Logo by Tom Purves

Overclocking refers to the practice of running one or many parts of your computer at a higher frequency/speed than they were rated for. Obviously, the idea of this is to make your computer run faster and to get more for your money. Few things are more attractive than the promise of something for nothing, and that's exactly what overclocking is...the promise of being able to buy a P166 and turn it into a P200 simply by changing a few jumper settings.

Why it works

Safety margins

Inherent in the design, specification and production of every chip or product is a margin of error. Companies do not simply design chips to run, say, at exactly 100 MHz. That would mean that if there was even a little error in production or design that affected the performance of each chip, the company would be stuck with a bunch of bad chips. Instead, a speed higher than the planned speed rating of the chip has to be targeted, so that whatever errors or difficulties are encountered, at the end of the day the company still has usable chips/components to sell.

So if a company like Intel wanted to design and sell a 100 MHz chip, for example, it might shoot for 120 MHz. Even if errors and variances in the design/production process caused the worst case chip to only run at 100 MHz, they would all still be good.

You also have to understand that it costs time and money to design to higher specifications, so you can't just design a 1,000 MHz chip and sell every single one as a good 100 MHz. You design just high enough so that you get an acceptable yield, i.e., so you can keep most of the chips and only have to throw a few bad ones away.

Economies of scale & market differentiation

There are two sides to this next issue. On the one hand, in this day and age it costs A LOT for a company like Intel to set up a fab plant to produce chips. Therefore, it would be prohibitively expensive to set up different plants or lines just to produce each different speed of chip.

However, the market WANTS different speeds of chips. Some people will never buy a computer at all unless it's cheap enough, and some people are willing to pay anything to get just a little extra speed. Selling something to these people is far better for a chip maker than selling nothing at all!

The solution to these two problems is to produce a whole range of chips from just a single line. You avoid the prohibitive setup costs of new fabs, but you still turn out the chips you need. Most of the cost in making microprocessors is in the setup...producing the chips is relatively cheap, by comparison. Every chip gets tested, but if P133's are in demand, it's quite possible for a chip capable of running at 166 MHz to go out as a P133 instead. After all, once the initial costs of setting up a fab and tuning it are paid out, each chip costs the same to make regardless of the speed it turns out to be, and more can always be made later.

First of all, overclocking is completely luck. Sometimes you will be lucky, and sometimes you won't. And that's that.

Don't be stupid. If you overclock your computer in one way or another, don't ask why it doesn't work, if it doesn't work. You KNOW why it's not working. It's because you overclocked it!

Don't ask something like "When I overclock, it works fine in DOS, but it crashes in Windows 95. What can I do to fix it?" Hardware is hardware. If your hardware is screwing up under Windows 95, it's screwing up under DOS too, and it's screwing up no matter what OS you run. It may appear to be stable in DOS, but the fact is, you're just lucky you haven't run the wrong program yet.

The question you should be asking is: What can I do to fix whatever problems were caused by overclocking? The easy answer is, of course, not to overclock, but that's unlikely to be an answer that you are looking for. I'll try to categorize the answers by the type of part in your computer you might be overclocking.

Basic procedure for overclocking the CPU

1. Prepare adequate cooling for your CPU. That means no crappy heatsinks/fans.

2. Read your motherboard manual and find the settings for bus speed and clock multiplier.

3. Simply turn off your computer and increment the CPU speed up by the smallest amount possible. At each increment, thoroughly test the CPU for anything out of the ordinary. You can try anything you want, really...I think Winstone 97 is great for stress testing an overclocked system. Run stuff for an hour or a night or whatever. If you notice anything different at all besides the extra speed, like weird behavior or crashes, clock back down a step and follow some of the troubleshooting guidelines farther down in this document.

4. After you've established a speed at which your system seems to be running ok, continue to use your system as usual and watch for anything out of the ordinary. Not all problems manifest themselves immediately and in my own personal experience a problem didn't show up for three days. Anything that is clearly different besides speed is a problem, even if it doesn't happen often. I know it's tempting to think "maybe that won't happen again?", but it always does and you get a sick feeling in your stomach until you clock back.

Yes, that's pretty much it. Overclocking is pretty easy, which is somewhat of a shame, actually, because a lot of idiots are doing it now and have no idea what is going on. Hopefully this page is making you less of an idiot! :-)

Questions

Is it safe?

