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 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.
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?