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Q & A: Overclocking
Overclocking: is it worth it?
Q: I don't want do try anything that might compromise my machine or my data. Is overclocking risky?
A: Overclocking can indeed be risky to your data and the hardware itself -- not to mention your PC's warranty if something goes wrong. You can burn chips out (permanently!) or crash hard by exceeding tolerances. With that said, it probably won't blow up or go up in smoke, if you are careful. You will, however, probably experience an increased number of crashes as you start to reach the limit of your hardware.
As well, a phenomenon known as electromigration -- a gradual deterioration of the CPU's core -- may occur more rapidly on an overclocked processor, due to higher heat levels. Thus, adequate cooling is a must on overclocked systems, and will very likely increase the life expectancy of any computer.
Officially, we don't recommend overclocking, although we'll admit that most of our personal-use machines are overclocked to some extent. We've seen at least two overclocked machines fail catastrophically, requiring an entirely new motherboard in one case, and a new CPU in the other.
Q: Okay, I'll try it. How do I do it?
A: There are two main methods of overclocking: Front Side Bus (FSB) speed manipulation, and changing multipliers. Unfortunately, this last method doesn't work on most of the processors you'll find. Both Intel and AMD have deliberately disabled the ability for their recent CPUs to change multiplier values (all Intel processors since the latter days of the Pentium II have been locked in this way ). In other words, they are unable to run at a different "clock multiplier" value than that for which it was designed. So, what is a clock multiplier? This is a circuit that takes the FSB value, multiplies it by a certain value and thus derives the clock speed of the processor. For example, let's say that your multiplier value is 3.5, and your bus is 100 MHz. That would make the clock speed of the CPU equal 350 MHz (3.5 x 100). Similarly, a 300 MHz Celeron has a multiplier value of 4.5 and a 66 MHz FSB (4.5 x 66 = 300). Because the multiplier is locked, the only way to overclock these processors is to change the FSB speed, say, from 66 MHz to 75, 83, 100 or even higher. Most Celeron 300 chips, for example, can easily reach FSB speeds of 100 with standard air cooling. With a sophisticated water-cooling system, they can go even higher.
Changing the multiplier may be possible, if your processor is very old (manufacturered before August 1998). Some motherboards, such as the BH-6 from Abit, can even change the multiplier value on some (but not all) Pentium II and Celeron family chips that Intel has specifically disabled in this way. (Note: this feature only works on Pentium II 350s and 400s produced before Aug.19th, 1998. After this date, all PIIs are locked at their respective multiplier and the BH6's so-called "SEL100/66#" feature cannot unlock it.). Thus, a 133 MHz chip (a 66 MHz FSB with a 2.0 multiplier) could be overclocked by changing the multiplier to 2.5 and/or be increasing the FSB speed to 75 MHz.
There are distinct disadvantages to overclocking via FSB manipulation. The main problem is that the PCI and AGP slots are designed to run at very specific frequencies. PCI cards expect to find a 33 MHz slot and most won't run reliably if the FSB speed is too far out of spec. Similarly, graphics cards connected to the AGP slot may misbehave or malfunction if overclocked too far. And extreme overclocking (say, by pushing a 33 MHz slot beyond 41.5 MHz) can cause hard drive failures, data corruption, or worse. There: you have been warned.
Overclocking enthusiasts often find that overclocked systems need a few tweaks to achieve optimum stability. These include: faster RAM, better cooling and increasing the voltage going to the chip.
Although most Pentium II and PIII class boards automatically set the CPU voltage, some motherboards (recent Abit boards, for example), allow you to manually set the voltage for optimum performance. For example, reports suggest that 2.10V is the optimum value for an overclocked 300 MHz Celeron-A that won't run reliably at the default 2.0 Volts. Be careful, though. As mentioned above, overclocked chips run hotter and can cause instability or cause problems, including premature hardware failure. Have we warned you enough times yet? Hard-core overclockers usually beef up the fans in their system, sometimes even adding up to three fans on the CPU itself. On K5 and Cyrix 6x86 systems, a so-called "Peltier cooler" -- a sort of mini refrigeration unit -- is often employed, according to Anand Shimpi, in his excellent article on overclocking. Great, as long as you don't mind a computer that sounds like a jet engine, as well as running like one.
