G5 memory: DualChannel or not

Discussion in 'Macintosh Computers' started by manitoubalck, Jul 21, 2003.

  1. manitoubalck macrumors 6502a


    Jul 17, 2003
    Adelaide, Australia
    Apple claim and I don't doubt that the new G5 supports up to 8GB of DDR 400. My misunderstanding however is that Apple claim that G5 has a memory bandwidth of 6.4GB/s which I don’t doubt, but is however twice what DDR 400 can offer, being 3.2 GB/s hence its other name DDR 3200.

    This leads me to the assumption that the new G5 runs Dual Channel DDR (developed by Nvidia.) Hence 4GB per channel if the memory is shared between both the processors, or 2GB per channel per processor to make up the 6.4GB/s bandwidth.

    These configurations are the only way without using the now defunct RD RAM (which had a frequencies of up to 1066MHz and a bandwidth of 8GB/s) to come even close to making use of the amazingly fast FSB of 1000MHz.

    If the second assumption is true about each processor accessing its own bank of ram Apple hasn't broken the 4GB barrier per processor.

    Either way these insights show that the 8GB of DDR 400 may not be all it’s cracked up to be.
  2. blogo macrumors 6502

    Apr 1, 2002
    Looks like they run dual channel
    Because even the low-end config on the apple store comes with 2x128mb ram modules.
  3. britboy macrumors 68030


    Nov 4, 2001
    Kent, UK
    It's definitely dual-channel DDR. The RAM slots on the mobo are split into two sections, with 8 slots in total (4 on each side). The only way to upgrade the RAM is by adding it in pairs. For the proof, just try going to the apple store and pricing up a G5. When you try and upgrade the RAM, you'll notice that it's all multiples of 2.
  4. AngryAngel macrumors regular

    Jul 12, 2003
    ...and the 8GB limit is only because 2GB DIMMs aren't made in volume yet.
  5. cc bcc macrumors 6502

    Jul 3, 2001
    It is dual channel, Apple just calls it 128 bit wide.

    It would make no sense at all for each cpu to have it's own ram.
    The OS distributes the threads to the cpu's, so if one cpu needs to access some information that happens to be stored on the other cpu's ram, it would first have to be copied. This happen countless times per second, inefficiency to the max. A chipset designer would be instantly fired for proposing a memory setup like this.
    A solution would be to keep double copies of all the data..

    Anyway, it is true dual channel ram. That means we went from effectively 167 MHz RAM to 800 MHz!

    (Man I'm so happy, today I ordered a 1.8 GHz G5 with Ati 9800 pro! Yeehaw!)
  6. Catfish_Man macrumors 68030


    Sep 13, 2001
    Portland, OR
    Re: G5 memory: DualChannel or not

    It's full 36 bit addressing (42 bit from the processor), so it can address up to 16GB of ram. They do use interleaved memory (and have been using it off and on since the early 90s). New Intel chipsets and nVidia nForce do the same thing.

Share This Page