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10.5: Use 4GB of RAM in older MacBook Pros Laptop Macs
I'm surprised this one hasn't come up earlier, actually. I recently purchased a 4GB Techworks upgrade kit for my previous-generation (2.33GHz) 17" MacBook Pro. From what I'd read, due to either limitations in the Intel logic board, or limitations in the EFI firmware, I'd only be able to access 3.3GB of it.

Not so. Running 10.5.1 Leopard, after installing the RAM, I found I could address all 4GB of it. As seen in both System Profiler and Activity Monitor, I have the full 4GB available, running in dual-channel mode.

Third-party SO-DIMM RAM is cheaper than ever at the moment, so previous-gen MacBook Pro owners who've installed Leopard have no excuse but to upgrade now, really.

[robg adds: I can't confirm this one, but MacTracker lists 3GB as the RAM limit for the 2.33GHz 17" machine. If you have 4GB of usable RAM in some other previous-generation MacBook Pro, please post in the comments...]

Update: Please read the comment below (by macavenger) for the full story on the RAM expansion limits. I've left this one comment and cleaned up all the rest, so that anyone searching can easily see the proper explanation.]
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RAM limitation technical details
Authored by: macavenger on Feb 06, '08 09:55:14AM

Ok, people, here's how this works. While the processor in older macbook pros is 64 bit, the chipset used to access the memory is only 32 bit (as mentioned in a previous post). This 32 bit limitation implies that it is only capable of addressing 2^32 bits=4294967296 bits=4 Gbit 1-byte memory locations, or 4 GBytes of ram (each of the 4 Gbit memory addresses is 1 Byte of actual RAM)

Now computers typically use what is called Memory Mapped I/O, which means that you send data to/from input or output devices by writing that data to a specific memory address which is mapped to the I/O device in question. This is done so that all data transfers look the same to the computer, and don't require any special programing-a read from ram is exactly the same (to the CPU) as a read from the hard drive, it's just looking at a different "memory" address that happens to be connected to a physical device. This is somewhat simplified, of course, as it has to address the proper area of the hard drive, but that is handled at a different level.

In order to make this work, a portion of that 4 GB memory space addressable by the memory controller chip has to be set aside for I/O operations - specifically the upper 700 some odd megs. There is nothing to prevent you from putting in 4 GB of RAM, rather than 3, nor will doing so cause any problems, and as the hint says the computer will SEE that you have 4 GB installed. However, this does not change the fact that it is unable to USE all of it for RAM. Apple could have said the machines supported up to 4 GB, and they would have been technically correct, as the system will "see" all 4 GB. This is, in fact, what many PC manufacturers do. Apple chose instead to claim only 3 GB supported in order to keep people from wasting their money buying extra RAM their computer would be unable to use.

Newer laptop machines (I'd have to check which models specifically) use the 64bit santa-rosa chipset, which being 64 bit can address MUCH more memory, effectively removing the 4 GB limitation. There is still a physical limitation as to the size of the ram chips and how many you can fit into the machine, of course, and there may be other limiting factors, but the addressable memory limitation is gone (at least for a while).

There. I knew that Computer engineering degree I worked so hard for would come in handy sometime :)

Aluminum iMac 20" 2.4 GHz/3GB/300GB HD

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10.5: Use 4GB of RAM in older MacBook Pros
Authored by: QuickSander on Dec 28, '10 02:36:34AM

I just bought 4 GB of RAM and installed it in my late 2006 white iMac 24" (revision B). This iMac only supports 3GB (Mac rumors RAM Buying guide).

In other words: The Activity monitor reports 3GB total while System profiler reports 4GB installed.

But, I created a 1GB RAM drive (which I use to store my Safari Cache directory) and my free available RAM did not lower 1 Byte. Thus, it seems you can use the full 4GB of RAM by:
- Letting the system use 3GB and;
- Create a RAM drive of 1GB (using hdiutil see this hint)

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