Will dual channel memory work with 2 almost identical pieces of ram?

Discussion in 'Asus' started by DC, Jan 28, 2005.

  1. DC

    DC Guest

    I have a p4p800e dlx md. I have one piece cosair xms 512 pc3200 and
    one piece cosair xms 512 pc3000. Both pieces have the same timing and
    are running at 333 mhz. Can I use dual channel with these?
    DC, Jan 28, 2005
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  2. DC

    lakesnow Guest

    I guess yes. I had two sticks of RAM from different no-brand makers. They
    were bought
    in different time, yet they work in dual channel as well.
    lakesnow, Jan 29, 2005
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  3. DC

    Paul Guest

    Well, they don't have exactly the same timing, in the sense that one
    is rated at PC3000 and the other at PC3200. But the distinction in
    this case is probably nothing to worry about. For simple timing
    differences, the slowest stick sets the timing.

    Matching is a function of the memory controller in question.

    An Nforce2 dual channel motherboard, needs virtually no matching
    at all. Each slot in the motherboard functions independently,
    and in true dual channel fashion, the two channels interleave
    (take turns providing 64 bits of data), to increase the
    data rate.

    The next level is the Intel chipset method. It is called
    "Uber Dimm", and the memory controller sends the exact same
    control signals to two sticks positioned for dual channel
    operation. This means the row/columns/banks of the two sticks
    have to match, as does their total memory size. If the
    rows/columns/banks match, but there are trivial differences
    in clock rate (PCxxxx), Tcas, Trcd etc., the BIOS can select
    the lowest common denominator setting that both DIMMs can
    accept, and the DIMMs will be run with those settings. (And
    in your case, as long as your PC3000 and PC3200 stick have the
    same rows/columns/banks, they should be workable in dual
    channel mode, barring any BIOS bugs.)

    The third level, is the AMD Athlon64 method. It is like
    the Intel method, only as far as I can tell, virtual single
    channel is missing. That means, if you have widely disparate
    sticks of RAM, then you run them in single channel mode, and
    that means using the sockets of only one of the two channels.
    In other words, on a four slot Athlon64 motherboard, you
    cannot place a 128, a 256, a 512, and a 1024MB stick, and have
    a single channel 1920MB configuration (like you could on an
    Intel 875/865 board). The best you could do is a 1536MB
    configuration, placing the two largest of those DIMMs in the
    two DIMM slots identified in the manual, as the ones to use
    in single channel mode.

    I think the Athlon64 method would likely prevent a matched
    pair of DIMMs, placed in a pair of slots that arranges them
    as a 128 bit wide "Uber DIMM", from working with a third
    stick placed in the remaining single channel slot. Once you
    decide to run dual channel, it means you either use two matching
    sticks, or use two pairs of sticks matched with one another.
    A matched pair of 512MB and a matched pair of 1024MB would give
    a 3GB configuration on the Athlon64 dual channel. (And interleave
    might be disabled with that 3GB configuration.)

    For both the Intel and the Athlon64 methods, there are
    opportunities to interleave between sets of DIMMs, to take
    advantage of the ability to keep memory pages open. If you
    use four 512MB double sided DIMMS, where rows/columns/banks
    match on all four sticks, then some lesser significant address
    bits can be used to control chip select on the DIMMs, and the
    resulting interleave pattern maximizes the number of pages
    being kept open at any one time. A page that is kept open,
    does not need to be reopened, so the overhead on next
    access is reduced.

    Some refererences:

    For Intel dual channel:

    865P memory guide

    875P memory guide

    For Athlon64:

    BIOS writer's guide:

    Socket 754 datasheet (single channel):

    Socket 939 datasheet (dual channel unbuffered):

    Socket 940 datasheet (dual channel registered):

    For Nforce2, there is no datasheet available for download, and
    user experience is the only guide as to what works. No idea
    how dual channel on the KT880 works.

    Paul, Jan 29, 2005
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