Clockspeeds and Overclocking


The original 8088 system had a single system clock which oscillated at 4.77MHz. This means that the system clock 'ticked over' at nearly 5 million cycles per second. (A 'tick' or 'cycle' is the smallest measurable unit of time on a PC in which anything can be done.) The problem with this approach is that all components ran at 4.77MHz: the CPU, the memory bus and I/O bus (which were, admittedly, all one bus in the 8088).

It turns out that improving the speed of the CPU is technologically a much easier task than speeding up the memory bus, which is turn easier than speeding up the I/O bus. Rather than limit all devices to that of the slowest bus, modern systems use multiple clocks running at different speeds. The speeds of all these clocks are set as a multiple (or fraction) of one 'master' clockspeed - the system clock. For example, the PCI bus should run at 33MHz. Thus on a system with a 66MHz system clock, the PCI bus is set to run one half the speed of the system clock. On a system with a 133MHz system clock, the PCI bus is set to 1/4 system clock speed. The term 'system clockspeed' is usually referring to the clockspeed of the memory bus (or frontside bus in DIB systems, which is most of them now), not the CPU clock speed. This is also sometimes referred to as the external CPU speed, i.e. the speed that the CPU talks to memory and other devices - hence the memory bus.

However, to confuse matters, with the arrival of DDR RAM which runs at twice the system clock speed, it can be said that the memory bus is actually running at twice the speed of the system clock. For example, the table below shows the various clockspeeds that my (now parted with) Athlon 2000+ system should be set to:

Device / Bus Clockspeed (MHz) Multiplier
'System clock'133x 1
Processor1666x 12.5
Memory bus (FSB)266x 2
Level 2 cache1666x 12.5
AGP Bus66x 1/2
PCI Bus33x 1/4
ISA Bus8.3PCI Bus / 4

In this case, there are no devices or buses that actually run at the speed of the system clock. The system clock is simply present as a reference point to set all the other clocks.

If you want to analyse the various clockspeeds being used in your system, you can use the useful utility WCPUID which you can download here or from the H.Oda! website.


Notice that I said above that the speeds listed in the table are what my Athlon 2000+ system should be running at. Well, now that you know a bit more about buses and clockspeeds, a couple of easy ways to overclock your system becomes apparent.

One approach is to change the clock multiplier generating the core frequency of your CPU. For example, let's say you have Intel Celeron 500. The system clock speed (memory bus speed) for the Celeron is 66MHz. To create a 500MHz core speed, a clock multiplier of 7.5 is used (since 7.5 x 66.6 = 500). So, you could increase the core speed of the CPU to 566MHz by simply increasing the clock multiplier to 8.5.

However, there are a couple of reasons why this approach isn't ideal, or even viable! Firstly, the core speed of the CPU is modern machines is rarely a limiting factor, since they run so much quicker than the memory bus anyway. (This is why, for example, the Intel Pentium 150 actually runs slower than the Pentium 133! This is because the bus speed of the former is 50MHz, with a clock-multiplier of 3x, while the latter is running at a much higher bus speed of 66MHz, clock-doubled.)

But the main reason for not taking this approach is that it is simply not usually possible. The chips have circuitry which enforces clock-locking. This means that the clock-multiplier is hard-wired into the CPU itself and cannot be easily changed without actually physically modifying the chip. Such modifications are possible, but not recommended for the faint of heart!

The much easier approach to overclocking is simply to increase the system clock speed. Most modern boards will allow you to select your system clock speed in the system BIOS. This approach has pros and cons. On the good side, if you increase the system clock speed, not only do you increase the core speed of your CPU, but you will also increase the speed of all the buses (including PCI and ISA). On the bad side, ditto. While your motherboard and chip may well be tolerant of increases in speed, your peripherals may not. Overclocking your PCI and ISA bus may well decrease the stability of your system.

Fortunately, many modern motherboards (such as my Abit AN7 Guru) allow you to fix the PCI and AGP frequencies while still allowing you to change the FSB frequency. Thus you can overclock your CPU without compromising your peripherals.

