PC Architecture.Chapter 18. Overclocking. A book by Michael B. Karbo

Copyright Michael Karbo and ELI Aps., Denmark, Europe.

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    Chapter 18. Overclocking

    The Pentium II was subjected to a lot of overclocking. It was found that many of Intelís CPUís 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 didnít work, but at factor of 4.5 it functioned superbly.


    System bus


    Internal clock


    66 MHz


    100 MHz

    Pentium MMX

    66 MHz


    166 MHz

    Pentium II

    66 MHz


    300 MHz

    Pentium II

    100 MHz


    600 MHz


    100 MHz


    800 MHz

    Pentium III

    133 MHz


    1200 MHz


    133 MHz x 2


    1733 MHz


    166 MHz x 2


    2167 MHz

    Pentium 4

    100 MHz x 4


    2200 MHz

    Pentium 4

    133 MHz x 4


    3066 MHz

    Pentium 4

    200 MHz x 4


    3600 MHz

    Figur 122. The CPUís 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:

    Figur 123. Setting the CPU voltage using the motherboardís Setup program.

    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.

    Figur 124. A gigantic cooler with two fans and pure silver contact surfaces. Silverado, a German product which is used for overclocking CPUís.

    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, donít like people overclocking their CPUís. 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 CPUís can be overclocked. In any case, you shouldnít expect your warranty to apply if you play around with overclocking.

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