How it's possible

With the Celeron 300A, we had a CPU that ran at the 66MHz FSB by default and was clock locked at a low enough clock multiplier so that forcing the 100MHz FSB operation of the CPU resulted in an attainable overclock. Therefore, a chip that was designed to run at 66MHz x 4.5 ended up working very nicely at 100MHz x 4.5, or 450MHz, a 50% increase in operating frequency. The on-die presence of the Celeron's L2 cache also helped remove the limitation the Pentium II/III's off-chip L2 cache placed on overclocking successes.

The same situation is present with the FC-PGA Pentium III 500E and 550E. Both chips feature low enough clock multipliers that forcing the 133MHz FSB operation on either of those chips would result in a healthy overclock without stretching the limits of the CPU. Keep in mind that the FC-PGA Pentium III is nothing more than the Slot-1 Pentium III, which is already available in clock speeds in excess of 733MHz and thus overclocking either of these two to around the 733MHz mark should not be difficult to attain.

Unlike their Slot-1 counterparts, the two currently available FC-PGA parts operate at a 1.60v core voltage (versus 1.65 for the Slot-1 versions) primarily because, in comparison, they run at fairly low clock speeds. As we just mentioned, the core itself is identical to those found on all of the other Pentium III CPUs, and thus paves the way for the exploitation of the overclocking capabilities of the two CPUs.

When dealing with officially supported FSB settings alone (like those are any fun), the target to hit with the 500E would be 667MHz, or 133MHz x 5.0 and, with the 550E, that target would be 733MHz, or 133MHz x 5.5. Note that, percentage wise, we are still not achieving the same 50% increase in clock speed that the old Celeron 266 and 300A CPUs virtually guaranteed us. Instead, the 500E/550E chips offer a 33% potential clock speed increase when using officially supported FSB settings to overclock the CPUs.

The options

As we are already aware of, there are a limited number of chipset solutions that offer official 133MHz FSB support: the i810E, i820, i840, VIA Apollo Pro 133 and 133A and thus only a limited number of options to overclock the CPUs without pushing the limits of the rest of your system. Why must you have a chipset with official 133MHz FSB support?

The main reason behind this need is that chipsets without official 133MHz FSB support do not allow for the ½ AGP clock divider that would allow the AGP bus to operate at ½ the FSB frequency; in the case of a 133MHz FSB, that would be 66MHz. All chipsets without official 133MHz FSB support only allow for 1/1 and 2/3 AGP clock to FSB frequency ratios, which means that running at the 133MHz FSB on a motherboard based on one of these chipsets would place the AGP bus clock at no less than 89MHz full 34% above the AGP operating frequency specification. While some AGP cards have no problem working at this frequency, for the most part, it is an unrealistic setting and isn't a viable option if you plan on using your AGP video card for anything more than simple 2D output (i.e. 3D games).

On the other hand, all chipsets that officially support the 133MHz FSB naturally support the ½ AGP clock divider to allow for the AGP bus frequency to stay as close to the 66MHz specification as possible.

The big question is, what options are there for 133MHz FSB boards? The i820 is always an option but with the current cost of RDRAM so incredibly high and the performance hit you take with SDRAM and a Memory Translator Hub that enables SDRAM support it is not worth the added cost.

At the same time, quite a few users still happen to be using BX motherboards without support for the 133MHz FSB. In fact, the 100MHz FSB Pentium IIIs make for the perfect upgrade path to BX board owners, so what options do we have if the 133MHz FSB isn't a reality?

And we can't forget the fact that the FC-PGA Pentium III only works in boards that support the updated VRM8.4 specification which, as of now, is limited to the newer i810E boards and currently unreleased motherboards.

Index The solutions

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