A primer on overclocking your PC at Sharky Extreme provides additional heatsink and fan recommendations.
CPU Voltages: the shocking truth
Q: What is the meaning of a "single plane" CPU versus a "dual plane" one?
A: Some CPUs, such as Intel's P54C or P54CS (classic Pentium), AMD's K5, and the IBM/Cyrix 6X86, use a single plane voltage regulator. In other words, the chip requires only one voltage. For a P54C, this is 3.4V (STD) or 3.5V (VRE). Other, newer processors, however, are said to be dual plane. These chips, including the AMD K6, Intel P55C and IBM/Cyrix 6X86MX, use dual voltages, yet are still referred to by a single voltage -- somewhat lower than their older siblings, due to improvements in fabrication techniques. The AMD K6-2, for example, is said to be a .25micron chip, with a dual plane voltage of 2.2V (dual). The older AMD K6-PR233, meanwhile, was a .35micron chip, running at 3.2V (dual).
Overclocking an OverDrive?
Q: Can I overclock a Pentium OverDrive processor?
A: Yes, but don't expect amazing gains. (Standard disclaimers apply.) Let's say, for example, you have a 150 MHz Intel Pentium OverDrive processor. By increasing your CPU external clock (bus) frequency to 66.8 MHz, you will increase the processor to the equivalent of a 166 MHz processor. (66 MHz x 2.5) We tested this with an Asus P5A-B motherboard with excellent results. However, when we pushed the system to 190 Mhz (75 MHz x 2.5) we began to see Windows protection errors, application crashes and other misbehavior. But all's well at 166!
Q: I want to use a Cyrix 6x86MX PR266. Is it supported by motherboards that support K6 and P54C?
A: The 6x86MX PR266 uses a 2.7V core voltage (at least the ones that we've seen do). So make sure that the motherboard that you get supports this voltage -- not all of them do. Your best bet is a board based on one of the chipsets that supports an 83.3MHz bus: the Ali Aladdin IV+ or V, SiS 5591, or VIA MVP3. See our Socket 7 motherboard specs listings for details.
Q: What chip would you get for a Slot 1 motherboard?
A: At this point, I'd go for a Malaysian-made Celeron 300A (with 128K of cache onboard). However, if I wanted a "real" Pentium II, I'd go for a 333MHz PII which is based on a .25 micron fabrication process compared to the .35 micron process used for the 300, 233 or 266 MHz chips. The 333 runs much cooler and, with a multiplier of 5, can be overclocked very high on a properly cooled system. Intel CPUs produced before Aug. 1998 are even more flexible, and can, with an Abit BH6 or BX6 board, defeat the multiplier lock that Intel has built into its recent chips, or using the technique listed on Tom's Hardware site (www.tomshardware.com).
However, if you are planning to go for a newer chip, go for one of the 450 MHz Pentium III models (with a 4.5 multiplier), preferably the easily overclockable OEM model labeled as "SL35D." These CPUs can usually (but not always) be overclocked to speeds close to 600 MHz using a board like the Abit BH6.
At a more economical level, I'd consider going with the Celeron 266 or 300. The 266, locked at a 4x multiplier, will usually do 400 MHz (4x multiplier X 100MHz bus speed), while the 300 will usually do 450 MHz, by simply changing the bus speed from the Celeron's default 66 MHz rate to settings such as 75, 83 or even 100 MHz. Thus, a Celeron 300A, with its 4.5x multiplier (4.5x66=300) would become 450 MHz when the bus speed is set to 100 MHz.
