X-bit labs

Newbie Overclocking Guide (page 7)

Category: CPU

by Doors4ever

[ 08/01/2007 | 06:47 PM ]

Overclocking Specifics for Intel Core CPUs

CPUs with the Core micro-architecture are the most appealing nowadays. They overclock well, so I’ll pay them most of my attention here.

An annoying thing you have to keep in your mind as you are overclocking a Core processor is the so-called FSB Wall. This term refers to the maximum bus frequency this sample of the CPU can work at. Thus, it is convenient to begin to overclock a Core processor with finding its FSB wall. To do this, reduce the multiplier to the minimum of x6 and see what FSB frequency your CPU can be overclocked to. Your CPU may be not stable at this frequency with the default multiplier, but at least you get a notion of what your CPU can potentially do.

For example, CPUs with a default bus frequency of 200MHz can be but seldom overclocked to over a 400MHz FSB. You should take this fact into account when you are choosing your CPU. There is no sense in overpaying for a senior and more expensive model if you can take a junior one and overclock it but note that the top frequency of a junior CPU with a default multiplier of x8 is likely to be limited by the FSB wall at 3.2GHz or something. Most likely, you will stop at 3.0-3.1GHz, which is low. You don’t want such limitations, do you? So, you may want to consider a CPU with an x9 multiplier instead.

As for CPUs with a default FSB frequency of 266 or 333MHz, people often buy a junior model with a multiplier of x7, but such CPUs may be limited not only by the FSB wall but also by the capabilities of your mainboard and memory. It is better to use CPUs with an x8 or higher multiplier, yet you have to face a new problem with them, the FSB Strap.

The FSB Strap is not a feature of the CPU, but of the mainboard and chipset. It is the frequency the chipset switches into another operation mode at, increasing the latencies and bringing about a performance hit. Gigabyte mainboards on the Intel P965 Express chipset slow down as soon as you try to overclock the CPU. ASUS mainboards on the same chipset deliver excellent performance up to a 400MHz FSB and then use the FSB strap. When testing an ASUS Striker Extreme mainboard on an Nvidia nForce 680i SLI chipset we found a performance hit on transitioning from a FSB frequency of 420MHz to 425MHz (for details see our article called ASUS Striker Extreme Mainboard Review). Mainboards on the Intel P35 Express chipset seem to be free from this drawback, judging by first tests (for details see our article called Asus P5K Deluxe Mainboard: Second Encounter).

Some non-overclocker-friendly mainboards on Intel’s 945 and 965 series chipsets do not support the FSB strap at all (se our Biostar TForce P965 Mainboard Review) and CPUs with a default 200MHz FSB can usually be overclocked on them to a 300MHz FSB or something. This can be corrected with a CPU modification called BSEL mod. By isolating and connecting contact pads on the CPU’s “belly”, the mainboard is made to think that the default FSB frequency of the CPU is 266MHz rather than 200MHz and behave appropriately at overclocking.

So, you should be aware of the FSB strap thing beforehand and try to avoid non-overclocker mainboards. You should pick up your mainboard considering the default multiplier of your CPU so that you didn’t hit the frequency range with reduced performance. Perhaps you will even have to lower the maximum FSB frequency a little to avoid this range. On the other hand, you shouldn’t have much fear of this thing. If your CPU can overclock far beyond 500MHz FSB, you shouldn’t care about the frequency of the FSP strap because the high frequency of the CPU will make up for any performance hits.

Overclocking Specifics of AMD CPUs

AMD processors can be overclocked like any other CPUs but you should reduce the frequency of the HyperTransport bus that connects the CPU with the chipset. It’s usually enough to set an x3 multiplier or a frequency of 600MHz, which is in fact the same thing.

Besides, with AMD processors the memory controller is integrated into the CPU. It means the resulting speed of the system doesn’t depend much on the employed chipset and will generally be the same in most cases. It means you can take almost any mainboard, except for those models that suit poorly for overclocking due to their limited BIOS options, sloppy PCB design, or other such reasons. Read a review of your mainboard to learn if the mainboard you’ve chosen is good for overclocking.

There is another difference from Intel’s CPUs and it is due to the integrated memory controller, too. The memory timings play a more important role for AMD processors, especially with DDR SDRAM. You should test your system in different modes to see if it’s better to reduce the timings rather than to increase the memory frequency.

Note also that AMD Athlon 64 X2 processors on the 65nm Brisbane core are slower than their 90nm Windsor precursors due to slower cache memory and fractional multipliers (see our article called AMD Athlon 64 X2 4800+ Anew: AMD Masters 65nm Technology). The memory frequency on the AMD platform is based on the CPU frequency and integer divisors and the real memory frequency may sometimes be much lower than set in the BIOS, leading to a performance hit. That’s why dual-core processors on the Windsor core are preferable for overclocking. Their overclockability isn’t inferior to that of their technologically advanced but slower mates.