Ride the Big Yellow Front Side Bus

Discussion in 'Abit' started by Derrick Regalia, Aug 28, 2003.

  1. Hey now that my goofy subject got your attention maybe you can help me. I
    have Abit NF7-S and XP2500+(Barton) cpu. I have been experimenting with
    overclocking the multiplier and the FSB.

    My question is: What is affected when I change FSB? I know obviously the
    CPU speed is affected (multiplier x fsb = mhz) and that the memory is
    affected (usually 2xfbs). Is anything else affected? (PCI, usb, etc.)

    Thanks, Derrick
    Derrick Regalia, Aug 28, 2003
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  2. Derrick Regalia

    Wes Newell Guest

    Your board should have locks for the PCI and AGP buses, so if you set
    them, they should remain constant. Other boards may differ dramtically. On
    some, it changes the PCI, AGP, and memory bus. So the components must be
    able to run at whatever the buses end up from changing the FSB. On my old
    KT7 board, it changes all of them. The pci bus is determined by FSB/3. The
    AGP bus is determined by PCIx2. And the memory bus is the same as the FSB
    setting or FSB+33. The PCI bus is supposed to be 33MHz and the AGP is
    supposed to 66MHz. Raising the FSB to 116 would put the pci bus at about
    38MHz and the AGP bus at 76MHz. That's the highest my components will
    support. The tops that I've heard fo is118Mhz for this board, but only
    from 1 person. Other chipsets may work in other ways. I'm told the
    SIS746FX chipset doesn't actually lock the PCI/AGP buses but always keeps
    them close to the nominal and never exceeding about 36Mhz on the PCI IIRC.
    And I've run it at about all FSB values from 70Mhz up to 185 MHz (higher
    is to much for my 13 multiplier chip) without a problem. Others have had
    it up to 220MHz without problems. So to get to the meat of the matter, it
    will depend on the MB and chipset. And before one starts changing the FSB,
    they need to be aware of what will happen. Otherwise, one may experience
    system crashes, burned up video cards, loss data on HD, etc. Things that
    aren't fun.:)
    Wes Newell, Aug 28, 2003
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  3. NF7 locks PCI bus speed at 33MHz thus any component that uses the PCI bus
    will not be overclocked. The AGP port can be overclocked separately in
    Softmenu and is not affected by FSB on the NF7.

    As you wrote, the FSB affects the CPU but using the multiplier & FSB
    together you can set the speed of the CPU to whatever it will do, within
    reason. Eg. 11x166, 10x183, 9x204, 8x229 are all pretty much spec' speed
    for a 2500+ (ie 1833MHz) With decent cooling a recent Barton 2500+ ought to
    hit 11x200 for 2.2GHz no problem (3200+ speed.) although you may well need
    to raise the Vcore a few notches for 100% stability.

    FSB does affect memory speed directly but, again, there is a
    multiplier/divider in Softmenu that you can use to alter the memory speed.
    Default is 'By SPD' & there are various multipliers and dividers available
    eg. 6/6, 6/5, 5/4, 4/5 Etc.

    The chipset is affected by almost any change you make in BIOS - for
    succesful overclocking you'll probably need to raise the chipset Voltage to
    1.7V and then the Southbridge gets very hot. I have found that this can
    lead to data corruption on the hard disks, even though the PCI bus ought to
    be at spec' 33MHz. Therefore I would recommend fitting a heatsink to the
    southbridge before upping the chipset Voltage.

    In general, the higher you can push the FSB the better overall system
    performance but it is more efficient with the memory running at the same
    speed. (ie. Synchronously.)

    My particular week 7 2003 Barton 2500+ doesn't like 200MHz FSB but it will
    do 12x192 for 2.3GHz 100% stable straight watercooled at 1.85Vcore...


    Nick M V Salmon, Aug 28, 2003
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