PCI Speed and Overclocking: Test Results

 PCI Bus Overclocking Tests
 Motherboard
CPU
 Chipset  PCI Speed at 220 Base FSB Setting  Functional PCI Lock
Asus P4C800-E
Pentium 4 3.2C
Intel 875p 33.3 YES
Asus P4S800D-E
Pentium 4 3.2C
SiS 655TX 33.3 YES
Soltek SL-PT880PRO
Pentium 4 3.2C
VIA PT880 33.3 YES
AOpen AK86-L
Athlon 64 3200+
VIA K8T800 36.6 NO
ECS 755A2
Athlon 64 3200+
SiS 755 36.6 NO
Soyo CK8 Dragon Plus
Athlon 64 3200+
nForce3-150 36.6 NO

As you will see in results with the new VIA PT880 board for Intel, early testing raised questions about whether the PCI/AGP lock was working. However, tests with PC Geiger showed the new VIA PT880, as well as the Intel 875P and SiS 655TX, all have a working PCI lock. We have seen outstanding overclocking results with the Intel chipsets and SiS 655TX chipsets; the working PCI lock certainly contributes to that outstanding overclocking performance.

The big surprise here is that none of the current chipsets for Athlon 64 have a working PCI lock, whether they claim one or not. This was a complete surprise and our first inclination was to throw out the results, since speeds on the Athlon 64 are derived in a very different way with Hypertransport. However, oscilloscope results and tests elsewhere pretty much support our findings. There does not appear to be a working PCI lock on any current chipsets for Athlon 64. Certainly, this contributes to the poorer overclocking that we have seen on A64 inasmuch as on-board component frequencies and AGP bus are related to PCI bus.

Please keep in mind that we have only measured PCI bus speed with PC Geiger. While we expect on-board component speeds and AGP to be affected also, we cannot say that with certainty on Athlon 64 systems. Some reports on the web claim that AGP frequency is fixed on both VIA and nVidia chipsets on the Athlon 64. We can neither confirm nor deny these claims with the simple tests that we have performed here. Also keep in mind that results are for 6 individual motherboards. We can generally assume these results apply to the chipset used, but in some cases, this may be a stretch. The ECS 755A2 is designed to be a value board, for example, and 'PCI lock' may simply not have been implemented on this board. On VIA and nVidia chipsets for Athlon 64, our results do mirror those measured objectively at other sites using other VIA and nVidia A64 boards and we are more confident applying these results to chipset assumptions.

We will continue to look at PCI speeds in future tests of new boards and chipsets, particularly those designed for Athlon 64 processors. For now, it certainly appears that a floating PCI bus is one of the factors holding back overclocking on all current Chipsets for the Athlon 64.

PCI Speed and Overclocking: Test Configuration
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  • gordon151 - Monday, February 16, 2004 - link

    All the chipsets for the A64 don't have PCI locks, why are you singling out VIA Icewind??? Reply
  • Wonga - Monday, February 16, 2004 - link

    I think it is a bit pathetic how none of the chipsets for the Athlon 64 can manage to get a working PCI lock in them.

    Come on SIS/VIA/nVidia, you can do better than that. How they can do it for the Pentium 4 but screw up on the Athlon 64 is pathetic.
    Reply
  • Icewind - Monday, February 16, 2004 - link

    The first SATA did rely on the PCI bridge but the newer ones have their own dedicated bus to the southbridge so it is not affected by OC. ATA is still attached to the PCI bus if I remember correctly. Reply
  • lebe0024 - Monday, February 16, 2004 - link

    What about harddrive controlers? Does SATA depend on the PCI bus? how about PATA? Reply
  • Icewind - Monday, February 16, 2004 - link

    Cool, no wonder my P4C800-E Deluxe is such a kick ass overclocker. Another reason why not to buy VIA chipset mobo's. Lets hope Nvidia can make a good 939 pin mobo later this year, VIA still leaves a bad taste in my mouth Reply
  • PrinceGaz - Monday, February 16, 2004 - link

    Would it have been possible to raise the FSB to 233 or 234 on those A64 chipsets to see if a 1/7 divider kicks in? I believe the A64 is partially multiplier-unlocked and can have it lowered because of the Cool n Quiet power-saving technology so the CPU itself shouldn't have been a problem.

    If current 8x AGP cards are so sensitive to AGP speed, why not use a slower AGP mode or use an older card which can tolerate a far wider range for some of the tests? I've got an old 2MX somewhere that was quite happy with a 75MHz AGP speed (150FSB on a KT266A chipset, equivalent to 225FSB on current boards) and may well have gone somewhat higher but my PC2100 memory couldn't take any more.

    Interesting article though, surprising about the nForce3 board.
    Reply

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