800 FSB CPU on 400 FSB motherboard?

Discussion in 'ECS' started by rjdriver, Nov 17, 2007.

  1. rjdriver

    rjdriver Guest

    I assume these are backwards compatible, but need to be sure. Will a
    Pentium 4 socket 478 CPU made for 800 front side bus work on a MB with a max
    FSB of 400/533?

    rjdriver, Nov 17, 2007
    1. Advertisements

  2. rjdriver

    Paul Guest

    Yes, with some caveats.

    Most retail processors have a locked multiplier. Say I have a 3GHz/FSB800
    processor. The input clock would be 200MHz (quad pumped to FSB800, as
    there are four data transfers per clock cycle). The multiplier is the
    ratio between the core clock and the input clock. 3000/200 = 15x.

    Now, plug the 15x processor, into an FSB400 motherboard. Input clock
    is 100MHz. 100 x 15 = 1.5GHz, or half the core speed!

    Using FSB533 speeds, the result is 133 x 15 = 2GHz.

    This makes retrofitting an FSB800 into an older, frequency limited
    motherboard, a relative waste of time. For example, my old S478
    board, with FSB400, is already running at 1.8GHz. If I put my FSB800
    processor in it, I'd actually be falling behind in core speed.

    So consider what is going to happen, when the input clock runs
    slower on the old motherboard, in your purchase decision.

    Also, check the BIOS on the FSB400/FSB533 motherboard, so you
    can set the input clock to as high a value as the board can handle.
    When the FSB800 processor is plugged in, it is hard for me
    to predict what the automatic setting will be. It might start
    working at FSB400, and you'd ideally want to be able to go into
    the BIOS and dial 133MHz, or even 150MHz. For some of those old
    boards, 160MHz is about as far as it can be pushed (see the bottom
    of this post, regarding warnings about this). There are
    some chipsets, that can be pushed all the way to 200MHz, which
    means your processor can run at full speed. But a manual
    clock setting capability in the BIOS, is an essential
    ingredient to finding out what is possible.

    (An Asus P4PE is an example of a board that just barely went
    from FSB533 as a normal clock, to FSB800. On that motherboard,
    you were restricted as to how many sticks of RAM that could be run
    while overclocking to that extent. One stick only.)


    The second issue, is the Prescott / Celeron D 90nm issue. The Prescott
    processor has a sense pin on the bottom of it. The purpose of the
    sense pin, is to detect old motherboards. If a Prescott 90nm
    processor is plugged into an older motherboard, the Prescott processor
    senses that the motherboard is the wrong type. It will refuse to start
    and the BIOS screen will remain black. No harm will come to the processor
    but it won't run.

    The processor type needed for this operation, is a Northwood 0.13u
    processor. They don't have a sense pin on the bottom, like the
    Prescott does. Look up the SLxxx or the order code on the processor,
    on processorfinder.intel.com , to find out what family your
    processor belongs to.

    Motherboards that are "Prescott ready", started with 865PE, 865G,
    875P, so that is the era where they started to appear. A board with
    an 845PE on it, is older, and will fail the sensor test if a Prescott
    is used.

    So, yes, it is possible, but there are limits to the performance
    level that can be achieved. If your FSB533/FSB400 board was a
    P4PE, you might make good use of your FSB800 processor. A lot
    of other motherboards, don't go all the way from FSB533 to FSB800
    under overclocking conditions, so your CPU core clock will not
    run at the full value.

    And as an overclocker, you have to know a bit about how the clock
    generator works. On 865 or higher motherboards, for example, the
    PCI/AGP clocks are locked to 33MHz and 66Mhz respectively. When
    the clock is locked (i.e. an independent source of clock signal
    for the PCI and the AGP bus), then you can dial the clock on the
    processor anywhere between 133 to 200MHz, without a care in the world.

    On older motherboards, the PCI clock is derived from the processor
    input clock. If the input clock is 133MHz, the PCI bus is 33Mhz.
    If the input clock is 150MHz, the PCI bus is 37.5Mhz. That is
    about as much overclocking as the PCI bus can safely take. If
    you set the input clock to 160MHz, the PCI bus becomes 40MHz,
    and your boot disk will be magically corrupted on the first
    boot (yippee).

    The ratio between input clock and PCI clock, changes with
    frequency range. But there are still "disallowed zones" of
    frequency, that an overclocker will not use. Safe values
    are 100Mhz, 133Mhz, probably 166Mhz, and 200Mhz (multiples of
    33MHz will most likely have a correct divider). The divider
    switchover point is a function of the clock generator chip.

    I cannot find a good example quickly, so I'll give you this one
    purely to illustrate what to look for.


    You can find more ICS clock generator parts listed here:


    In the table on page 1 of the ics9250 document, the first entry
    is 133MHz CPU clock and 33MHz PCI. In brackets, it says "CPU / 4",
    meaning the clockgen is using a 4 divider. But in the very same table,
    another selection is 133MHz CPU clock, and 44.33 PCI. That
    will "crash and burn", so to speak. 120 / 40 would also be
    a bad value. An example of a good choice, is 112 / 37.33,
    because the PCI is just up to its safe limit. So some of
    the choices are disallowed.

    As an overclocker, you have a couple choices for test conditions.
    Install your processor, and play with the BIOS. Use anything
    other than your Windows hard drive for testing! You can use
    a memtest86+ boot floppy, or a Linux LiveCD like Knoppix or
    Ubuntu. Leave the hard drive disconnected if you want.
    Or, clone a copy of the hard drive (make a backup copy),
    and then experiment with a hard drive you know is "safe"
    to corrupt. You can then copy the clone back, if there
    is damage. In any case, don't try any "stupid" values of
    input clock to your CPU, using your Windows hard drive
    as the "test case". And yes, it can be difficult to get
    info on every clock generator chip in existence. I hope the
    above archived link, will give you access to a good chunk
    of them. There are other brands besides ICS.

    Paul, Nov 17, 2007
    1. Advertisements

  3. rjdriver

    rjdriver Guest


    Paul, thanks for the wealth of information. It's appreciated.

    rjdriver, Nov 19, 2007
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.