SillyDog701

[Antony]

A friend pointed this out to me.

The Intel-based iMac, first released in early 2006 (17-inch 1.83GHz Intel Core Duo and 20-inch 2.0GHz Intel Core Duo) came with
512MB (single SODIMM) 667MHz DDR2 SDRAM (PC2-5300), supports up to 2GB.

Then in mid 2006, the same models (17-inch 1.83GHz Intel Core Duo and 20-inch 2.0GHz Intel Core Duo) shipped with
512MB (in two SODIMMs) 667MHz DDR2 SDRAM (PC2-5300), supports up to 2GB.

Then, in late 2006 (released in August), the Intel-based iMacs come with Intel Core 2 Duo, all come with more memory except the entry level iMac (17-inch 1.83GHz Intel Core 2 Duo) which has 512MB (2x256MB) 667MHz DDR2 SDRAM (PC2-5300), supports up to 2GB.
The 17-inch, 20-inch, 24-inch iMac (1.83GHz, 2.0GHz, or 2.16GHz Intel Core 2 Duo) come with 1GB (2x512MB) 667MHz DDR2 SDRAM (PC2-5300), supports up to 3GB.

Now, according to this Apple support document (#304284), the each memory slot will support up to a 2 GB SO-DIMM, but the iMac will only support 3 GB total memory.

If you do install a 2 GB SO-DIMM in both the bottom and top memory slots of the computer, the About This Mac window and Apple System Profiler will both show that you have 4 GB of SDRAM installed. However, Activity Monitor and other similar applications will reveal that only 3 GB of SDRAM has been addressed for use by the computer.

That's an interesting limitaion, I wonder why.

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[Don_HH2K]

If I'm not mistaken, an entire 32-bit system is capable of allocating 4GB of RAM to the entire system, and less memory (usually 2GB or 3GB) to one or more processes.

The reference for the /3GB switch in Windows boot.ini files might explain a bit more about this.

When Leopard comes out in 64-bits (and I assume it will), you should be able to address up to 8TB of memory for any given process.

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[Antony]

Thanks Don.

I found some explanations from this MacNN thread

From what I've read, it is because of a limitation in the 945 chipset (at least the 945G/GM, unsure about 945PM) where it can't address all 4GB of memory due to how some of that memory is remapped for things like PCI/PCIe bus addressing (the 945 chipset supposedly does this so that 32-bit Operating Systems can see and use that hardware). Only 3.2GB is addressable by the OS and applications. The Crestline chipset (part of the codename Santa Rosa platform) will probably address this limitation (March 2007).

The chipset in the current iMacs (945PM, 945GM in the low-end) is the one for Yonah (original Core Duo) that also has support for Merom (Core 2 Duo). Apple just popped a Merom in there to stay current. A bigger update would take longer. The 965 chipset family supports more RAM, but that's a desktop chipset intended for Conroe. Don't know if it will support Merom, and in any case it's a bigger retooling.

Short version: Next update will support more, either with 965M (Santa Rosa) or Conroe with 965-chipset.

My PowerMac G4 has only 2GB of RAM.

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