Additionally, these older Celerons' lack of L2 cache makes them more stable when overclocked. Users routinely report getting between 400 and 448 MHz out of a Celeron 266 using a board such as the Abit BH-6. (The heatsink/fan that comes with the boxed version of the Celeron is suitable for running the chip as high as 448 MHz, however, most overclockers prefer to add their own hatsink/fan combos to OEM CPUs or disassembled retail units). As we've mentioned elsewhere, please remember that the Celeron 300, intended for a 66 MHz bus, is not always capable of running at a 100 MHz bus speed -- you may have better reliability with an 83 or 75 MHz bus clock setting. Because the Celeron 300 is locked at a 4.5x multiplier, some Celerons will are more reliable at 374 (4.5 x 83MHz) than 450 (4.5x100) MHz.
In our tests, most Celeron and Celeron-A chips, particularly the 266 and 300 MHz models, are well-suited to overclocking. Although you can boost the bus speed beyond 66 MHz on most brands of motherboards, you will obtain the best results with motherboards from Abit and other suppliers that can boost the voltage a few notches as well.
At www.anandtech.com, Anand Lal Shimpi has a RealVideo video (25Kb Stream) showing exactly how to install and overclock a Celeron-A in a system based on an Abit BH6 board. (Anand sets the voltage to 2.10V, citing increased reliability and safety.) Anand also has detailed charts and reviews describing other important factors to consider when choosing a motherboard and CPU.
Indeed, Jerry Adamson (jada...@ix.netcom.com) of BusterVision notes that the Celeron-A is basically a .25mu PII with a different cache setup. In effect, he says, it is a PII slowed down by Intel not because of performance reasons, but to fit in a lower cost market segment. In essence, a 350-450MHz chip for a 100MHz bus taken down to 300 for a 66MHz bus, not because for some technical reasons that it can't run on the 100MHz bus at 350-450MHz, but simply for economic reasons.
Says Adamson, "In the time I have been monitoring hardware sites about Celeron 300a to 450 overclocking, I have not yet heard of a single case of a hardware failure related to this operation. Some of these jokers are using their 450s as around the clock renderers for 3D Studio Max ! Granted it's only been a couple of months, but my experience has been that reliability issues with overclocking are almost nonexistant after this period of successful use."
It is worth reminding users that reliability is not guaranteed, a CPU's warranty is voided by overclocking and the Celeron-A has only 128K of L2 cache. However, The Celeron 300a's 128K of L2 cache runs at the core CPU speed -- not one-half of the core CPU speed, as is the case with the PII. This helps boost performance close to Pentium II speeds.
Finally, it's worth mentioning that some reports suggest that early runs of the Pentium III were among the least overclockable Intel processors company has yet produced. Although, according to a PC Week report, Intel claimed it had not "bus locked" the chip, PC Week found that the chips it tested could not be run at bus speeds or multiplier values other than those intended by the manufacturer -- a situation corroborated by some (but not all) other sources, too. Fortunately, later runs of the PIII -- particularly the SL35D revision produced in mid-1999 -- improved this situation.
Q: Is it possible to overclock an Apple iMac?
A: Yes. See the iMac ClockUp page for details and heed the warnings there. Note, however, that we've seen an Apple PowerPC machine fail catastrophically (burning out the motherboard) when overclocked just 20 percent faster. And, tellingly, former Mac CPU upgrade supplier Newer Technology recently got out of the clock-chip overclocking business.
Q:How do I overclock my Front Side Bus?
A: Generally, you use jumpers on your motherboard, or, if your mobo supports it, via a "CPU SoftMenu" such as that provided on the Abit BX6, BH6, AOpen AX6BPlus, etc. An interesting alternative is to use SoftFSB. This tool allows you to set the FSB speed via software. The author of the program says there is a way to make it start with Windows and auto-set the Bus speed everytime it is restarted. Here's what you do:
Unzip the SoftFSB program, Start the program at least once. That will allow the SoftFSB.ini file to be made. Open the .ini file, On line 6 under the SoftFSB menu, there's a
- nExtMode=0
- now, change the value to 1 as in
- nExtMode=1
Now close the file and restart the SoftFSB.exe file. If you have done all of that correctly, you will now see 2 new buttons under the [ Get FSB ] button. Get your FSB now, and the AUTO button will be enabled. Now configure it to your personal liking. For example, you might set it to a 100MHz bus to make a Celeron 300A run at 450MHz.
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