Over Clocking Processors

By Merle Nicholson, President, Tampa PC Users Group


An interesting thing is going on in the computer gaming world right now with "over clocking" Pentium II Celeron processors. There’s quite a lot being circulated about this lately, so I thought I would explain what is going on.

To put it as simply as possible, over clocking is running a computer above its intended speed. Why would you do that? It all boils down to money. Right now the cost of an Intel Pentium Celeron 300a processor is $100, and an Intel Pentium II 450 processor is $550. So instead of buying the expensive version, just buy the Celeron and over clock it to 450. Why isn’t everyone doing this? What are the risks? Read on; all is explained here.

Hobbyists have been over clocking computers for a long time now, and I’ve never really understood what there was to gain. Back when we were all on 486 computers, much was made of the new AMD 486-80 that could sometimes be run at 100. That’s a 25% increase in processing power. Why bother? Would you see the difference? My opinion was then that it wasn’t worth the work. You had to have a motherboard capable of running it faster in the first place; in other words, make the settings that make it possible. And you had to add a fan inside to keep it cool because it ran hotter. And it didn’t always work.

The price to pay – when it didn’t work - was instability. Sometimes the processor wouldn’t work at all, but many times it would work, but badly. You would have program faults and the deadly Windows "blue screen" errors that forced you to reboot. And sometimes you would get spontaneous reboots. But that’s about the extent of the risks, and if you don’t like the way it behaves just put it back.

Back to the present, the issues and risks are identical to what they were five years ago. But one thing has changed this time: the gain is greater.

Intel and other chip manufacturers would prefer that their processors not be over clocked. Their main concern is that an unscrupulous manufacturer would buy cheaper processors for their computers and pass them off as more powerful machines— an opportunity to cheat the public. And obviously it would cut into the revenue stream. So with the Pentium II they go to some trouble to make sure they can’t be over clocked. And that takes some technical explanation. Stick with me here; I’ll try to make it painless.

A processor’s power is mainly dependent on two things; the capability of the internal program, and the speed at which it is run. The capability of the "instruction set" – that’s the internal program, is slowly evolving with each new processor type. Two things have to be accomplished here. One is that the processor does all the things it used to do so it can run all the older programs, and that any new instructions are useful enough so that there’s interest in software development to take advantage of them. MMX is a very good example of this. MMX doesn’t hurt anything, and as programs are available to utilize the capability of MMX, some significant performance gains are possible.

I said two things – the other is the speed at which the processor is being run. All other things being equal (the instruction set) the faster a processor is run, the more it can do in a given time (the speed). Why not just build a Pentium II 10,000 then, instead of just messing around with 300’s or 450’s? The answer is just evolving technology. They’re basically making them as fast as the technology allows. And there’s been a constant revolution in manufacturing techniques for many years. If they were capable of building a compatible, say, Pentium 600, and they could make it for a price that would make it worth it, it would be done. And they’re working on it; you can count on that.

Every once in a while they figure that a major change in chip architecture is needed to take advantage of current technology, so we have a major change like the P II. And the next one is already available - the Xeon, and AMD is coming out with a 500 MHz K7 midyear.

So, back to speed. Motherboards have a clock. They run a speed measured in frequency called Hertz, and since the range we’re dealing with is in millions, the term is in Mega-Hertz (MHz). It’s simply the number of "ticks" per second, and the processor does a certain number of instructions at every "tick" of the clock. The Motherboard supplies the clock and normally has several settings that can be made so that it will accommodate more than one speed of processor. So a manufacturer will use the same motherboard for a Pentium 233 and a 266 or 300, for instance.

Motherboards have basic "board speeds"; that is the speed that the board ships information around to its several components. The most important function is the speed at which the processor ships data to the memory. Over time, Pentiums used 25 , 60 and 66 MHz bus speeds. Other speeds needed by the other processors have been 75 and 83.3. Now we’re using 100 MHz for many computers. And the new K7 will be using a 200 MHz bus. Each of these speed increases required more technology development and expense.

The board must also be capable of handling a given range of processor speeds, and that is set by a "multiplier". The multiplier is a function of the board speed, and to get say a 266, you have to multiply 66.667 times four. And a 400 MHz CPU is run with a multiplier of four also, but uses a 100 MHz bus instead. Boards typically have multipliers of 2x through 5x or more; in half steps; 2.5, 3, 3.5 etc.

Now back to over clocking and the Celeron. Intel reportedly builds into their processors the capability of detecting both the board speed and the multiplier. But the story goes that someone decided that detecting the board speed of a 300 wasn’t necessary. And then it was discovered that the Celeron 300 would actually run at 450 if you reset the board speed from its normal 66 to 100. The multiplier stays the same at 4.5. Successful over clocking at a 50% increase is unheard of. The catch is that the processor voltage must be increased from the normal 2.0 volts to 2.2 or 2.3 volts. And that’s the tricky part. And it also increases the risk – of burning out your $100 processor. But an increase from 300 to 450 is very much noticeable, especially if you’re running games.

So first you have to buy a board and processor. And you have to get PC-100 memory. Sorry, you can’t use your old memory for this. And do your homework and some research on this. There’s a lot available at Tom’s Hardware page at http://www.tomshardware.com and several other sites. At Tom’s, there’re good pictures available, and you’ll need them, because to set the voltage on most boards, you must actually paint fingernail polish on some of the pins on the processor board.

PC gaming enthusiasts discovered this. There are people out there who routinely experiment with this. But this particular discovery was pretty noteworthy because the 300a is at the bottom end of the processor food chain. So if you would rather put your money into game software and still have the power of a machine that would cost $500 more, it has a lot of appeal.

If you’re interested in doing this, you have some research to do, but there’s a lot of help out there. People like to write about what they’re doing, and the Internet is the perfect place to publish this information. u