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xRichard Amies

Overclocking your PC

An Introduction

July 2000

Page One | Page Two | Page Three | Page Four

Section 2: How to Overclock my machine (Part 2)

To start investigating overclocking your system, you will need your motherboard/mainboard manual, as there are literally thousands of different jumper settings and each board is different. We cannot give a complete guide to every motherboard here!

Something to always bear in mind is that there is no set guide as to how much voltage to use. My personal rule of thumb is 20% extra voltage maximum, and that is with extra cooling in place. 20% is a relatively safe upper limit. If you feel like risking your system then try more, but I'd be pretty happy to use 20% extra voltage on any chip I see here. Needless to say, before increasing the voltage your processor is using, you need to ensure it is properly cooled using a decent cooling system. Standard heatsink and fan combinations are generally pretty poor. Generally, if the heatsink is large (and I mean large!), and the fan is rated at above 4000rpm, it will be okay.

1. Introduction - What is Overclocking?

2. How to Overclock my machine (Part 1)

How to Overclock my machine (Part 2)

3. How to maintain stability and fix problems

4. Conclusion

The original Socket 7 Pentiums (P75 - P200, P166MMX - P233MMX) are probably the easiest for first time clockers to overclock. With a few exceptions, namely most P133 chips and some of the P200MMX chips, can be overclocked by just increasing the clock multiplier. The speed of the processor is based on the following equation:

CPU Speed = Bus Speed (System Speed) * Clock Multiplier

All Intel Socket 7 processors used the 66.666MHz bus speed, with the exception of the P90, P120 and P150 processors. If you have, lets say, a P166 processor, by using the above equation we can find out what it is set to :

166MHz / 66.666 = 2.5

To overclock this system, all we need do is adjust the multiplier to 3, and we have 200MHz. If the
system fails at this speed, then we either try giving the system more voltage gently, not just jumping up to a sky-high voltage. It may need only 0.1v extra to run stable. If that doesn't work, then we can try dropping the bus speed to 60MHz, and keeping the multiplier at 3, which will give us 180MHz. By doing the overclock this way, we probably won't need any extra voltage/cooling, but we lose overall system performance.

Generally speaking, if the only way to overclock your system is to drop the bus speed down then it isn't worth it. The bus speed is how fast the system talks to itself; the hard drives, the memory, your graphics card and sound card speed are directly related to it. Try and keep it to 66.666MHz.

On some later motherboards, there are options of 75MHz/83.333MHz that can be investigated once you feel comfortable with the whole overclocking idea, but using these speeds also overclocks your memory, hard drives, video card, sound card, etc, and are best left alone until you do feel truly comfortable with what you are doing.

Going back to the P133 and the P200MMX chips, which I said might be difficult, Intel is to blame on this I'm afraid. They started doing something called 'Multiplier Locking', which means the processor is limited to the clock speed that Intel sets at the manufacturing lab. The only way to overclock these chips is to increase the bus speed, which then in turn increases the entire system speed and can make troubleshooting very difficult! Best left alone for the novice.

The subject of 'Multiplier Locking' takes us on to the Pentium 2 and Celeron processors. All these
processors were locked, and the only way to overclock them is to increase the bus speed (Front Side Bus or FSB) and therefore, the whole system. With later boards this isn't such a problem as they have what is known as AGP/PCI dividers, which can slow down the system speed from half the speed of the bus to either a third or a quarter, depending on the speed you are using.

The AGP speed (for your graphics card) is designed to run at 66.666MHz, the PCI speed (sound cards, modems, etc) is designed to run at 33.333MHz. By looking at the table below we can see how increasing the bus speed to the processor affects the speed for the rest of the system.

FSB Speed PCI Speed AGP Speed

66.666MHz 33.333MHz 66.666MHz
75MHz 37.5MHz 75MHz
83MHz 41.5MHz 83MHz
100MHz 33.333MHz 66.666MHz
112MHz 37.333MHz 74.666MHz
124MHz 41.333MHz 82.666MHz

Up to 100MHz FSB speed, the AGP runs at full bus speed and the PCI runs at half the bus speed. At speeds of 100MHz and over (in general, but not in all cases), the PCI speed drops to a third of the FSB speed, and the AGP speed drops to two-thirds of FSB speed. Therefore, the easiest jump for your system is from 66.666MHz to 100MHz. But, unfortunately, that is one of the hardest jumps for your processor! For example, a PII 233 uses a multiplier of 3.5 (233 / 66.666 = 3.5) so to jump up to the 100MHz FSB speed would be asking the processor to do 350MHz! But since most of the 233 processors were originally intended to be 333MHz processors anyway, this isn't such a huge overclock is it?

Continued . . . (Section 3)

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