PCI Speed and Overclocking: How We Tested

At stocks speeds and a x1 setting on the Geiger, the PCI bus is reported as 33.3MHz:

The shot above was actually taken on an Asus P4C800-E running at a 250 (1000FSB) setting. When PCI lock is working, the PCI bus stays around the default of 33.3. AGP speed is a 2X multiple of PCI in the most common setup.

When the PCI bus floats, or is unlocked, the PCI speed floats with FSB settings. You can see this here where the base FSB is set at 210:

The PCI is 1/6 the base setting of 210, or 35. This level of overclock is generally not a problem even when the PCI bus floats.

Raising the base FSB to 220, PCI increases to 36.6, or an unlocked AGP of 73.

This is often a problem for current AGP cards. Our ATI Radeon 9800 PRO tests cards generally fail in intensive benchmarking above an unlocked base setting of around 218-220.

Index PCI Speed and Overclocking: Test Configuration


View All Comments

  • slewis5150 - Sunday, February 29, 2004 - link

    How is this going to relate to pci eXpress? Reply
  • Pumpkinierre - Thursday, February 19, 2004 - link

    So, Wesley#24, does this mean that some of the low maximum overclocks AT has shown for the A64 might have been as a result of PCI bus failure. Wouldnt it be possible to have a go at 234MHz for the Aopen and some of these other chipsets which supposedly had a PCI lock (nf3,SIS) but may be, like VIA, a dividor transition. Reply
  • Wesley Fink - Wednesday, February 18, 2004 - link

    #23 - As we were testing in the article, none of the current Athlon64 chipsets have working PCI locks - and that includes nVidia and SiS (at least on the ECS). Some of the board makers do have a feature where a 1/7 multiplier kicks in at 233 and the PCI bus drops back to spec. We found that to be the case on the AOpen AK86-L which uses the VIA chipset. Reply
  • Jeff7181 - Wednesday, February 18, 2004 - link

    Ummmm... I was pretty sure Sis and nVidia both had PCI/AGP locks and Via is the only one who can't get it right. Is this untrue? Reply
  • Xentropy - Tuesday, February 17, 2004 - link

    Is this article going to be updated with the new information about 1/7 dividers and which boards have them? Not everyone reads the comments and would see the details. Reply
  • bigtoe33 - Tuesday, February 17, 2004 - link

    Guys I bought the PCI geiger for Wesley from www.scan.co.uk They still show stock but I have just been informed its a discontinued product. If you want one I would grab one now. Reply
  • soki - Tuesday, February 17, 2004 - link

    Good Job Wesley! Reply
  • Pumpkinierre - Monday, February 16, 2004 - link

    Thank you Wesley! Reply
  • Wesley Fink - Monday, February 16, 2004 - link

    As many have suggested as a possibility, it does appear different multipliers kick-in at 234 on the AOpen AK86-L. At a 233 setting PC Geiger shows PCI speed as 38.8, while at 234 the PCI readout drops to 33.2.

  • TrogdorJW - Monday, February 16, 2004 - link

    Is there any chance that a BIOS hack or something could adjust the PCI clock later in the boot process? You stated that you didn't load the OS and only booted into the BIOS. This seems a bit of a quick-and-dirty test, especially with the A-Open having acheived much higher results than other boards.

    My suggestion, in addition to including higher bus speeds, is to also get to the point where the OS is actually being loaded. You don't need to let it boot completely, but at least let it start. Get well beyond the BIOS POST test before assuming that the PCI speed being reported is "final".

    On the other hand, the people questioning whether or not ALC655 and various other devices would run at 43 MHz... that's actually quite possible. Back in the day, I had a Pentium MMX 200 running at 250 on an A-Bit IT5H motherboard, which ended up being an 83 MHz bus and AGP and a 41.5 MHz PCI. (Ahhh... the good old Intel 430 HX chipset. Those were the days! /nostalgia.)

    Later, I had Celeron 300A through 366 all running on 83.3 MHz bus, giving the same overclocked PCI and AGP. Finally, I am STILL running an A-bit BE6-II Rev. 2.0 motherboard on a 133 MHz bus. That motherboard has a 1/4 PCI divider, so PCI is in spec, but the AGP is running 2/3 and is at 88.9 MHz! I have a GeForce 4 MX 440 in it and it serves as my movie/music entertainment center.

    Having been an overclocker since the days of the Pentium 166, I feel quite safe in saying that the "overclockability" of devices varies greatly. Some ATA/IDE setups can't handle more than 5 MHz out of spec (38 MHz or so), while others can clearly handle AGP and/or PCI speeds up to 33% out of spec. Only thorough testing will say for sure what any device can handle, and that's way beyond the ability for any one company/website to test.

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