The CPU uses the FSB (front side bus, also called the system bus) to communicate with system memory and peripherals. (The backside bus, on the other hand, connects the Pentium II to the L2 cache inside the Pentium II cartridge.) Pentium IIs rated at 350 MHz and faster operates by default at a front side bus speed of 100 MHz, while Pentium IIs rated at 333 MHz and slower run at 66 MHz.

Peripherals residing on the PCI bus, however, are designed to operate at 33 MHz. The clock divider sets the clock rate for the PCI bus as a fraction of the front side bus speed. On motherboards with a front side bus speed of 100 MHz, for instance, the clock divider is set to 1/3 that speed, to deliver a PCI bus clock speed of 33 MHz. On motherboards with a front side bus speed of 66 MHz, the clock divider is set to 1/2 that speed.

The motherboard chipset controls the clock multiplier, which, in conjunction with the FSB speed, determines the core speed of the CPU. Multiply the FSB speed by the clock multiplier to get the CPU speed. For instance, a 350-MHz Pentium II runs at a default FSB speed of 100 MHz with a clock multiplier of 3.5 (3.5 x 100 = 350).

By manipulating the clock multiplier and/or the FSB speed, you can increase the core speed at which the CPU runs. Here's a list of default settings for Pentium IIs.

Optimal Speed for Your CPU
Remember just because your neighbor or brother or whoever can achieve a certain speed with their identical CPU doesn't mean that yours should be able to hit that same level. Each and every CPU is unique. Considering these small cores have 8 million or more trace routes, it should come as no surprise that no two CPU cores behave the same under stress.

In addition, the Level 2 cache, which operates at approximately one half the speed of the CPU, is matched by Intel to the speed of the CPU core to which it is connected. There's usually a good deal of leeway, but the individual speed of your cache is a key limiting factor in overclocking your CPU.

The goal of overclocking is to find your CPU's comfort zone--the speed at which it will run reliably, without system errors or data corruption. Here are some common, relatively conservative settings.

Note: To run reliably at a 100-MHz FSB, you need to have 100-MHz SDRAM installed in your system. Also note that in many of the cases listed above, increasing the FSB speed requires you to lower the clock multiplier on your system.

Since August 1998, Intel has been locking the clock multiplier on its CPUs, so it probably will not be possible to change the multiplier when overclocking a 350-MHz, 400-MHz, or 450-MHz Pentium II. If you try, the CPU will either refuse to boot the machine, or it will boot it up at 1/3 its proper speed. To get around this limitation, a Pentium II overclocker's only remaining option is to increase the speed of the front side bus.

I have a Pentium II 400-Mhz and a motherboard that is capable of 66-100-Mhz FSB (the cheapest in it's class), so I thought that I would not be able to overclock my PC anymore because of the locked clock multiplier. And I thought that I should replace my motherboard with a 133-Mhz capable in order for me to increase the FSB. But to my surprise, I was able to change the clock multiplier and I was able to run my PC at 500-Mhz but it's not so stable. I think the problem is with the memory modules so I just have to settle with 450-Mhz (4.5 x 100-Mhz) until I have the money to buy a good quality SDRAM. My Pentium II 400-Mhz runs well at 450-Mhz (4.5 multiplier) and it's rock stable. I don't have to add any cooling device because it runs cool with the standard fan and with the side panel of my tower case removed. The performance is great and it runs just like or even faster than a 450-Mhz non-overclocked Pentium II.

Unfortunately, it's not always so easy to set the clock multiplier and FSB speed. You will need to do some research to determine the proper method for your particular system.

If you're lucky, your motherboard includes a BIOS utility for configuring bus speeds and multiplier options. This is the best situation, because it facilitates experimentation. Unfortunately, only a few motherboards currently offer this option, notably Abit's BH6 and BX6 boards.

If your motherboard doesn't have a BIOS utility, it might utilize DIP switches to select the settings for both the CPU multiplier and bus speed selection. DIP switches are relatively easy to set, as long as you can get information on the proper settings.

In the absence of either of these two methods for setting CPU options, you'll almost certainly find the venerable old jumper on your motherboard.

Jumpers are relatively easy to set, but they require a case that allows access from a variety of openings. Nothing is more frustrating than having a jumper slip out of your hands to disappear somewhere inside the case.

Check the manual that came with your motherboard or your system for details on whether to use BIOS settings, DIP switches, or jumpers. Some systems also have this information written on a sheet pasted inside the system case. Failing that, your motherboard manufacturer's Web site might have the required details.

1. Make sure you have not set the motherboard to exceed the maximum multiplier setting of your particular CPU. Otherwise it will boot at half its listed core speed and you won't see any performance gains. Also, be sure that the FSB speed setting is correct, and that you've lowered the clock multiplier setting, if necessary.

