2007 aluminum iMac ever be able to use more than 4GB RAM?

Discussion in 'Apple' started by Jamie Kahn Genet, Aug 15, 2008.

  1. I've just upgraded from 2GB to 4GB RAM (cost me USD$90 + $12
    international shipping from Transintl.com in the US to me in New
    Zealand) and the speed boost when launching and switching apps is great!
    I only wish I'd done it sooner! :)

    However I'd have loved to have gone beyond only 4GB. Is this a hardwired
    limit by Apple in the iMac range? Or just untested? Or unsupported in
    OSX for this model, but possible with another OS?

    Curiously yours,
    Jamie Kahn Genet
    Jamie Kahn Genet, Aug 15, 2008
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  2. Jamie Kahn Genet

    David Empson Guest

    4 GB memory modules do exist in the DDR2 SO-DIMM form factor, but so far
    I've only spotted PC2-5300, which wouldn't work in the current iMac (but
    might in the current MacBook Pro).

    The main question is what the memory controller supports, and Intel's
    detailed specification may clarify that. (I haven't gone hunting for

    Apple's official specification is that the memory controller in the
    latest iMac only supports 1 GB and 2 GB SO-DIMMs, so the maximum total
    RAM capacity is 4 GB.


    That specification should be taken with a grain of salt - Apple has been
    known to only admit support for configurations they have tested or which
    existed when a model was released.

    The operating system certainly doesn't have a 4 GB limit - the current
    Mac Pro supports up to 32 GB.

    The total address space of the computer is greater than 4 GB (RAM plus
    ROM plus I/O), which is only possible on a 64-bit processor such as the
    Core2 Duo, so the processor clearly isn't imposing a limit on the RAM
    size at this point.
    David Empson, Aug 15, 2008
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  3. Jamie Kahn Genet

    Dane Guest

    Dane, Aug 15, 2008
  4. Jamie Kahn Genet

    David Empson Guest

    That particular 8 GB memory module is fully buffered and ECC, and is a
    full size DIMM. None of those characteristics are compatible with an
    iMac, Mac Mini or any Mac laptop. They are PC2-5300, so they might work
    in a first generation Mac Pro or Intel Xserve, but not in the latest
    models, which require PC2-6400.

    According to Apple's specifications, the memory controller in the
    original Mac Pro doesn't support DIMMs larger tan 2 GB, and the current
    model supports up to 4 GB per DIMM. This gives maximum memory capacities
    (with eight slots) of 16 GB and 32 GB respectively. (Again, this may not
    be the whole truth, only what Apple has tested.)

    If those memory modules work in an original Mac Pro, eight of them would
    give a total RAM capacity of 64 GB. A steal at US$13504 (compared to
    Apple's prices).
    David Empson, Aug 16, 2008
  5. That I've encountered several times over the years with certain Macs
    such as the early CRT iMacs able to support more RAM than Apple
    offically stated.
    Of course, but - and perhaps I'm being paranoid - I wouldn't be
    surprised if Apple sets a software/firmware based limit for certain
    models. Certainly they hobbled many a 68k and PPC Mac in the past.

    Hmmm... after re-reading the above I'm forced to admit it does sound
    rather daft :-D NM...

    Jamie Kahn Genet
    Jamie Kahn Genet, Aug 16, 2008
  6. Jamie Kahn Genet

    Warren Oates Guest

    Apple advertised that our very ancient but still running Quadra 630
    would only take 36 megs (hah! remember those days?), but when 128 meg
    simms became available, I put one in and the machine saw all 132 megs
    (it had 4 megs soldered in (hah!)).
    Warren Oates, Aug 16, 2008
  7. Jamie Kahn Genet

    David Empson Guest

    I've now found the datasheet for what I think is the correct chipset
    series (Mobile Intel 965 Express) and it clearly states that the chipset
    supports a maximum of 4 GB of memory.


    None of Intel's chipsets in this product line support more than 4 GB
    RAM. This means that the current iMac, MacBook Pro, MacBook and Mac Mini
    cannot support more than 4 GB RAM in total.

    As for the Mac Pro:

    The current model uses the 5400 chipset, which has a limit of 128 GB
    according to Intel. This limit is for "Up to 8 rows and 4 DIMMs per
    memory channel". Judging from Apple's documentation, the Mac Pro has
    four memory channels, but it only has enough slots for two DIMMs per
    memory channel. In theory this means the current Mac Pro could support
    64 GB using eight 8 GB FB-DIMMs, which disagrees with Apple's stated
    limit of 32 GB.

    I haven't spotted which chipset is used by the original Mac Pro.
    David Empson, Aug 16, 2008
  8. Jamie Kahn Genet

    David Empson Guest

    I'd be extremely surprised if they were still doing that sort of trick.
    Consider that you can run Windows (and Linux) natively on the same
    computer. It would be a rather bad look if a competing operating system
    could address more memory than Apple's own operting system on the same
    David Empson, Aug 16, 2008
  9. Cheers for that info, David.
    Jamie Kahn Genet, Aug 17, 2008
  10. Well Apple hobbled lots of Macs with hardware that could have run much
    faster, e.g. the Mac IIvi

    Many of the 'road apples' were intentionally hobbled as well in order to
    make them fit into the product line
    Jamie Kahn Genet, Aug 17, 2008
  11. Jamie Kahn Genet

    David Empson Guest

    Now I need to correct myself. The 965 Express chipset only has an 800
    MHz frontside bus, so it can't be what they are using in the current
    iMac models, which have a 1066 MHz frontside bus.

    This probably means that Apple switched to using "desktop" chipsets for
    the iMac - they were known to be using "laptop" chipsets in earlier iMac

    Since the previous generation 2.8 GHz iMac model was described as having
    a "Core 2 Extreme" processor, this change might have occurred with the
    first aluminium iMac, possibly only with the 2.8 GHz model, but that
    model also had an 800 MHz frontside bus, which doesn't rule out the use
    of a "laptop" chipset.

    If they are using a "desktop" chipset now, then variants like the Intel
    975X Express chipset _do_ support 8 GB using two 4 GB DIMMs.

    It would be hard to confirm this without pulling apart an iMac and
    seeing which chipset it is actually using (or finding out through
    internal diagnostic mechanisms, if there are any). I couldn't see
    anything useful in the output of the 'ioreg' command line tool for
    identifying the chipset, and certainly nothing in System Profiler.

    The issue then for a current iMac is whether anyone makes 4 GB PC2-6400
    SODIMMs, and whether Apple has done anything to prevent them being used.

    With the mid 2007 iMac, the 2.8 GHz model is more likely to support 4 GB
    SODIMMs than the slower models, but my gut feeling is that Apple will
    have used the same chipset over the whole product line. I have seen 4 GB
    PC2-5300 SODIMMs (from Crucial), so this might be a worthy experiment if
    someone is willing to risk it.

    No change to my previous analysis for the laptops or the Mac Mini - they
    are probably using laptop chipsets at present, so the 4 GB limit still
    David Empson, Aug 17, 2008
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