustments is necessary for getting the most from the chip. For about 100fsb chips, you should be able to get a fair bit out, especially from the lower speed chips.
Raising the voltage can help increase the overclocking potential, but don't do it more than a few 0.1V, due to the large heat output. Don't do it without a powerful heatsink and fan. More advanced cooling techniques would provide better overclocking, but what's the point on such and old chip ?
Performance increase by raising the speed is high particularly in games due to its powerful floating point instructions. Raising the bus speed and dropping the multiplier if neccesary is a better idea to overclock since most chips can handle much higher bus speeds than they are rated for. This will overclock the whole system, so with good memory (pc100) and a good motherboard it will provide a much better increase. In many cases, having a lower CPU speed at 100 bus speed is better than a higher CPU speed at 66 bus speed. Beware of the bus speed of 83 though - this is particularly stressful on the pci and agp buses and may result in data corruption.
AMDs first mainstream chip - The classic Athlon
The Athlon was a much better seller than the K6 series. Once again, they are tough to overclock, partly due to heat, but mostly due to cache memory speed, and the locked multiplier. To save cost on production, (which would have been a lot when the chips were available) AMD ran the cache of memory on the cpu differently to the main chip speed by using dividers. Hence, raising the chip speed results in cache memory instability. Overclocking just by motherboard is hard due to this. However, devices are available (called golden fingers) that allow you to adjust the multiplier, voltage and sometimes dividers, result in easier overclocking. Using just a motherboard, you would be lucky to get 50MHz extra. Using one of these devices should result in a lot more.
Raising the bus speed is limited by the agp bus usually, since it does not have the dividers to work above about 120MHz, but most never get close to that. Otherwise, it should be ok as long as you use the right divider for the pci bus and good pc100 memory or pc133. However, you need a motherboard with lots of 100-120 MHz adjustments, and there wasn't many of them.
The best way to overclock an Athlon is with both multiplier and cache divider settings. This cannot be done from the motherboard, but with a card that attaches to the cpu itself. To do this, the cpu needs to be removed from its case (check the web for how to do this). With this setup, you should be able to reach 750MHz with a 600MHz chip, 700MHz with below 600MHZ chips and 800-900MHz with above 600MHz chips as long as you have a good cooling system. The cache divider can also be adjusted with a program for windows, but you must be able to get into windows first (which is rare).
As long as the cache memory speed is kept at the same speed or higher, performance should equally quite a bit with each step. When the cache divider is dropped to gain more speed, the increase will drop a bit.
AMDs big success: the Athlon Thunderbird and Duron
The Thunderbird and Duron series was the first set of AMD chips that started to seriously contend with Intel, due to its performance, price and mass production. Overclocking is very easy, especially if you have a good motherboard with voltage, bus speed and multiplier adjustments that is capable to cope with high bus speeds. However, most CPUs below 1.2Ghz and above a certain age are all multiplier locked. This is however unlockable. There are numerous sites that tell you how to connect the l1 bridges on the top of the cpu to unlock it again, with equipment as simple as a HB pencil. However, the pencil approach deteriorates over time, so I suggest using a conductive pen or buying one unlocked online. Heat is however a big problem. You will need a very good heatsink and fan to get past 1ghz, unless you get a lucky chip. If your cpu temperature gets to above about 55C under 100% load, you will need a more powerful cooler to get further. The voltage is easily adjustable on most boards, and none really allow you to raise it too high to harm the chip, unless your cooling is not adequate.
Adjusting the bus speed is the best way to overclock, as long as the motherboard can take it. If you have a new ddr memory board (AMD 760/SIS 735/745/VIA kt266/NVidia Nforce chipsets) or a board based on the kt133a chipset, you should be fine up to about 166MHz, as long as the rest of the system can stand it. Bad news if you own a kt133 or the older AMD chipset - you will be lucky to hit 115. This will still however give you about 100 MHz more. The cpu itself should provide about 200 MHz more than its rated speed in most cases, with some chips (1GHz 100fsb marked AXIA for instance) going much higher. Going above 100 bus speed needs at least good pc100, but most boards allow you to run 33MHz different memory speed to the bus speed if necessary. The motherboards for these chips are fine for bus speeds up to 133, above that the pci and agp buses will run out of spec.
Increasing the multiplier is easy to do when the multiplier is unlocked, with most boards having the settings. This is the best way to do it on a kt133 or the older AMD chipset, due to the low overclockablity of the chipset itself when overclocking using the bus speed. However, if you have one of the other chipsets, I would suggest lowering the multiplier and raising the bus speed to at least 133, as long as your memory can take it. As before, don't expect more than about 200 MHz extra unless you have a really good chip and/or a excellent cooling system. You should manage 100MHz if you cannot adjust the voltage.
Performance with overclocking these chips should rise nicely with each step, but you may find that due to the speed of these chips, your games may be limited most by your graphics card. Unless you run some very cpu intensive programs, a 1ghz cpu should be all you need.
AMDs newest chip: The Athlon4 (XP and MP)
The new Athlon 4 is bascially an improvement on the Thunderbird. As a result, most of what i said about the Thunderbird can be applied in the same way. All XPs are locked, and so far, all MPs are unlocked. Unlocking is a much harder process now as they have not just cut the bridges now, they have dug them out. Now you have to fill in the hole and then reconnect the bridges. Heat is less of a problem now due to the more effcient core, but due to the rise in clock speed, you still need a good heatsink and fan. Results so far are very good, with most chips easily making the 200MHz overclock, some even at default voltage ! All chips come at a 133 bus speed, so it is hard to overclock without affecting the PCI and AGP speeds. As before, i suggest raising the multiplier to overclock it, though raising the bus speed will also increase memory peformance - just make sure your PCI, AGP and memory can handle it!
Last modified: 01/06/2005