In theory, it is possible to damage your chip.

However, in my own experience, yes, it is safe. I obviously cannot take responsibility for any damage you do to your own chip, but for many many people it does work. The best advice I can give you is to find out other people's experiences with overclocking and to read as much as possible. The next bit of advice I can give you is to always overclock in the smallest increments possible. It is almost completely assured that you will see minor failures at lower clock speeds, not a catastrophic failure. If you are stupid enough to clock higher after that, well, don't be surprised if your chip goes POOF! Ditto if you are stupid enough to try clocking a P100 to 166 MHz without going up the intermediate clock speeds and testing them thoroughly first.

The CPU

This is the one part of your computer people are most likely to overclock. It's fairly obvious why...it's the heart of your computer. Nevertheless, several things can go wrong when you overclock your CPU:

1. The CPU might overheat. This is because when you run the chip at faster speeds, it also generates more heat. Too much heat can change the electrical characteristics of the chip and make it unreliable. Any time the system seems to be stable upon cold booting but starts exhibiting reproducible errors after running for a while, the problem is usually overheating.

i. Put some heat sink grease/thermal compound between the CPU and the heatsink. This stuff should always be there, regardless of whether or not you are trying to overclock. The grease forms a complete seal between the CPU and the heatsink and allows heat to be transmitted more efficiently. Without it, the imperfect seal formed by sticking those two not-so-flat surfaces together is a very poor conductor of heat, and the thin areas where the heatsink fails to make contact with the CPU can actually act to insulate heat rather than conducting it away. You can get heat sink compound at any local Radio Shack. Use as little as possible...only just enough to form a seal between the CPU and heatsink.

ii. Get a good heatsink/fan combo. If you don't have one already, get one. Ask for a ball-bearing fan...these last longer than sleeve-bearing fans and often spin faster too.

iii. Be sure your case is adequately ventilated and proper airflow can occur. You want cool air to be blowing in over the CPU's at all times. Sometimes this might involve buying a second fan for your case, or even just running the computer with the case off to determine if you actually have a heat problem.

iv. If you are really serious about cooling the CPU, you might try a Peltier cooler for your CPU. This is essentially a miniature refrigiration unit that actively cools your CPU. Very expensive sometimes, and you might be better off just spending that money on a better CPU! The Peltier cooler transmits heat from one side of the cooler to the other, actively cooling the CPU. The problem is that this process itself generates a lot of its own heat, which must be dissipated as well! So you really need a really good fan and heatsink on the hot side of the Peltier cooler. God forbid that your fan should break down or that you should buy a poorly designer Peltier cooler, because your CPU will overheat in a jiffy! Many people have found that their CPU's stay cooler with just a nice big heatsink and fan. I actually don't think using a Peltier cooler is a good idea at all and I personally wouldn't use one. But obviously it all depends. Just be aware of the risks.

2. The CPU may simply be unable to handle the higher frequency. Despite all the cooling in the world, a chip may simply be physically unable to handle the higher speed. Gates inside the chip may be unable to switch fast enough at the appropriate frequency, for example.

i. Try increasing the voltage to the CPU. Increasing the voltage decreases the rise/fall time of gates in the CPU, among other things, which can possibly bring a CPU back to normal operation if indeed the switching time is a problem. Of course, increasing the voltage increases the amount of heat that the CPU generates, so be prepared with adequate cooling.

The motherboard

A popular way to increase the performance of your computer in the past year or so has been to increase the speed of the motherboard to 75 or 83 MHz instead of the usual 60 or 66 MHz.

In case you don't know how chips and motherboards work together nowadays, here's a little introduction. There used to be a time not so long ago when chips and motherboards actually ran at the same speed, like with the Pentium 60 or 66. But as Intel continued to ramp up the speed of their chips, motherboard manufacturers couldn't keep the pace. The simple fact behind this is that it's much harder and expensive to engineer a big part with lots of physical connections (like a motherboard) to run at high speeds than a small integrated part like a chip. So Intel began running the chip at multiples of the bus speed, like 1.5x or 2.0x, while motherboards hit an apparent brick wall at 60/66 MHz bus speeds.

This is OK, of course, but if processors get faster and faster while motherboards continue to stay at 60/66 MHz, each memory access outside the chip becomes a major bottleneck. It doesn't matter whether your chip runs at 60 MHz or 180 MHz, the time to access main memory is the same for both and both processors will stall for the same amount of time.