Also bear in mind that if your system has DDR RAM, there will be a limit to how far it can be overclocked. PC2100 RAM is only rated to run up to 266MHz, while PC2700 is rated at 333MHz. However, you can purchase so-called 'overclocking' PC2100 DDR-RAM which should be more tolerant of clock speed tweaks.

The table below shows the actual speed of my old Athlon 2000+ system with overclocking:

Device / Bus Clockspeed (MHz) Multiplier
'System clock'138.6x 1
Processor1733x 12.5
Memory bus (FSB)277.3x 2
Level 2 cache1733x 12.5
AGP Bus69.3x 1/2
PCI Bus34.7x 1/4
ISA Bus8.7PCI Bus / 4

Note that the AGP, PCI and ISA buses are running faster than specification. This speed is totally stable on my machine, and rates my CPU core speed at the same as an Athlon 2100+ (1733MHz). However, my system runs signicantly faster than an Athlon 2100+ because the buses are running quicker.

I hear many people saying, "So what? That's a lot of effort for such a small gain."

If you think about it, changing a setting in your motherboard is really no effort at all. Bear in mind that when I purchased my XP2000+ (mid 2002), it was near top-end. You should always consider the price-per-MHz. Going on from a 1900+ to a 2000+ was negligible in terms of cost, but going up to a 2100+ or faster would have required a much bigger wallet! So overclocking is sometimes a much more viable option, if you really want the speed. Of course, now the 2100+ is hardly expensive anymore, but you get the point.

Of course, in this case I've only overclocked my system by some 4%. That's nothing. Buy any computer magazine and check out a CPU or motherboard performance article, and you'll see that some people take overclocking to extremes. For example, I read an article a month or so back where they overclocked a Pentium 4 25% by increasing the FSB from 400 to 500MHz!

If you can get away with a 25% clockspeed improvement through overclocking, then think of how much money you could be saving.

Potential hazards and cooling

There are some important points to bear in mind. Firstly, overclocking means that your CPU will draw more power. If you plan to increase your clockspeed significantly, then you may also need to increase your CPU's voltage setting. Secondly, increasing clockspeed results in more heat. If your CPU overheats, your system will become unstable and, worse still, may not start at all. (In this case, you will need to let the system cool down and then reset the clockspeed to a more sensible figure in the BIOS. If you can't get into the BIOS, then there are failsafe methods to reset the BIOS defaults.) To complicate matters, increasing the chip voltage will also generate more heat, so overclockers must carefully balance these two competing factors. Whatever you do, make sure you get some beefy chip cooling hardware. This is the cooler I was using on my old Athlon 2000. It was cool, but it's no where near the top of the cooling league.

My Athlon XP2000+ cooling fan My Athlon XP2000+ cooling fan

My funky CPU fan

My current system uses significantly more cooling...

Gigabyte Cooler Gigabyte Cooler

Of course, the best thing about these fans is that they glow. While I may think my fan is funky, real overclockers would look down their noses and say something like, "You call that a CPU cooler? Na, THIS is a CPU cooler!" (See image below.) Like I said, some people really take this stuff to extremes. Sure, you can cool a car engine with a water system, so why not a PC? Makes perfect sense to me. But if you really are considering using a water-cooled system, then make sure you keep an eye on the algae that could be growing in your tubes!

This is serious cooling

Ideally, you should make sure that your case is also well ventilated. My box has three chassis fans: front and back to create a stream of air through the case, and another mounted in the side to extract hot air from the vicinity of the CPU itself.

One last note: overclocking is more of a bonus than a given. If you can significantly overclock your system without stability problems, then consider yourself fortunate. Bear in mind that overclocking may not only destabilise your system, but could also harm components. Even if there are no apparent problems, overclocking will shorten the lifespan of your system. On the otherhand, you'll probably build a new one before it gives up the ghost anyway... For more tips on overclocking, visit

What's next

Now move on to look at the chipset.