2. Double-check that your heatsinks and fans are properly installed and plugged in. Replace your fan & heat sink if necessary to prevent overheating.

3. If you're shooting for an FSB speed beyond 100 MHz, disconnect any nonessential peripherals, such as network cards, SCSI controllers, PCI sound cards, and any other components that run on the PCI bus. This will allow you to properly diagnose errors later in the process. Also, change your UDMA IDE HD's mode to "PIO Mode 2" or "DMA Mode 1." This will decrease hard drive performance, but it will help protect your hard drive.

The reason for taking these precautions is simple: PCI peripherals are designed to operate at a bus speed of only 33 MHz (1/3 of 100-MHz or 1/2 of 66-MHz FSB). It's very possible that one, if not all, of your PCI peripherals might become unstable running in a bus clocked higher than 33 MHz. There's a very real possibility that you could lose all the data on your hard drive.

4. Change all settings in the BIOS to reflect the most compatible environment. All settings regarding the shadowing or caching of the video BIOS or RAM should be turned off. All SDRAM timings should be set to the lowest possible numbers. Set your 8- and 16-bit I/O recovery speeds to their maximum numbers (4 and 8, respectively).

Here are some tips for troubleshooting your overclocked system. Of course, we advocate a better-safe-than-sorry approach: If you can't get your system to run reliably in an overclocked state, you're much better off returning to a standard configuration. It's just not worth the risk to your system and data.

Typically, if you get the machine to boot, count its memory, and give you a close or correct megahertz indicator for your CPU, you're on your way to success. (Note: The new core speed, expressed in megahertz (MHz), often will not be correctly indicated in the boot-up screen if it's a nonstandard number, such as 372 MHz.) If the machine refuses to boot up at all, there are two possible problems: there isn't enough voltage going to your CPU, or your CPU has simply failed to run at the speed you've set it to run.

When you increase the megahertz level request, the CPU sometimes needs to draw more power to achieve that new setting. Abit's BX6 and BH6 are the only 440BX motherboards that allow you to increase the Pentium II's voltage to give it the extra juice that it needs. If you have the BX6, then you'll want to adjust the voltage in the Soft Menu II BIOS upward from the stock level. A stock Pentium II requires either 2.0v or 2.8v to operate at its rated megahertz level, depending upon whether it's a 0.35-micron Klamath or a 0.25-micron Deschutes CPU.

So, for instance, you might want to run a 300-MHz Pentium II at 3.0v, rather than the default 2.8v, when overclocked to 350 MHz. However, you'll need to experiment a lot, as 3.0v may be more or less than you need, depending upon your specific CPU. Fortunately, the BX6 will not let you go too far on a "more power" binge--when it comes to voltage settings, you can do only a little tweaking either way. Increasing the voltage going to your CPU is quite dangerous and should only be attempted with extreme caution.

Several problems can occur during the loading of Windows 95 or 98. This is also where the problem of overheating comes into play. If your machine has booted up, displays the correct or near-correct megahertz level for your overclocked CPU, and then proceeds into the Windows load screen only to freeze, you can try two corrective measures.

First, go into the system BIOS and lower the hard drive's PIO level one notch below its previous setting. (This is only for people using a higher-than-100-MHz FSB speed in their overclocking efforts.)

Second, reduce the CPU's heat by taking the cover off the case and removing all nonessential PCI components.

These two steps might not seem like much, but aside from increasing the voltage of the CPU, they're most likely all you've got left. In many cases, it's possible to realize as much as a 15-degree Fahrenheit reduction in internal system temperature simply by taking the cover off the case. For maximum stability, many die-hard overclockers run their machines full-time with no cover on the case. As long as you take time each month to clean the system of dust, you should have no problem running your machine topless. However, be aware that some cases are designed to take advantage of the airflow created by a closed case.

After taking these steps, if the machine still refuses to boot into Windows, it's more than likely that you've set your CPU to a speed beyond its limits. Cooling usually solves some of the problems, but if you are able to boot into Windows only to suffer from crashing and system instability, you've clocked your CPU too high. Either try a more conservative overclocked setting, or return to your CPU's default settings.

One other instance where the machine boots properly into Windows but then proceeds to develop strange errors could be caused by the quality of the SDRAM in the machine. If you're clocking your FSB to 100 MHz or 112 MHz, make sure you are using 100-MHz SDRAM. Even in the PC-100 SDRAM subcategory, however, you must choose between a maximum CAS latency timing of "3" or "2." Memory that supports a CAS latency timing of 2 can sustain the 133-MHz FSB overclocking option of the Intel 440BX chipset. CAS 3 SDRAM, however, can't handle any bus speed beyond 112 MHz.