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According to apple's latest documentation (at the time of writing), the cap on the newest MacBook Pros is 8GB RAM.

The Mac Pros, however, can take up to 32GB RAM.

So, why do the MacBooks have this cap? Is it a hardware issue, because I don't think it's a software one.

The OS X that runs on the Mac Pros is the same that runs on the MacBook Pros. Being 64-bit software on top of a 64-bit Architecture (both the MacBooks and the Mac Pros), it's obvious that the OS can use as much as 32 GB of RAM.

Is it simply because manufactures don't make RAM sticks larger than 4GB for laptops?

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I have 16GB in my early 2011 Macbook Pro, as well as 512GB Samsung SSD. It works very well. I look at the new 13" Macbook pros ... the screen and the lighter weight are interesting, but I like running virtual machines and not being able to use 16GB is a showstopper. Hopefully this option will emerge in later releases. PS My RAM is from Crucial. –  user35699 Nov 30 '12 at 11:12
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2 Answers

up vote 18 down vote accepted

The limitation imposed is based on the chipsets used on the computers motherboard. For example, the noted Apple maximums you mentioned for the MacBook Pro and the Mac Pro, are smaller than the maximums that those machines can address in practice. The MacBook Pro tested maximum is 16 GB according to EveryMac's MacBook Pro max RAM listing . And as for the Mac Pro its maximum is 128 GB according to EveryMac's MacPro max RAM listing.

As noted on Intel's Specs, the CPU supports a different maximum than the chipset on the motherboard does. The Xeon X5670 used in the Mac Pro supports up to 288 GB.

And as for the i7s e.g. a i7-2860QM used in the MacBook Pros it supports up to 32 GB.

So these limitations come from engineering decisions made by Apple based on what kinds of chipsets are selected to be installed on the motherboard and what those chips support is what enfoces the maximum amount of RAM that a particular Mac can address at or below what the CPU can actually address.

As for why Apple under rates their maximum numbers for RAM on some Macs, thats for Apple to know and for us to wonder. However, admittedly it is a nice practice to under promise and over deliver for whatever reason Apple has. Although, the Mac Pro supports more RAM than Mac OS X can address, according to OWC's testing on Mac Pros, where they discovered Mac OS X unofficially will not address more than 96GB of RAM,but other 64-bit operating system can get to the full 128GB.

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+1 ahh... I didn't know it had to do with the CPU too. Interesting. Great answer, thanks! –  andy May 21 '12 at 6:37
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Anyone who knows isn't saying. But apparently they can use up to 16GB. Other World Computing tests various machines for their actual (not specified) maximum useable memory and sells a 16GB memory kit for the current MacBook Pro.

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+1 interesting robert. How does your answer work with MrDaniel's do you think. I'm quite interested... would love it if I could indeed get noticeable better performance from more RAM (I currently have 8GB). –  andy May 22 '12 at 2:57
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I'd love to know - I can't speak to the difference from personal experience; only that they claim the machines will accept 2x of their memories. Here's a blog article from OWC about their 2011 MBP upgrades. It's certainly worth an ask. –  JRobert May 22 '12 at 16:25
    
Cool, thanks for the heads up JRobert! –  andy Jun 5 '12 at 1:22
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