Chapter 18. Overclocking

The Pentium II was subjected to a lot of overclocking. It was found that many of Intel’s CPU’s could be clocked at a higher factor than they were designed for.

If you had a 233 MHz Pentium II, you could set up the motherboard to, for example, run at 4.5 x 66 MHz, so that the processor ran at 300 MHz. I tried it myself for a while, it worked well. At a factor of 5 it didn’t work, but at factor of 4.5 it functioned superbly.

CPU

System bus	Clock factor	Internal clock frequency
Pentium	66 MHz	1.5	100 MHz
Pentium MMX	66 MHz	2.5	166 MHz
Pentium II	66 MHz	4.5	300 MHz
Pentium II	100 MHz	6	600 MHz
Celeron	100 MHz	8	800 MHz
Pentium III	133 MHz	9	1200 MHz
AthlonXP	133 MHz x 2	13	1733 MHz
AthlonXP+	166 MHz x 2	13	2167 MHz
Pentium 4	100 MHz x 4	22	2200 MHz
Pentium 4	133 MHz x 4	23	3066 MHz
Pentium 4	200 MHz x 4	18	3600 MHz

Figur 122. The CPU’s internal clock frequency is locked to the system bus frequency.

Overclocking the system bus

Another method of overclocking was to turn up the system bus clock frequency. In the early versions of the Pentium II, the system bus was at 66 MHz, which suited the type of RAM used at that time.

You could increase the bus speed, for example to 68 or 75 MHz, depending on how fast your RAM was. This type of tuning makes both the CPU and RAM faster, since it is the actual system clock speed which is increased.

The disadvantage is that the system clock in these motherboard architectures also controls the I/O bus, which runs synchronously with the system bus. PCI bus devices (which we will come to in a later chapter) cannot handle being overclocked very much; otherwise faults can occur, for example in reading from the hard disk.

Overclocking typically requires a higher voltage for the CPU, and most motherboards can be set up to supply this:

Many still use the same kind of overclocking on the Athlon XP and Pentium 4. The system clock has to be able to be adjusted in increments, which it can on many motherboards.

Example using the Pentium 4

A Pentium 4 processor is designed for a system clock of 200 MHz. If you can have a 3200 MHz model with a 200 MHz system bus, it can theoretically be clocked up to 4000 MHz by turning up the system clock. However, the processor needs a very powerful cooling system to operate at the increased frequencies:

System clock	CPU clock
200 MHz	3200 MHz
230 MHz	3700 MHz
250 MHz	4000 MHz

Fig. 125. Overclocking a Pentium 4 processor.

The manufacturers, Intel and AMD, don’t like people overclocking their CPU’s. They have sometimes attempted to prevent this by building a lock into the processors, so that the processor can only work at a specific clock frequency. In other cases the CPU’s can be overclocked. In any case, you shouldn’t expect your warranty to apply if you play around with overclocking.