Overclocking socket 775 Pentium 4 processors has some particularities, comparing to overclocking done in other platforms.

The first difficulty is that since the first Pentium 4 the clock multiplier is locked. Only engineering samples (that are made for testing) and the Extreme Edition models have the unlocked clock multiplier. Engineering samples are identified with the letters ”ES“ and are not available on the market. Some manufacturers, such as ABIT, have created motherboards that can bypass that protection against overclocking.

Thus, as you can see, unless you have an engineering sample, a Pentium 4 Extreme Edition or an ABIT motherboard, the only way to overclock a Pentium 4 processor is by increasing its external clock (front side bus - FSB).

On Intel i865 and i875P chipsets and chipsets from other manufacturers (NVIDIA, SiS, VIA, ULi, etc) you just have to keep on increasing the front side bus until you find the limit the processor tolerates. On Intel i915P and i925X chipsets, Intel created a protection against overclocking, making socket 775 Pentium 4 processors overclocking extremely difficult.

That protection limits the overclocking to up to 10% of the standard front side bus clock. This happens because the buses (PCI Express, SATA and the link between the chipset’s north and south bridges) use the same clock generator and, hence, their frequencies vary according to the clock used. It causes problems to SATA and PCI Express devices, since there is no way of locking their frequencies. SATA devices, for example, have problems in working at frequencies higher than 110 MHz, and NVIDIA video cards tolerate the maximum of 120 MHz on PCI Express bus.

The protection against overclocking monitors the link between the chipset’s north and south bridges. Every time that link’s frequency goes beyond 10% of the standard clock, the board turns itself off or simply doesn’t turn on. A way of overcoming this blockade is by increasing the chipset voltage, which allows gains of 10 to 15 MHz.

Some manufactures, such as ASUS, ABIT and DFI, have created mechanisms in their motherboards that attempt to correct those problems. In practice, those mechanisms try to keep the PCI Express bus operating clock below 120 MHz by altering the PCI Express and SATA bus multipliers during initialization, which allows the use of PCI Express video cards.

In Figure 1 you see the case of the DFI LanParty 925X-T2 motherboard, where the PCI Express bus clock can be configured in a fixed value. If you are overclocking the socket 775 platform, the ideal thing to do is to change the PCI Express bus clock from ”Auto“ to a fixed value, as 100 MHz, which for sure won’t cause problems with PCI Express devices installed to the board. This DFI board allows you to configure the PCI Express clock from 100 MHz to 140 MHz in increments of 1 MHz.

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Figure 1: Options to lock the PCI Express bus clock.

Yet we can only achieve more aggressive overclocks when we use boards with i875 and i865 chipsets (or chipsets from other manufacturers) or systems using PCI Express video cards from ATI, that tolerate high frequencies better or even PCI video cards. As the SATA bus fails in high frequencies, hardcore overclockers only use regular IDE hard disks (parallel).