Adding cache helped to alleviate this problem somewhat. The cache is high speed memory which buffers the most frequently accessed information from the main memory. When a chip makes a request for something from memory, the cache typically already has it about 85-95% of the time. So the cache, you see, makes a big difference.

Most chips have a small amount of cache built into them, running at the clock speed of the chip. This cache is called the Level 1, or L1, cache. A much larger amount of cache, usually 256KB or 512KB, also is used and is referred to as the L2 cache. In Pentiums, the L2 cache is on the motherboard and runs at the bus speed (usually 60/66 MHz). In Pentium Pro's, the L2 cache is actually bonded to the chip and is running at the same speed as the chip (currently anywhere from 150-200!)

Anyhow, times change and it's a couple of years later. It seems as if motherboard manufacturers are now actually capable of producing motherboards that work reliably at 75/83 MHz. However, Intel only guarantees their chipsets to work up to 66 MHz, and since Intel produces basically the only motherboard chipsets of consequence, motherboard manufacturers do not officially support the 75/83 Mhz settings. That doesn't mean we end users can't try them, though!

Now that you understand how bus speeds on the motherboard work, you should now also understand how overclocking bus speeds affect performance.

All other things being equal, the reason performance is affected so greatly in Pentium-based systems when you overclock the CPU is because the speed of the path to the L2 cache increases. Yes, the speed to main memory and to your peripherals also increases, but if 85-95% of your memory accesses are to the cache, you tell me which makes more of a difference. This is the reason why Pentium 166's running at 83 MHz x 2 can often outperform 200 MHz Pentium's running at 66 MHz x 3. The memory bottleneck is significantly widened.

Now consider Pentium Pro's. Given what you know from above, what should you conclude the effect of overclocking the bus would be on a Pentium Pro? The answer is, overclocking the bus speed on a Pentium Pro does not affect performance by nearly as much as it does on a Pentium. The L2 cache is already running at chip speed, up to an incredible 200 MHz (or 233 if you're overclocking :-) )! Far better than Pentium-based systems can ever hope to achieve at the present time. Furthermore, the L2 cache is independent of the bus speed (unless you are using the bus speed as a means to achieve some other chip speed). So don't make the common mistake of thinking that overclocking the bus affects Pentium Pro's as much as it does Pentium (as many people do). It's simply not very important to choose a Pentium Pro motherboard that runs at 75 MHz. All other things being equal, however, a higher bus speed can only be a good thing. Just keep in mind that all other things are rarely equal.

What to do when you overclock the bus and your computer fails. There are way too many possible ways for your computer to fail when you overclock your bus to 75/83 MHz to list them all. This is because you are overclocking all your peripherals (since typically PCI and ISA is tied to the bus speed in some manner), your memory, your motherboard, and often your chip as well. With the popularity of non-parity RAM, detecting memory problems (very common when overclocking the bus) is very very difficult. By the way, I do believe memory problems are probably the most common faliure when overclocking your bus speed, so look hard at that first.

At any rate, you must apply some common sense and technique.

i. Attempt to completely isolate the problem. Remember that the problem can be damn near anything. So remove every component that you possibly can to make sure your basic components are stable. You should have essentially nothing but a motherboard, CPU, video card, one hard drive, some RAM, a keyboard and a mouse. You MUST have a stable foundation to work from if you want any chance of diagnosing the problem. If you have a stable system, you can start adding peripherals one by one to determine which one is giving you trouble.

ii. Set all BIOS settings to the safest settings possible. Turn off all possible optimizations and reenable them one by one. This applies especially to memory timings and hard drive PIO mode settings.

iii. Try just overclocking the CPU first. If you are overclocking both the CPU and your bus speed simultaneously, it can be very difficult to diagnose potential problems. Try overclocking just the CPU and nothing else to the target speed you want to get...this will let you know if your CPU is causing problems. Obviously not every speed setting can be reached at the different bus speeds, so you may have to test at a setting a little higher than what you actually plan on using.

iv. Use different parts. It's difficult to do, but sometimes this really is your only option. If you or a friend have extra video cards or motherboards, etc., lying around, you can try switching out these essential parts of the computer to see if they are causing the problem. Hopefully, it won't come to that, because this will get very time consuming.